Mercurial > cpython-lru_cache-weakref
annotate Doc/tut.tex @ 4566:6403c94de50d legacy-trunk
Added safeguard against failure in __del__.
| author | Guido van Rossum <guido@python.org> |
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| date | Wed, 27 Nov 1996 19:39:58 +0000 |
| parents | 847445aeab2d |
| children | 9ac9ea848d7d |
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1 \documentstyle[twoside,11pt,myformat]{report} |
| 112 | 2 |
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3 \title{Python Tutorial} |
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4 |
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5 \input{boilerplate} |
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6 |
| 112 | 7 \begin{document} |
| 8 | |
| 9 \pagenumbering{roman} | |
| 10 | |
| 11 \maketitle | |
| 12 | |
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13 \input{copyright} |
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14 |
| 112 | 15 \begin{abstract} |
| 16 | |
| 17 \noindent | |
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18 Python is a simple, yet powerful programming language that bridges the |
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19 gap between C and shell programming, and is thus ideally suited for |
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20 ``throw-away programming'' |
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21 and rapid prototyping. Its syntax is put |
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22 together from constructs borrowed from a variety of other languages; |
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23 most prominent are influences from ABC, C, Modula-3 and Icon. |
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25 The Python interpreter is easily extended with new functions and data |
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26 types implemented in C. Python is also suitable as an extension |
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27 language for highly customizable C applications such as editors or |
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28 window managers. |
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30 Python is available for various operating systems, amongst which |
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31 several flavors of {\UNIX}, the Apple Macintosh, MS-DOS, Windows |
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32 (3.1(1), '95 and NT flavors), OS/2, and others. |
| 112 | 33 |
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34 This tutorial introduces the reader informally to the basic concepts |
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35 and features of the Python language and system. It helps to have a |
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36 Python interpreter handy for hands-on experience, but as the examples |
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37 are self-contained, the tutorial can be read off-line as well. |
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38 |
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39 For a description of standard objects and modules, see the {\em Python |
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40 Library Reference} document. The {\em Python Reference Manual} gives |
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41 a more formal definition of the language. |
| 112 | 42 |
| 43 \end{abstract} | |
| 44 | |
| 45 \pagebreak | |
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46 { |
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47 \parskip = 0mm |
| 112 | 48 \tableofcontents |
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49 } |
| 112 | 50 |
| 51 \pagebreak | |
| 52 | |
| 53 \pagenumbering{arabic} | |
| 54 | |
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55 |
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56 \chapter{Whetting Your Appetite} |
| 112 | 57 |
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58 \section{Disclaimer} |
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59 |
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60 Now that there are several books out on Python, this tutorial has lost |
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61 its role as the only introduction to Python for most new users. It |
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62 takes time to keep a document like this up to date in the face of |
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63 additions to the language, and I simply don't have enough time to do a |
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64 good job. Therefore, this version of the tutorial is almost unchanged |
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65 since the previous release. This doesn't mean that the tutorial is |
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66 out of date --- all the examples still work exactly as before. There |
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67 are simply some new areas of the language that aren't covered. |
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68 |
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69 To make up for this, there are some chapters at the end cover |
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70 important changes in recent Python releases, and these are up to date |
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71 with the current release. |
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72 |
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73 \section{Introduction} |
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74 |
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75 If you ever wrote a large shell script, you probably know this |
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76 feeling: you'd love to add yet another feature, but it's already so |
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77 slow, and so big, and so complicated; or the feature involves a system |
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78 call or other function that is only accessible from C \ldots Usually |
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79 the problem at hand isn't serious enough to warrant rewriting the |
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80 script in C; perhaps because the problem requires variable-length |
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81 strings or other data types (like sorted lists of file names) that are |
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82 easy in the shell but lots of work to implement in C; or perhaps just |
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83 because you're not sufficiently familiar with C. |
| 112 | 84 |
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85 In such cases, Python may be just the language for you. Python is |
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86 simple to use, but it is a real programming language, offering much |
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87 more structure and support for large programs than the shell has. On |
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88 the other hand, it also offers much more error checking than C, and, |
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89 being a {\em very-high-level language}, it has high-level data types |
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90 built in, such as flexible arrays and dictionaries that would cost you |
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91 days to implement efficiently in C. Because of its more general data |
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92 types Python is applicable to a much larger problem domain than {\em |
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93 Awk} or even {\em Perl}, yet many things are at least as easy in |
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94 Python as in those languages. |
| 112 | 95 |
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96 Python allows you to split up your program in modules that can be |
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97 reused in other Python programs. It comes with a large collection of |
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98 standard modules that you can use as the basis of your programs --- or |
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99 as examples to start learning to program in Python. There are also |
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100 built-in modules that provide things like file I/O, system calls, |
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101 sockets, and even a generic interface to window systems (STDWIN). |
| 112 | 102 |
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103 Python is an interpreted language, which can save you considerable time |
| 112 | 104 during program development because no compilation and linking is |
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105 necessary. The interpreter can be used interactively, which makes it |
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106 easy to experiment with features of the language, to write throw-away |
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107 programs, or to test functions during bottom-up program development. |
| 112 | 108 It is also a handy desk calculator. |
| 109 | |
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110 Python allows writing very compact and readable programs. Programs |
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111 written in Python are typically much shorter than equivalent C |
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112 programs, for several reasons: |
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113 \begin{itemize} |
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114 \item |
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115 the high-level data types allow you to express complex operations in a |
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116 single statement; |
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117 \item |
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118 statement grouping is done by indentation instead of begin/end |
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119 brackets; |
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120 \item |
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121 no variable or argument declarations are necessary. |
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122 \end{itemize} |
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124 Python is {\em extensible}: if you know how to program in C it is easy |
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125 to add a new built-in |
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126 function or |
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127 module to the interpreter, either to |
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128 perform critical operations at maximum speed, or to link Python |
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129 programs to libraries that may only be available in binary form (such |
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130 as a vendor-specific graphics library). Once you are really hooked, |
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131 you can link the Python interpreter into an application written in C |
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132 and use it as an extension or command language for that application. |
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133 |
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134 By the way, the language is named after the BBC show ``Monty |
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135 Python's Flying Circus'' and has nothing to do with nasty reptiles... |
| 112 | 136 |
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137 \section{Where From Here} |
| 112 | 138 |
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139 Now that you are all excited about Python, you'll want to examine it |
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140 in some more detail. Since the best way to learn a language is |
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141 using it, you are invited here to do so. |
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142 |
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143 In the next chapter, the mechanics of using the interpreter are |
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144 explained. This is rather mundane information, but essential for |
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145 trying out the examples shown later. |
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146 |
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147 The rest of the tutorial introduces various features of the Python |
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148 language and system though examples, beginning with simple |
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149 expressions, statements and data types, through functions and modules, |
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150 and finally touching upon advanced concepts like exceptions |
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151 and user-defined classes. |
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152 |
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153 When you're through with the tutorial (or just getting bored), you |
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154 should read the Library Reference, which gives complete (though terse) |
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155 reference material about built-in and standard types, functions and |
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156 modules that can save you a lot of time when writing Python programs. |
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157 |
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158 |
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159 \chapter{Using the Python Interpreter} |
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160 |
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161 \section{Invoking the Interpreter} |
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162 |
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163 The Python interpreter is usually installed as {\tt /usr/local/bin/python} |
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164 on those machines where it is available; putting {\tt /usr/local/bin} in |
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165 your {\UNIX} shell's search path makes it possible to start it by |
| 112 | 166 typing the command |
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167 |
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168 \bcode\begin{verbatim} |
| 112 | 169 python |
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170 \end{verbatim}\ecode |
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171 % |
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172 to the shell. Since the choice of the directory where the interpreter |
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173 lives is an installation option, other places are possible; check with |
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174 your local Python guru or system administrator. (E.g., {\tt |
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175 /usr/local/python} is a popular alternative location.) |
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176 |
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177 The interpreter operates somewhat like the {\UNIX} shell: when called |
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178 with standard input connected to a tty device, it reads and executes |
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179 commands interactively; when called with a file name argument or with |
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180 a file as standard input, it reads and executes a {\em script} from |
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181 that file. |
| 112 | 182 |
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183 A third way of starting the interpreter is |
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184 ``{\tt python -c command [arg] ...}'', which |
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185 executes the statement(s) in {\tt command}, analogous to the shell's |
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186 {\tt -c} option. Since Python statements often contain spaces or other |
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187 characters that are special to the shell, it is best to quote {\tt |
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188 command} in its entirety with double quotes. |
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189 |
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190 Note that there is a difference between ``{\tt python file}'' and |
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191 ``{\tt python $<$file}''. In the latter case, input requests from the |
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192 program, such as calls to {\tt input()} and {\tt raw_input()}, are |
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193 satisfied from {\em file}. Since this file has already been read |
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194 until the end by the parser before the program starts executing, the |
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195 program will encounter EOF immediately. In the former case (which is |
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196 usually what you want) they are satisfied from whatever file or device |
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197 is connected to standard input of the Python interpreter. |
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198 |
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199 When a script file is used, it is sometimes useful to be able to run |
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200 the script and enter interactive mode afterwards. This can be done by |
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201 passing {\tt -i} before the script. (This does not work if the script |
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202 is read from standard input, for the same reason as explained in the |
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203 previous paragraph.) |
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204 |
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205 \subsection{Argument Passing} |
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206 |
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207 When known to the interpreter, the script name and additional |
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208 arguments thereafter are passed to the script in the variable {\tt |
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209 sys.argv}, which is a list of strings. Its length is at least one; |
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210 when no script and no arguments are given, {\tt sys.argv[0]} is an |
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211 empty string. When the script name is given as {\tt '-'} (meaning |
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212 standard input), {\tt sys.argv[0]} is set to {\tt '-'}. When {\tt -c |
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213 command} is used, {\tt sys.argv[0]} is set to {\tt '-c'}. Options |
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214 found after {\tt -c command} are not consumed by the Python |
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215 interpreter's option processing but left in {\tt sys.argv} for the |
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216 command to handle. |
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217 |
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218 \subsection{Interactive Mode} |
| 112 | 219 |
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220 When commands are read from a tty, the interpreter is said to be in |
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221 {\em interactive\ mode}. In this mode it prompts for the next command |
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222 with the {\em primary\ prompt}, usually three greater-than signs ({\tt |
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223 >>>}); for continuation lines it prompts with the |
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224 {\em secondary\ prompt}, |
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225 by default three dots ({\tt ...}). Typing an EOF character |
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226 (Control-D on {\UNIX}, Control-Z on DOS or Windows) |
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227 at the primary prompt causes the interpreter to exit with a zero exit |
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228 status. |
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230 The interpreter prints a welcome message stating its version number |
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231 and a copyright notice before printing the first prompt, e.g.: |
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232 |
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233 \bcode\begin{verbatim} |
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234 python |
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235 Python 1.4 (Oct 25 1996) [GCC 2.7.2] |
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236 Copyright 1991-1996 Stichting Mathematisch Centrum, Amsterdam |
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237 >>> |
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238 \end{verbatim}\ecode |
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239 |
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240 \section{The Interpreter and its Environment} |
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241 |
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242 \subsection{Error Handling} |
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243 |
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244 When an error occurs, the interpreter prints an error |
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245 message and a stack trace. In interactive mode, it then returns to |
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246 the primary prompt; when input came from a file, it exits with a |
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247 nonzero exit status after printing |
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248 the stack trace. (Exceptions handled by an {\tt except} clause in a |
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249 {\tt try} statement are not errors in this context.) Some errors are |
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250 unconditionally fatal and cause an exit with a nonzero exit; this |
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251 applies to internal inconsistencies and some cases of running out of |
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252 memory. All error messages are written to the standard error stream; |
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253 normal output from the executed commands is written to standard |
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254 output. |
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256 Typing the interrupt character (usually Control-C or DEL) to the |
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257 primary or secondary prompt cancels the input and returns to the |
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258 primary prompt.% |
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259 \footnote{ |
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260 A problem with the GNU Readline package may prevent this. |
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261 } |
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262 Typing an interrupt while a command is executing raises the {\tt |
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263 KeyboardInterrupt} exception, which may be handled by a {\tt try} |
| 112 | 264 statement. |
| 265 | |
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266 \subsection{The Module Search Path} |
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267 |
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268 When a module named {\tt spam} is imported, the interpreter searches |
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269 for a file named {\tt spam.py} in the current directory, |
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270 and then in the list of directories specified by |
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271 the environment variable {\tt PYTHONPATH}. This has the same syntax as |
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272 the {\UNIX} shell variable {\tt PATH}, i.e., a list of colon-separated |
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273 directory names. When {\tt PYTHONPATH} is not set, or when the file |
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274 is not found there, the search continues in an installation-dependent |
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275 default path, usually {\tt .:/usr/local/lib/python}. |
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276 |
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277 Actually, modules are searched in the list of directories given by the |
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278 variable {\tt sys.path} which is initialized from the directory |
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279 containing the input script (or the current directory), {\tt |
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280 PYTHONPATH} and the installation-dependent default. This allows |
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281 Python programs that know what they're doing to modify or replace the |
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282 module search path. See the section on Standard Modules later. |
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283 |
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284 \subsection{``Compiled'' Python files} |
| 112 | 285 |
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286 As an important speed-up of the start-up time for short programs that |
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287 use a lot of standard modules, if a file called {\tt spam.pyc} exists |
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288 in the directory where {\tt spam.py} is found, this is assumed to |
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289 contain an already-``compiled'' version of the module {\tt spam}. The |
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290 modification time of the version of {\tt spam.py} used to create {\tt |
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291 spam.pyc} is recorded in {\tt spam.pyc}, and the file is ignored if |
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292 these don't match. |
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293 |
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294 Normally, you don't need to do anything to create the {\tt spam.pyc} file. |
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295 Whenever {\tt spam.py} is successfully compiled, an attempt is made to |
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296 write the compiled version to {\tt spam.pyc}. It is not an error if |
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297 this attempt fails; if for any reason the file is not written |
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298 completely, the resulting {\tt spam.pyc} file will be recognized as |
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299 invalid and thus ignored later. The contents of the {\tt spam.pyc} |
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300 file is platform independent, so a Python module directory can be |
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301 shared by machines of different architectures. (Tip for experts: |
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302 the module {\tt compileall} creates {\tt .pyc} files for all modules.) |
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303 |
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304 \subsection{Executable Python scripts} |
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305 |
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306 On BSD'ish {\UNIX} systems, Python scripts can be made directly |
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307 executable, like shell scripts, by putting the line |
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308 |
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309 \bcode\begin{verbatim} |
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310 #! /usr/local/bin/python |
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311 \end{verbatim}\ecode |
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312 % |
| 112 | 313 (assuming that's the name of the interpreter) at the beginning of the |
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314 script and giving the file an executable mode. The {\tt \#!} must be |
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315 the first two characters of the file. |
| 112 | 316 |
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317 \subsection{The Interactive Startup File} |
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318 |
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319 When you use Python interactively, it is frequently handy to have some |
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320 standard commands executed every time the interpreter is started. You |
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321 can do this by setting an environment variable named {\tt |
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322 PYTHONSTARTUP} to the name of a file containing your start-up |
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323 commands. This is similar to the {\tt .profile} feature of the UNIX |
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324 shells. |
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325 |
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326 This file is only read in interactive sessions, not when Python reads |
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327 commands from a script, and not when {\tt /dev/tty} is given as the |
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328 explicit source of commands (which otherwise behaves like an |
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329 interactive session). It is executed in the same name space where |
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330 interactive commands are executed, so that objects that it defines or |
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331 imports can be used without qualification in the interactive session. |
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332 You can also change the prompts {\tt sys.ps1} and {\tt sys.ps2} in |
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333 this file. |
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334 |
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335 If you want to read an additional start-up file from the current |
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336 directory, you can program this in the global start-up file, e.g. |
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337 \verb\execfile('.pythonrc')\. If you want to use the startup file |
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338 in a script, you must write this explicitly in the script, e.g. |
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339 \verb\import os;\ \verb\execfile(os.environ['PYTHONSTARTUP'])\. |
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340 |
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341 \section{Interactive Input Editing and History Substitution} |
| 112 | 342 |
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343 Some versions of the Python interpreter support editing of the current |
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344 input line and history substitution, similar to facilities found in |
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345 the Korn shell and the GNU Bash shell. This is implemented using the |
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346 {\em GNU\ Readline} library, which supports Emacs-style and vi-style |
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347 editing. This library has its own documentation which I won't |
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348 duplicate here; however, the basics are easily explained. |
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349 |
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350 Perhaps the quickest check to see whether command line editing is |
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351 supported is typing Control-P to the first Python prompt you get. If |
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352 it beeps, you have command line editing. If nothing appears to |
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353 happen, or if \verb/^P/ is echoed, you can skip the rest of this |
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354 section. |
| 112 | 355 |
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356 \subsection{Line Editing} |
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357 |
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358 If supported, input line editing is active whenever the interpreter |
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359 prints a primary or secondary prompt. The current line can be edited |
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360 using the conventional Emacs control characters. The most important |
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361 of these are: C-A (Control-A) moves the cursor to the beginning of the |
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362 line, C-E to the end, C-B moves it one position to the left, C-F to |
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363 the right. Backspace erases the character to the left of the cursor, |
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364 C-D the character to its right. C-K kills (erases) the rest of the |
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365 line to the right of the cursor, C-Y yanks back the last killed |
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366 string. C-underscore undoes the last change you made; it can be |
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367 repeated for cumulative effect. |
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369 \subsection{History Substitution} |
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370 |
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371 History substitution works as follows. All non-empty input lines |
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372 issued are saved in a history buffer, and when a new prompt is given |
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373 you are positioned on a new line at the bottom of this buffer. C-P |
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374 moves one line up (back) in the history buffer, C-N moves one down. |
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375 Any line in the history buffer can be edited; an asterisk appears in |
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376 front of the prompt to mark a line as modified. Pressing the Return |
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377 key passes the current line to the interpreter. C-R starts an |
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378 incremental reverse search; C-S starts a forward search. |
| 112 | 379 |
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380 \subsection{Key Bindings} |
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381 |
| 112 | 382 The key bindings and some other parameters of the Readline library can |
| 383 be customized by placing commands in an initialization file called | |
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384 {\tt \$HOME/.inputrc}. Key bindings have the form |
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385 |
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386 \bcode\begin{verbatim} |
| 112 | 387 key-name: function-name |
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388 \end{verbatim}\ecode |
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389 % |
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390 or |
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391 |
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392 \bcode\begin{verbatim} |
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393 "string": function-name |
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394 \end{verbatim}\ecode |
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395 % |
| 112 | 396 and options can be set with |
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397 |
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398 \bcode\begin{verbatim} |
| 112 | 399 set option-name value |
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400 \end{verbatim}\ecode |
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401 % |
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402 For example: |
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403 |
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404 \bcode\begin{verbatim} |
| 112 | 405 # I prefer vi-style editing: |
| 406 set editing-mode vi | |
| 407 # Edit using a single line: | |
| 408 set horizontal-scroll-mode On | |
| 409 # Rebind some keys: | |
| 410 Meta-h: backward-kill-word | |
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411 "\C-u": universal-argument |
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412 "\C-x\C-r": re-read-init-file |
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413 \end{verbatim}\ecode |
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414 % |
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271
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415 Note that the default binding for TAB in Python is to insert a TAB |
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416 instead of Readline's default filename completion function. If you |
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417 insist, you can override this by putting |
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418 |
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419 \bcode\begin{verbatim} |
| 112 | 420 TAB: complete |
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421 \end{verbatim}\ecode |
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422 % |
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423 in your {\tt \$HOME/.inputrc}. (Of course, this makes it hard to type |
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424 indented continuation lines...) |
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425 |
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426 \subsection{Commentary} |
| 112 | 427 |
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428 This facility is an enormous step forward compared to previous |
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429 versions of the interpreter; however, some wishes are left: It would |
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430 be nice if the proper indentation were suggested on continuation lines |
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431 (the parser knows if an indent token is required next). The |
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432 completion mechanism might use the interpreter's symbol table. A |
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433 command to check (or even suggest) matching parentheses, quotes etc. |
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434 would also be useful. |
| 112 | 435 |
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436 |
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437 \chapter{An Informal Introduction to Python} |
| 112 | 438 |
| 439 In the following examples, input and output are distinguished by the | |
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440 presence or absence of prompts ({\tt >>>} and {\tt ...}): to repeat |
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441 the example, you must type everything after the prompt, when the |
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442 prompt appears; lines that do not begin with a prompt are output from |
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443 the interpreter.% |
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444 \footnote{ |
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445 I'd prefer to use different fonts to distinguish input |
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446 from output, but the amount of LaTeX hacking that would require |
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447 is currently beyond my ability. |
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448 } |
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449 Note that a secondary prompt on a line by itself in an example means |
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450 you must type a blank line; this is used to end a multi-line command. |
| 112 | 451 |
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452 \section{Using Python as a Calculator} |
| 112 | 453 |
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454 Let's try some simple Python commands. Start the interpreter and wait |
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455 for the primary prompt, {\tt >>>}. (It shouldn't take long.) |
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456 |
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457 \subsection{Numbers} |
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458 |
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459 The interpreter acts as a simple calculator: you can type an |
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460 expression at it and it will write the value. Expression syntax is |
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461 straightforward: the operators {\tt +}, {\tt -}, {\tt *} and {\tt /} |
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462 work just like in most other languages (e.g., Pascal or C); parentheses |
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463 can be used for grouping. For example: |
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464 |
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465 \bcode\begin{verbatim} |
| 112 | 466 >>> 2+2 |
| 467 4 | |
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468 >>> # This is a comment |
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469 ... 2+2 |
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470 4 |
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471 >>> 2+2 # and a comment on the same line as code |
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472 4 |
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473 >>> (50-5*6)/4 |
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474 5 |
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475 >>> # Integer division returns the floor: |
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476 ... 7/3 |
| 112 | 477 2 |
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478 >>> 7/-3 |
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479 -3 |
| 112 | 480 >>> |
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481 \end{verbatim}\ecode |
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482 % |
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483 Like in C, the equal sign ({\tt =}) is used to assign a value to a |
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484 variable. The value of an assignment is not written: |
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485 |
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486 \bcode\begin{verbatim} |
| 112 | 487 >>> width = 20 |
| 488 >>> height = 5*9 | |
| 489 >>> width * height | |
| 490 900 | |
| 491 >>> | |
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492 \end{verbatim}\ecode |
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493 % |
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494 A value can be assigned to several variables simultaneously: |
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495 |
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496 \bcode\begin{verbatim} |
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497 >>> x = y = z = 0 # Zero x, y and z |
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498 >>> x |
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499 0 |
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500 >>> y |
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501 0 |
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502 >>> z |
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503 0 |
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504 >>> |
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505 \end{verbatim}\ecode |
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506 % |
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507 There is full support for floating point; operators with mixed type |
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508 operands convert the integer operand to floating point: |
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509 |
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510 \bcode\begin{verbatim} |
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511 >>> 4 * 2.5 / 3.3 |
| 112 | 512 3.0303030303 |
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513 >>> 7.0 / 2 |
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514 3.5 |
| 112 | 515 >>> |
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516 \end{verbatim}\ecode |
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517 |
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518 \subsection{Strings} |
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519 |
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520 Besides numbers, Python can also manipulate strings, enclosed in |
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521 single quotes or double quotes: |
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522 |
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523 \bcode\begin{verbatim} |
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524 >>> 'spam eggs' |
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525 'spam eggs' |
| 112 | 526 >>> 'doesn\'t' |
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527 "doesn't" |
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528 >>> "doesn't" |
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529 "doesn't" |
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530 >>> '"Yes," he said.' |
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531 '"Yes," he said.' |
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532 >>> "\"Yes,\" he said." |
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533 '"Yes," he said.' |
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534 >>> '"Isn\'t," she said.' |
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535 '"Isn\'t," she said.' |
| 112 | 536 >>> |
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537 \end{verbatim}\ecode |
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538 % |
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539 Strings are written the same way as they are typed for input: inside |
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540 quotes and with quotes and other funny characters escaped by backslashes, |
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541 to show the precise value. The string is enclosed in double quotes if |
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542 the string contains a single quote and no double quotes, else it's |
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543 enclosed in single quotes. (The {\tt print} statement, described later, |
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544 can be used to write strings without quotes or escapes.) |
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545 |
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546 Strings can be concatenated (glued together) with the {\tt +} |
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547 operator, and repeated with {\tt *}: |
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548 |
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549 \bcode\begin{verbatim} |
| 112 | 550 >>> word = 'Help' + 'A' |
| 551 >>> word | |
| 552 'HelpA' | |
| 553 >>> '<' + word*5 + '>' | |
| 554 '<HelpAHelpAHelpAHelpAHelpA>' | |
| 555 >>> | |
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556 \end{verbatim}\ecode |
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557 % |
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558 Strings can be subscripted (indexed); like in C, the first character of |
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559 a string has subscript (index) 0. |
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560 |
| 112 | 561 There is no separate character type; a character is simply a string of |
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562 size one. Like in Icon, substrings can be specified with the {\em |
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563 slice} notation: two indices separated by a colon. |
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564 |
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565 \bcode\begin{verbatim} |
| 112 | 566 >>> word[4] |
| 567 'A' | |
| 568 >>> word[0:2] | |
| 569 'He' | |
| 570 >>> word[2:4] | |
| 571 'lp' | |
|
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572 >>> |
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573 \end{verbatim}\ecode |
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574 % |
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575 Slice indices have useful defaults; an omitted first index defaults to |
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576 zero, an omitted second index defaults to the size of the string being |
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577 sliced. |
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578 |
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579 \bcode\begin{verbatim} |
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580 >>> word[:2] # The first two characters |
| 112 | 581 'He' |
|
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582 >>> word[2:] # All but the first two characters |
| 112 | 583 'lpA' |
|
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584 >>> |
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585 \end{verbatim}\ecode |
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586 % |
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587 Here's a useful invariant of slice operations: \verb\s[:i] + s[i:]\ |
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588 equals \verb\s\. |
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589 |
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590 \bcode\begin{verbatim} |
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591 >>> word[:2] + word[2:] |
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592 'HelpA' |
| 112 | 593 >>> word[:3] + word[3:] |
| 594 'HelpA' | |
| 595 >>> | |
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596 \end{verbatim}\ecode |
|
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597 % |
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598 Degenerate slice indices are handled gracefully: an index that is too |
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599 large is replaced by the string size, an upper bound smaller than the |
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600 lower bound returns an empty string. |
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601 |
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602 \bcode\begin{verbatim} |
| 112 | 603 >>> word[1:100] |
| 604 'elpA' | |
| 605 >>> word[10:] | |
| 606 '' | |
| 607 >>> word[2:1] | |
| 608 '' | |
| 609 >>> | |
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610 \end{verbatim}\ecode |
|
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611 % |
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612 Indices may be negative numbers, to start counting from the right. |
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613 For example: |
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614 |
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615 \bcode\begin{verbatim} |
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616 >>> word[-1] # The last character |
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617 'A' |
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618 >>> word[-2] # The last-but-one character |
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619 'p' |
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620 >>> word[-2:] # The last two characters |
| 112 | 621 'pA' |
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622 >>> word[:-2] # All but the last two characters |
| 112 | 623 'Hel' |
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624 >>> |
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625 \end{verbatim}\ecode |
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626 % |
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627 But note that -0 is really the same as 0, so it does not count from |
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628 the right! |
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629 |
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630 \bcode\begin{verbatim} |
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631 >>> word[-0] # (since -0 equals 0) |
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632 'H' |
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633 >>> |
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634 \end{verbatim}\ecode |
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635 % |
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636 Out-of-range negative slice indices are truncated, but don't try this |
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637 for single-element (non-slice) indices: |
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638 |
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639 \bcode\begin{verbatim} |
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640 >>> word[-100:] |
| 112 | 641 'HelpA' |
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642 >>> word[-10] # error |
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643 Traceback (innermost last): |
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644 File "<stdin>", line 1 |
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645 IndexError: string index out of range |
| 112 | 646 >>> |
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647 \end{verbatim}\ecode |
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648 % |
| 112 | 649 The best way to remember how slices work is to think of the indices as |
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650 pointing {\em between} characters, with the left edge of the first |
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651 character numbered 0. Then the right edge of the last character of a |
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652 string of {\tt n} characters has index {\tt n}, for example: |
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653 |
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654 \bcode\begin{verbatim} |
| 112 | 655 +---+---+---+---+---+ |
| 656 | H | e | l | p | A | | |
| 657 +---+---+---+---+---+ | |
| 658 0 1 2 3 4 5 | |
| 659 -5 -4 -3 -2 -1 | |
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660 \end{verbatim}\ecode |
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661 % |
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662 The first row of numbers gives the position of the indices 0...5 in |
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663 the string; the second row gives the corresponding negative indices. |
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664 The slice from \verb\i\ to \verb\j\ consists of all characters between |
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665 the edges labeled \verb\i\ and \verb\j\, respectively. |
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666 |
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667 For nonnegative indices, the length of a slice is the difference of |
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668 the indices, if both are within bounds, e.g., the length of |
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669 \verb\word[1:3]\ is 2. |
| 112 | 670 |
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671 The built-in function {\tt len()} returns the length of a string: |
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672 |
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673 \bcode\begin{verbatim} |
| 112 | 674 >>> s = 'supercalifragilisticexpialidocious' |
| 675 >>> len(s) | |
| 676 34 | |
| 677 >>> | |
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678 \end{verbatim}\ecode |
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679 |
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680 \subsection{Lists} |
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681 |
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682 Python knows a number of {\em compound} data types, used to group |
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683 together other values. The most versatile is the {\em list}, which |
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684 can be written as a list of comma-separated values (items) between |
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685 square brackets. List items need not all have the same type. |
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686 |
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687 \bcode\begin{verbatim} |
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688 >>> a = ['spam', 'eggs', 100, 1234] |
| 112 | 689 >>> a |
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690 ['spam', 'eggs', 100, 1234] |
| 112 | 691 >>> |
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692 \end{verbatim}\ecode |
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693 % |
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694 Like string indices, list indices start at 0, and lists can be sliced, |
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695 concatenated and so on: |
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696 |
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697 \bcode\begin{verbatim} |
| 112 | 698 >>> a[0] |
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699 'spam' |
| 112 | 700 >>> a[3] |
| 701 1234 | |
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702 >>> a[-2] |
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703 100 |
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704 >>> a[1:-1] |
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705 ['eggs', 100] |
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706 >>> a[:2] + ['bacon', 2*2] |
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707 ['spam', 'eggs', 'bacon', 4] |
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708 >>> 3*a[:3] + ['Boe!'] |
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709 ['spam', 'eggs', 100, 'spam', 'eggs', 100, 'spam', 'eggs', 100, 'Boe!'] |
| 112 | 710 >>> |
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711 \end{verbatim}\ecode |
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712 % |
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713 Unlike strings, which are {\em immutable}, it is possible to change |
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714 individual elements of a list: |
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715 |
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716 \bcode\begin{verbatim} |
| 112 | 717 >>> a |
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718 ['spam', 'eggs', 100, 1234] |
| 112 | 719 >>> a[2] = a[2] + 23 |
| 720 >>> a | |
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721 ['spam', 'eggs', 123, 1234] |
| 112 | 722 >>> |
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723 \end{verbatim}\ecode |
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724 % |
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725 Assignment to slices is also possible, and this can even change the size |
| 112 | 726 of the list: |
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727 |
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728 \bcode\begin{verbatim} |
| 112 | 729 >>> # Replace some items: |
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730 ... a[0:2] = [1, 12] |
| 112 | 731 >>> a |
| 732 [1, 12, 123, 1234] | |
| 733 >>> # Remove some: | |
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734 ... a[0:2] = [] |
| 112 | 735 >>> a |
| 736 [123, 1234] | |
| 737 >>> # Insert some: | |
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738 ... a[1:1] = ['bletch', 'xyzzy'] |
| 112 | 739 >>> a |
| 740 [123, 'bletch', 'xyzzy', 1234] | |
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741 >>> a[:0] = a # Insert (a copy of) itself at the beginning |
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742 >>> a |
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743 [123, 'bletch', 'xyzzy', 1234, 123, 'bletch', 'xyzzy', 1234] |
| 112 | 744 >>> |
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745 \end{verbatim}\ecode |
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746 % |
| 112 | 747 The built-in function {\tt len()} also applies to lists: |
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748 |
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749 \bcode\begin{verbatim} |
| 112 | 750 >>> len(a) |
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751 8 |
| 112 | 752 >>> |
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753 \end{verbatim}\ecode |
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754 % |
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755 It is possible to nest lists (create lists containing other lists), |
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756 for example: |
|
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757 |
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758 \bcode\begin{verbatim} |
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759 >>> q = [2, 3] |
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760 >>> p = [1, q, 4] |
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761 >>> len(p) |
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762 3 |
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763 >>> p[1] |
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764 [2, 3] |
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765 >>> p[1][0] |
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766 2 |
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767 >>> p[1].append('xtra') # See section 5.1 |
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768 >>> p |
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769 [1, [2, 3, 'xtra'], 4] |
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770 >>> q |
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771 [2, 3, 'xtra'] |
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772 >>> |
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773 \end{verbatim}\ecode |
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774 % |
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775 Note that in the last example, {\tt p[1]} and {\tt q} really refer to |
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776 the same object! We'll come back to {\em object semantics} later. |
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777 |
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778 \section{First Steps Towards Programming} |
| 112 | 779 |
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780 Of course, we can use Python for more complicated tasks than adding |
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781 two and two together. For instance, we can write an initial |
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782 subsequence of the {\em Fibonacci} series as follows: |
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783 |
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784 \bcode\begin{verbatim} |
| 112 | 785 >>> # Fibonacci series: |
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786 ... # the sum of two elements defines the next |
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787 ... a, b = 0, 1 |
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788 >>> while b < 10: |
| 112 | 789 ... print b |
| 790 ... a, b = b, a+b | |
| 791 ... | |
| 792 1 | |
| 793 1 | |
| 794 2 | |
| 795 3 | |
| 796 5 | |
| 797 8 | |
| 798 >>> | |
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799 \end{verbatim}\ecode |
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800 % |
| 112 | 801 This example introduces several new features. |
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802 |
| 112 | 803 \begin{itemize} |
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804 |
| 112 | 805 \item |
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806 The first line contains a {\em multiple assignment}: the variables |
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807 {\tt a} and {\tt b} simultaneously get the new values 0 and 1. On the |
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808 last line this is used again, demonstrating that the expressions on |
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809 the right-hand side are all evaluated first before any of the |
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810 assignments take place. |
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811 |
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812 \item |
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813 The {\tt while} loop executes as long as the condition (here: {\tt b < |
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814 10}) remains true. In Python, like in C, any non-zero integer value is |
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815 true; zero is false. The condition may also be a string or list value, |
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816 in fact any sequence; anything with a non-zero length is true, empty |
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817 sequences are false. The test used in the example is a simple |
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818 comparison. The standard comparison operators are written the same as |
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819 in C: {\tt <}, {\tt >}, {\tt ==}, {\tt <=}, {\tt >=} and {\tt !=}. |
|
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820 |
| 112 | 821 \item |
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822 The {\em body} of the loop is {\em indented}: indentation is Python's |
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823 way of grouping statements. Python does not (yet!) provide an |
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824 intelligent input line editing facility, so you have to type a tab or |
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825 space(s) for each indented line. In practice you will prepare more |
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826 complicated input for Python with a text editor; most text editors have |
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827 an auto-indent facility. When a compound statement is entered |
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828 interactively, it must be followed by a blank line to indicate |
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829 completion (since the parser cannot guess when you have typed the last |
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830 line). |
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831 |
| 112 | 832 \item |
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833 The {\tt print} statement writes the value of the expression(s) it is |
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834 given. It differs from just writing the expression you want to write |
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835 (as we did earlier in the calculator examples) in the way it handles |
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836 multiple expressions and strings. Strings are printed without quotes, |
|
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837 and a space is inserted between items, so you can format things nicely, |
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838 like this: |
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839 |
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840 \bcode\begin{verbatim} |
| 112 | 841 >>> i = 256*256 |
| 842 >>> print 'The value of i is', i | |
| 843 The value of i is 65536 | |
| 844 >>> | |
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845 \end{verbatim}\ecode |
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846 % |
| 112 | 847 A trailing comma avoids the newline after the output: |
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848 |
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849 \bcode\begin{verbatim} |
| 112 | 850 >>> a, b = 0, 1 |
| 851 >>> while b < 1000: | |
| 852 ... print b, | |
| 853 ... a, b = b, a+b | |
| 854 ... | |
| 855 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 | |
| 856 >>> | |
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857 \end{verbatim}\ecode |
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858 % |
| 112 | 859 Note that the interpreter inserts a newline before it prints the next |
| 860 prompt if the last line was not completed. | |
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861 |
| 112 | 862 \end{itemize} |
| 863 | |
|
929
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864 |
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865 \chapter{More Control Flow Tools} |
| 112 | 866 |
|
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867 Besides the {\tt while} statement just introduced, Python knows the |
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868 usual control flow statements known from other languages, with some |
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869 twists. |
| 112 | 870 |
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871 \section{If Statements} |
| 112 | 872 |
| 873 Perhaps the most well-known statement type is the {\tt if} statement. | |
| 874 For example: | |
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875 |
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876 \bcode\begin{verbatim} |
| 112 | 877 >>> if x < 0: |
| 878 ... x = 0 | |
| 879 ... print 'Negative changed to zero' | |
|
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880 ... elif x == 0: |
| 112 | 881 ... print 'Zero' |
|
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882 ... elif x == 1: |
| 112 | 883 ... print 'Single' |
| 884 ... else: | |
| 885 ... print 'More' | |
| 886 ... | |
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887 \end{verbatim}\ecode |
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888 % |
| 112 | 889 There can be zero or more {\tt elif} parts, and the {\tt else} part is |
|
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890 optional. The keyword `{\tt elif}' is short for `{\tt else if}', and is |
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891 useful to avoid excessive indentation. An {\tt if...elif...elif...} |
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892 sequence is a substitute for the {\em switch} or {\em case} statements |
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893 found in other languages. |
| 112 | 894 |
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895 \section{For Statements} |
| 112 | 896 |
|
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897 The {\tt for} statement in Python differs a bit from what you may be |
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898 used to in C or Pascal. Rather than always iterating over an |
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899 arithmetic progression of numbers (like in Pascal), or leaving the user |
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900 completely free in the iteration test and step (as C), Python's {\tt |
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901 for} statement iterates over the items of any sequence (e.g., a list |
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902 or a string), in the order that they appear in the sequence. For |
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903 example (no pun intended): |
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904 |
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905 \bcode\begin{verbatim} |
| 112 | 906 >>> # Measure some strings: |
|
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907 ... a = ['cat', 'window', 'defenestrate'] |
| 112 | 908 >>> for x in a: |
| 909 ... print x, len(x) | |
| 910 ... | |
| 911 cat 3 | |
| 912 window 6 | |
| 913 defenestrate 12 | |
| 914 >>> | |
|
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915 \end{verbatim}\ecode |
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916 % |
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917 It is not safe to modify the sequence being iterated over in the loop |
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918 (this can only happen for mutable sequence types, i.e., lists). If |
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919 you need to modify the list you are iterating over, e.g., duplicate |
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920 selected items, you must iterate over a copy. The slice notation |
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921 makes this particularly convenient: |
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922 |
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923 \bcode\begin{verbatim} |
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924 >>> for x in a[:]: # make a slice copy of the entire list |
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925 ... if len(x) > 6: a.insert(0, x) |
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926 ... |
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927 >>> a |
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928 ['defenestrate', 'cat', 'window', 'defenestrate'] |
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929 >>> |
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930 \end{verbatim}\ecode |
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931 |
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932 \section{The {\tt range()} Function} |
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933 |
| 112 | 934 If you do need to iterate over a sequence of numbers, the built-in |
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935 function {\tt range()} comes in handy. It generates lists containing |
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936 arithmetic progressions, e.g.: |
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937 |
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938 \bcode\begin{verbatim} |
| 112 | 939 >>> range(10) |
| 940 [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] | |
| 941 >>> | |
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942 \end{verbatim}\ecode |
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943 % |
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944 The given end point is never part of the generated list; {\tt range(10)} |
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945 generates a list of 10 values, exactly the legal indices for items of a |
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946 sequence of length 10. It is possible to let the range start at another |
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947 number, or to specify a different increment (even negative): |
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948 |
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949 \bcode\begin{verbatim} |
| 112 | 950 >>> range(5, 10) |
| 951 [5, 6, 7, 8, 9] | |
| 952 >>> range(0, 10, 3) | |
| 953 [0, 3, 6, 9] | |
| 954 >>> range(-10, -100, -30) | |
| 955 [-10, -40, -70] | |
| 956 >>> | |
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957 \end{verbatim}\ecode |
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958 % |
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959 To iterate over the indices of a sequence, combine {\tt range()} and |
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960 {\tt len()} as follows: |
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961 |
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962 \bcode\begin{verbatim} |
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963 >>> a = ['Mary', 'had', 'a', 'little', 'lamb'] |
| 112 | 964 >>> for i in range(len(a)): |
| 965 ... print i, a[i] | |
| 966 ... | |
| 967 0 Mary | |
| 968 1 had | |
| 969 2 a | |
| 970 3 little | |
|
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971 4 lamb |
| 112 | 972 >>> |
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973 \end{verbatim}\ecode |
| 112 | 974 |
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975 \section{Break and Continue Statements, and Else Clauses on Loops} |
| 112 | 976 |
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977 The {\tt break} statement, like in C, breaks out of the smallest |
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978 enclosing {\tt for} or {\tt while} loop. |
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979 |
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980 The {\tt continue} statement, also borrowed from C, continues with the |
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981 next iteration of the loop. |
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982 |
| 112 | 983 Loop statements may have an {\tt else} clause; it is executed when the |
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984 loop terminates through exhaustion of the list (with {\tt for}) or when |
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985 the condition becomes false (with {\tt while}), but not when the loop is |
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986 terminated by a {\tt break} statement. This is exemplified by the |
| 2013 | 987 following loop, which searches for prime numbers: |
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988 |
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989 \bcode\begin{verbatim} |
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990 >>> for n in range(2, 10): |
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991 ... for x in range(2, n): |
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992 ... if n % x == 0: |
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993 ... print n, 'equals', x, '*', n/x |
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994 ... break |
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995 ... else: |
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996 ... print n, 'is a prime number' |
| 112 | 997 ... |
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998 2 is a prime number |
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999 3 is a prime number |
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1000 4 equals 2 * 2 |
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1001 5 is a prime number |
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1002 6 equals 2 * 3 |
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1003 7 is a prime number |
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1004 8 equals 2 * 4 |
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1005 9 equals 3 * 3 |
| 112 | 1006 >>> |
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1007 \end{verbatim}\ecode |
| 112 | 1008 |
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1009 \section{Pass Statements} |
| 112 | 1010 |
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1011 The {\tt pass} statement does nothing. |
| 112 | 1012 It can be used when a statement is required syntactically but the |
| 1013 program requires no action. | |
| 1014 For example: | |
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1015 |
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1016 \bcode\begin{verbatim} |
| 112 | 1017 >>> while 1: |
| 1018 ... pass # Busy-wait for keyboard interrupt | |
| 1019 ... | |
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1020 \end{verbatim}\ecode |
| 112 | 1021 |
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1022 \section{Defining Functions} |
| 112 | 1023 |
| 1024 We can create a function that writes the Fibonacci series to an | |
| 1025 arbitrary boundary: | |
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1026 |
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1027 \bcode\begin{verbatim} |
| 112 | 1028 >>> def fib(n): # write Fibonacci series up to n |
| 1029 ... a, b = 0, 1 | |
|
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1030 ... while b < n: |
| 112 | 1031 ... print b, |
| 1032 ... a, b = b, a+b | |
| 1033 ... | |
| 1034 >>> # Now call the function we just defined: | |
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1035 ... fib(2000) |
| 112 | 1036 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597 |
| 1037 >>> | |
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1038 \end{verbatim}\ecode |
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1039 % |
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1040 The keyword {\tt def} introduces a function {\em definition}. It must |
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1041 be followed by the function name and the parenthesized list of formal |
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1042 parameters. The statements that form the body of the function starts at |
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1043 the next line, indented by a tab stop. |
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1044 |
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1045 The {\em execution} of a function introduces a new symbol table used |
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1046 for the local variables of the function. More precisely, all variable |
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1047 assignments in a function store the value in the local symbol table; |
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1048 whereas |
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1049 variable references first look in the local symbol table, then |
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1050 in the global symbol table, and then in the table of built-in names. |
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1051 Thus, |
| 2013 | 1052 global variables cannot be directly assigned a value within a |
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1053 function (unless named in a {\tt global} statement), although |
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1054 they may be referenced. |
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1055 |
| 112 | 1056 The actual parameters (arguments) to a function call are introduced in |
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1057 the local symbol table of the called function when it is called; thus, |
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1058 arguments are passed using {\em call\ by\ value}.% |
| 112 | 1059 \footnote{ |
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1060 Actually, {\em call by object reference} would be a better |
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1061 description, since if a mutable object is passed, the caller |
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1062 will see any changes the callee makes to it (e.g., items |
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1063 inserted into a list). |
| 112 | 1064 } |
| 1065 When a function calls another function, a new local symbol table is | |
| 1066 created for that call. | |
| 1067 | |
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1068 A function definition introduces the function name in the |
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1069 current |
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1070 symbol table. The value |
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1071 of the function name |
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1072 has a type that is recognized by the interpreter as a user-defined |
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1073 function. This value can be assigned to another name which can then |
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1074 also be used as a function. This serves as a general renaming |
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1075 mechanism: |
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1076 |
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1077 \bcode\begin{verbatim} |
| 112 | 1078 >>> fib |
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1079 <function object at 10042ed0> |
| 112 | 1080 >>> f = fib |
| 1081 >>> f(100) | |
| 1082 1 1 2 3 5 8 13 21 34 55 89 | |
| 1083 >>> | |
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1084 \end{verbatim}\ecode |
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1085 % |
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1086 You might object that {\tt fib} is not a function but a procedure. In |
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1087 Python, like in C, procedures are just functions that don't return a |
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1088 value. In fact, technically speaking, procedures do return a value, |
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1089 albeit a rather boring one. This value is called {\tt None} (it's a |
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1090 built-in name). Writing the value {\tt None} is normally suppressed by |
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1091 the interpreter if it would be the only value written. You can see it |
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1092 if you really want to: |
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1093 |
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1094 \bcode\begin{verbatim} |
| 112 | 1095 >>> print fib(0) |
| 1096 None | |
| 1097 >>> | |
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1098 \end{verbatim}\ecode |
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1099 % |
| 112 | 1100 It is simple to write a function that returns a list of the numbers of |
| 1101 the Fibonacci series, instead of printing it: | |
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1102 |
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1103 \bcode\begin{verbatim} |
| 112 | 1104 >>> def fib2(n): # return Fibonacci series up to n |
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1105 ... result = [] |
| 112 | 1106 ... a, b = 0, 1 |
|
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1107 ... while b < n: |
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1108 ... result.append(b) # see below |
| 112 | 1109 ... a, b = b, a+b |
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1110 ... return result |
| 112 | 1111 ... |
| 1112 >>> f100 = fib2(100) # call it | |
| 1113 >>> f100 # write the result | |
| 1114 [1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89] | |
| 1115 >>> | |
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1116 \end{verbatim}\ecode |
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1117 % |
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1118 This example, as usual, demonstrates some new Python features: |
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1119 |
| 112 | 1120 \begin{itemize} |
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1121 |
| 112 | 1122 \item |
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1123 The {\tt return} statement returns with a value from a function. {\tt |
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1124 return} without an expression argument is used to return from the middle |
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1125 of a procedure (falling off the end also returns from a procedure), in |
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1126 which case the {\tt None} value is returned. |
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1127 |
| 112 | 1128 \item |
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1129 The statement {\tt result.append(b)} calls a {\em method} of the list |
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1130 object {\tt result}. A method is a function that `belongs' to an |
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1131 object and is named {\tt obj.methodname}, where {\tt obj} is some |
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1132 object (this may be an expression), and {\tt methodname} is the name |
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1133 of a method that is defined by the object's type. Different types |
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1134 define different methods. Methods of different types may have the |
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1135 same name without causing ambiguity. (It is possible to define your |
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1136 own object types and methods, using {\em classes}, as discussed later |
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1137 in this tutorial.) |
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1138 The method {\tt append} shown in the example, is defined for |
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1139 list objects; it adds a new element at the end of the list. In this |
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1140 example |
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1141 it is equivalent to {\tt result = result + [b]}, but more efficient. |
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1142 |
| 112 | 1143 \end{itemize} |
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1144 |
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1145 |
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1146 \chapter{Odds and Ends} |
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1147 |
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1148 This chapter describes some things you've learned about already in |
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1149 more detail, and adds some new things as well. |
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1150 |
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1151 \section{More on Lists} |
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1152 |
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1153 The list data type has some more methods. Here are all of the methods |
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1154 of lists objects: |
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1155 |
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1156 \begin{description} |
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1157 |
| 112 | 1158 \item[{\tt insert(i, x)}] |
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1159 Insert an item at a given position. The first argument is the index of |
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1160 the element before which to insert, so {\tt a.insert(0, x)} inserts at |
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1161 the front of the list, and {\tt a.insert(len(a), x)} is equivalent to |
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1162 {\tt a.append(x)}. |
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1163 |
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1164 \item[{\tt append(x)}] |
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1165 Equivalent to {\tt a.insert(len(a), x)}. |
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1166 |
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1167 \item[{\tt index(x)}] |
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1168 Return the index in the list of the first item whose value is {\tt x}. |
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1169 It is an error if there is no such item. |
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1170 |
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1171 \item[{\tt remove(x)}] |
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1172 Remove the first item from the list whose value is {\tt x}. |
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1173 It is an error if there is no such item. |
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1174 |
| 112 | 1175 \item[{\tt sort()}] |
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1176 Sort the items of the list, in place. |
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1177 |
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1178 \item[{\tt reverse()}] |
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1179 Reverse the elements of the list, in place. |
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1180 |
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1181 \item[{\tt count(x)}] |
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1182 Return the number of times {\tt x} appears in the list. |
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1183 |
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1184 \end{description} |
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1185 |
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1186 An example that uses all list methods: |
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1187 |
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1188 \bcode\begin{verbatim} |
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1189 >>> a = [66.6, 333, 333, 1, 1234.5] |
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1190 >>> print a.count(333), a.count(66.6), a.count('x') |
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1191 2 1 0 |
| 112 | 1192 >>> a.insert(2, -1) |
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1193 >>> a.append(333) |
| 112 | 1194 >>> a |
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1195 [66.6, 333, -1, 333, 1, 1234.5, 333] |
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1196 >>> a.index(333) |
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1197 1 |
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1198 >>> a.remove(333) |
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1199 >>> a |
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1200 [66.6, -1, 333, 1, 1234.5, 333] |
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1201 >>> a.reverse() |
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1202 >>> a |
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1203 [333, 1234.5, 1, 333, -1, 66.6] |
| 112 | 1204 >>> a.sort() |
| 1205 >>> a | |
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1206 [-1, 1, 66.6, 333, 333, 1234.5] |
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1207 >>> |
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1208 \end{verbatim}\ecode |
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1209 |
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1210 \section{The {\tt del} statement} |
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1211 |
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1212 There is a way to remove an item from a list given its index instead |
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1213 of its value: the {\tt del} statement. This can also be used to |
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1214 remove slices from a list (which we did earlier by assignment of an |
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1215 empty list to the slice). For example: |
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1216 |
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1217 \bcode\begin{verbatim} |
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1218 >>> a |
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1219 [-1, 1, 66.6, 333, 333, 1234.5] |
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1220 >>> del a[0] |
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1221 >>> a |
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1222 [1, 66.6, 333, 333, 1234.5] |
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1223 >>> del a[2:4] |
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1224 >>> a |
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1225 [1, 66.6, 1234.5] |
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1226 >>> |
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1227 \end{verbatim}\ecode |
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1228 % |
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1229 {\tt del} can also be used to delete entire variables: |
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1230 |
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1231 \bcode\begin{verbatim} |
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1232 >>> del a |
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1233 >>> |
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1234 \end{verbatim}\ecode |
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1235 % |
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1236 Referencing the name {\tt a} hereafter is an error (at least until |
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1237 another value is assigned to it). We'll find other uses for {\tt del} |
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1238 later. |
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1239 |
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1240 \section{Tuples and Sequences} |
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1241 |
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1242 We saw that lists and strings have many common properties, e.g., |
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1243 indexing and slicing operations. They are two examples of {\em |
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1244 sequence} data types. Since Python is an evolving language, other |
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1245 sequence data types may be added. There is also another standard |
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1246 sequence data type: the {\em tuple}. |
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1247 |
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1248 A tuple consists of a number of values separated by commas, for |
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1249 instance: |
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1250 |
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1251 \bcode\begin{verbatim} |
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1252 >>> t = 12345, 54321, 'hello!' |
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1253 >>> t[0] |
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1254 12345 |
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1255 >>> t |
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1256 (12345, 54321, 'hello!') |
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1257 >>> # Tuples may be nested: |
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1258 ... u = t, (1, 2, 3, 4, 5) |
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1259 >>> u |
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1260 ((12345, 54321, 'hello!'), (1, 2, 3, 4, 5)) |
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1261 >>> |
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1262 \end{verbatim}\ecode |
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1263 % |
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1264 As you see, on output tuples are alway enclosed in parentheses, so |
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1265 that nested tuples are interpreted correctly; they may be input with |
|
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1266 or without surrounding parentheses, although often parentheses are |
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1267 necessary anyway (if the tuple is part of a larger expression). |
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1268 |
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1269 Tuples have many uses, e.g., (x, y) coordinate pairs, employee records |
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1270 from a database, etc. Tuples, like strings, are immutable: it is not |
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1271 possible to assign to the individual items of a tuple (you can |
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1272 simulate much of the same effect with slicing and concatenation, |
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1273 though). |
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1274 |
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1275 A special problem is the construction of tuples containing 0 or 1 |
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1276 items: the syntax has some extra quirks to accommodate these. Empty |
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1277 tuples are constructed by an empty pair of parentheses; a tuple with |
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1278 one item is constructed by following a value with a comma |
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1279 (it is not sufficient to enclose a single value in parentheses). |
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1280 Ugly, but effective. For example: |
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1281 |
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1282 \bcode\begin{verbatim} |
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1283 >>> empty = () |
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1284 >>> singleton = 'hello', # <-- note trailing comma |
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1285 >>> len(empty) |
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1286 0 |
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1287 >>> len(singleton) |
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1288 1 |
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1289 >>> singleton |
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1290 ('hello',) |
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1291 >>> |
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1292 \end{verbatim}\ecode |
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1293 % |
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1294 The statement {\tt t = 12345, 54321, 'hello!'} is an example of {\em |
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1295 tuple packing}: the values {\tt 12345}, {\tt 54321} and {\tt 'hello!'} |
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1296 are packed together in a tuple. The reverse operation is also |
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1297 possible, e.g.: |
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1298 |
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1299 \bcode\begin{verbatim} |
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1300 >>> x, y, z = t |
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1301 >>> |
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1302 \end{verbatim}\ecode |
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1303 % |
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1304 This is called, appropriately enough, {\em tuple unpacking}. Tuple |
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1305 unpacking requires that the list of variables on the left has the same |
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1306 number of elements as the length of the tuple. Note that multiple |
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1307 assignment is really just a combination of tuple packing and tuple |
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1308 unpacking! |
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1309 |
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1310 Occasionally, the corresponding operation on lists is useful: {\em list |
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1311 unpacking}. This is supported by enclosing the list of variables in |
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1312 square brackets: |
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1313 |
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1314 \bcode\begin{verbatim} |
|
2063
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1315 >>> a = ['spam', 'eggs', 100, 1234] |
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1316 >>> [a1, a2, a3, a4] = a |
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1317 >>> |
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1318 \end{verbatim}\ecode |
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1319 |
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1320 \section{Dictionaries} |
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1321 |
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1322 Another useful data type built into Python is the {\em dictionary}. |
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1323 Dictionaries are sometimes found in other languages as ``associative |
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1324 memories'' or ``associative arrays''. Unlike sequences, which are |
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1325 indexed by a range of numbers, dictionaries are indexed by {\em keys}, |
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1326 which are strings (the use of non-string values as keys |
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1327 is supported, but beyond the scope of this tutorial). |
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1328 It is best to think of a dictionary as an unordered set of |
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1329 {\em key:value} pairs, with the requirement that the keys are unique |
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1330 (within one dictionary). |
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1331 A pair of braces creates an empty dictionary: \verb/{}/. |
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1332 Placing a comma-separated list of key:value pairs within the |
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1333 braces adds initial key:value pairs to the dictionary; this is also the |
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1334 way dictionaries are written on output. |
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1335 |
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1336 The main operations on a dictionary are storing a value with some key |
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1337 and extracting the value given the key. It is also possible to delete |
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1338 a key:value pair |
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1339 with {\tt del}. |
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1340 If you store using a key that is already in use, the old value |
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1341 associated with that key is forgotten. It is an error to extract a |
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1342 value using a non-existent key. |
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1343 |
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1344 The {\tt keys()} method of a dictionary object returns a list of all the |
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1345 keys used in the dictionary, in random order (if you want it sorted, |
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1346 just apply the {\tt sort()} method to the list of keys). To check |
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1347 whether a single key is in the dictionary, use the \verb/has_key()/ |
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1348 method of the dictionary. |
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1349 |
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1350 Here is a small example using a dictionary: |
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1351 |
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1352 \bcode\begin{verbatim} |
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1353 >>> tel = {'jack': 4098, 'sape': 4139} |
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1354 >>> tel['guido'] = 4127 |
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1355 >>> tel |
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1356 {'sape': 4139, 'guido': 4127, 'jack': 4098} |
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1357 >>> tel['jack'] |
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1358 4098 |
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1359 >>> del tel['sape'] |
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1360 >>> tel['irv'] = 4127 |
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1361 >>> tel |
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1362 {'guido': 4127, 'irv': 4127, 'jack': 4098} |
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1363 >>> tel.keys() |
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1364 ['guido', 'irv', 'jack'] |
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1365 >>> tel.has_key('guido') |
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1366 1 |
| 112 | 1367 >>> |
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1368 \end{verbatim}\ecode |
| 112 | 1369 |
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1370 \section{More on Conditions} |
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1371 |
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1372 The conditions used in {\tt while} and {\tt if} statements above can |
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1373 contain other operators besides comparisons. |
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1374 |
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1375 The comparison operators {\tt in} and {\tt not in} check whether a value |
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1376 occurs (does not occur) in a sequence. The operators {\tt is} and {\tt |
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1377 is not} compare whether two objects are really the same object; this |
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1378 only matters for mutable objects like lists. All comparison operators |
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1379 have the same priority, which is lower than that of all numerical |
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1380 operators. |
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1381 |
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1382 Comparisons can be chained: e.g., {\tt a < b == c} tests whether {\tt a} |
|
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1383 is less than {\tt b} and moreover {\tt b} equals {\tt c}. |
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1384 |
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1385 Comparisons may be combined by the Boolean operators {\tt and} and {\tt |
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1386 or}, and the outcome of a comparison (or of any other Boolean |
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1387 expression) may be negated with {\tt not}. These all have lower |
|
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1388 priorities than comparison operators again; between them, {\tt not} has |
|
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1389 the highest priority, and {\tt or} the lowest, so that |
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1390 {\tt A and not B or C} is equivalent to {\tt (A and (not B)) or C}. Of |
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1391 course, parentheses can be used to express the desired composition. |
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1392 |
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1393 The Boolean operators {\tt and} and {\tt or} are so-called {\em |
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1394 shortcut} operators: their arguments are evaluated from left to right, |
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1395 and evaluation stops as soon as the outcome is determined. E.g., if |
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1396 {\tt A} and {\tt C} are true but {\tt B} is false, {\tt A and B and C} |
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1397 does not evaluate the expression C. In general, the return value of a |
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1398 shortcut operator, when used as a general value and not as a Boolean, is |
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1399 the last evaluated argument. |
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1400 |
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1401 It is possible to assign the result of a comparison or other Boolean |
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1402 expression to a variable. For example, |
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1403 |
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1404 \bcode\begin{verbatim} |
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1405 >>> string1, string2, string3 = '', 'Trondheim', 'Hammer Dance' |
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1406 >>> non_null = string1 or string2 or string3 |
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1407 >>> non_null |
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1408 'Trondheim' |
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1409 >>> |
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1410 \end{verbatim}\ecode |
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1411 % |
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1412 Note that in Python, unlike C, assignment cannot occur inside expressions. |
|
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1413 |
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1414 \section{Comparing Sequences and Other Types} |
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1415 |
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1416 Sequence objects may be compared to other objects with the same |
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1417 sequence type. The comparison uses {\em lexicographical} ordering: |
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1418 first the first two items are compared, and if they differ this |
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1419 determines the outcome of the comparison; if they are equal, the next |
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1420 two items are compared, and so on, until either sequence is exhausted. |
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1421 If two items to be compared are themselves sequences of the same type, |
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1422 the lexicographical comparison is carried out recursively. If all |
|
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1423 items of two sequences compare equal, the sequences are considered |
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1424 equal. If one sequence is an initial subsequence of the other, the |
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1425 shorted sequence is the smaller one. Lexicographical ordering for |
|
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replace ASCII by macro call
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1426 strings uses the \ASCII{} ordering for individual characters. Some |
|
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1427 examples of comparisons between sequences with the same types: |
|
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1428 |
|
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1429 \bcode\begin{verbatim} |
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1430 (1, 2, 3) < (1, 2, 4) |
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1431 [1, 2, 3] < [1, 2, 4] |
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1432 'ABC' < 'C' < 'Pascal' < 'Python' |
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1433 (1, 2, 3, 4) < (1, 2, 4) |
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1434 (1, 2) < (1, 2, -1) |
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1435 (1, 2, 3) = (1.0, 2.0, 3.0) |
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1436 (1, 2, ('aa', 'ab')) < (1, 2, ('abc', 'a'), 4) |
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1437 \end{verbatim}\ecode |
|
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1438 % |
|
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1439 Note that comparing objects of different types is legal. The outcome |
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1440 is deterministic but arbitrary: the types are ordered by their name. |
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1441 Thus, a list is always smaller than a string, a string is always |
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1442 smaller than a tuple, etc. Mixed numeric types are compared according |
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1443 to their numeric value, so 0 equals 0.0, etc.% |
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1444 \footnote{ |
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1445 The rules for comparing objects of different types should |
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1446 not be relied upon; they may change in a future version of |
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1447 the language. |
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1448 } |
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1449 |
|
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1450 |
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1451 \chapter{Modules} |
| 112 | 1452 |
|
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1453 If you quit from the Python interpreter and enter it again, the |
| 112 | 1454 definitions you have made (functions and variables) are lost. |
| 1455 Therefore, if you want to write a somewhat longer program, you are | |
| 1456 better off using a text editor to prepare the input for the interpreter | |
|
1767
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Lots of small corrections by Andrew Kuchling (plus all new rotor docs)
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1457 and running it with that file as input instead. This is known as creating a |
|
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1458 {\em script}. As your program gets longer, you may want to split it |
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1459 into several files for easier maintenance. You may also want to use a |
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1460 handy function that you've written in several programs without copying |
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1461 its definition into each program. |
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1462 |
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1463 To support this, Python has a way to put definitions in a file and use |
| 112 | 1464 them in a script or in an interactive instance of the interpreter. |
|
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1465 Such a file is called a {\em module}; definitions from a module can be |
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1466 {\em imported} into other modules or into the {\em main} module (the |
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1467 collection of variables that you have access to in a script |
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1468 executed at the top level |
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1469 and in calculator mode). |
| 112 | 1470 |
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1471 A module is a file containing Python definitions and statements. The |
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1472 file name is the module name with the suffix {\tt .py} appended. Within |
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1473 a module, the module's name (as a string) is available as the value of |
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1474 the global variable {\tt __name__}. For instance, use your favorite text |
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1475 editor to create a file called {\tt fibo.py} in the current directory |
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1476 with the following contents: |
|
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1477 |
|
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1478 \bcode\begin{verbatim} |
| 112 | 1479 # Fibonacci numbers module |
| 1480 | |
| 1481 def fib(n): # write Fibonacci series up to n | |
| 1482 a, b = 0, 1 | |
|
1934
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1483 while b < n: |
| 112 | 1484 print b, |
| 1485 a, b = b, a+b | |
| 1486 | |
| 1487 def fib2(n): # return Fibonacci series up to n | |
|
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1488 result = [] |
| 112 | 1489 a, b = 0, 1 |
|
1934
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1490 while b < n: |
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1491 result.append(b) |
| 112 | 1492 a, b = b, a+b |
|
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1493 return result |
|
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1494 \end{verbatim}\ecode |
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1495 % |
|
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1496 Now enter the Python interpreter and import this module with the |
| 112 | 1497 following command: |
|
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1498 |
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1499 \bcode\begin{verbatim} |
| 112 | 1500 >>> import fibo |
| 1501 >>> | |
|
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1502 \end{verbatim}\ecode |
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1503 % |
| 112 | 1504 This does not enter the names of the functions defined in |
| 1505 {\tt fibo} | |
|
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1506 directly in the current symbol table; it only enters the module name |
| 112 | 1507 {\tt fibo} |
| 1508 there. | |
| 1509 Using the module name you can access the functions: | |
|
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1510 |
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1511 \bcode\begin{verbatim} |
| 112 | 1512 >>> fibo.fib(1000) |
| 1513 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 | |
| 1514 >>> fibo.fib2(100) | |
| 1515 [1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89] | |
|
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1516 >>> fibo.__name__ |
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1517 'fibo' |
| 112 | 1518 >>> |
|
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1519 \end{verbatim}\ecode |
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1520 % |
| 112 | 1521 If you intend to use a function often you can assign it to a local name: |
|
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1522 |
|
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1523 \bcode\begin{verbatim} |
| 112 | 1524 >>> fib = fibo.fib |
| 1525 >>> fib(500) | |
| 1526 1 1 2 3 5 8 13 21 34 55 89 144 233 377 | |
| 1527 >>> | |
|
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1528 \end{verbatim}\ecode |
| 112 | 1529 |
|
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1530 \section{More on Modules} |
| 112 | 1531 |
| 1532 A module can contain executable statements as well as function | |
| 1533 definitions. | |
| 1534 These statements are intended to initialize the module. | |
| 1535 They are executed only the | |
|
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1536 {\em first} |
| 112 | 1537 time the module is imported somewhere.% |
| 1538 \footnote{ | |
|
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1539 In fact function definitions are also `statements' that are |
|
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1540 `executed'; the execution enters the function name in the |
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1541 module's global symbol table. |
| 112 | 1542 } |
| 1543 | |
| 1544 Each module has its own private symbol table, which is used as the | |
| 1545 global symbol table by all functions defined in the module. | |
| 1546 Thus, the author of a module can use global variables in the module | |
| 1547 without worrying about accidental clashes with a user's global | |
| 1548 variables. | |
| 1549 On the other hand, if you know what you are doing you can touch a | |
| 1550 module's global variables with the same notation used to refer to its | |
| 1551 functions, | |
| 1552 {\tt modname.itemname}. | |
| 1553 | |
| 1554 Modules can import other modules. | |
| 1555 It is customary but not required to place all | |
| 1556 {\tt import} | |
| 1557 statements at the beginning of a module (or script, for that matter). | |
| 1558 The imported module names are placed in the importing module's global | |
| 1559 symbol table. | |
| 1560 | |
| 1561 There is a variant of the | |
| 1562 {\tt import} | |
| 1563 statement that imports names from a module directly into the importing | |
| 1564 module's symbol table. | |
| 1565 For example: | |
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1566 |
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1567 \bcode\begin{verbatim} |
| 112 | 1568 >>> from fibo import fib, fib2 |
| 1569 >>> fib(500) | |
| 1570 1 1 2 3 5 8 13 21 34 55 89 144 233 377 | |
| 1571 >>> | |
|
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1572 \end{verbatim}\ecode |
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1573 % |
| 112 | 1574 This does not introduce the module name from which the imports are taken |
| 1575 in the local symbol table (so in the example, {\tt fibo} is not | |
| 1576 defined). | |
| 1577 | |
| 1578 There is even a variant to import all names that a module defines: | |
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1579 |
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1580 \bcode\begin{verbatim} |
| 112 | 1581 >>> from fibo import * |
| 1582 >>> fib(500) | |
| 1583 1 1 2 3 5 8 13 21 34 55 89 144 233 377 | |
| 1584 >>> | |
|
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1585 \end{verbatim}\ecode |
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1586 % |
| 112 | 1587 This imports all names except those beginning with an underscore |
|
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1588 ({\tt _}). |
| 112 | 1589 |
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1590 \section{Standard Modules} |
| 112 | 1591 |
|
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1592 Python comes with a library of standard modules, described in a separate |
|
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1593 document (Python Library Reference). Some modules are built into the |
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1594 interpreter; these provide access to operations that are not part of the |
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1595 core of the language but are nevertheless built in, either for |
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1596 efficiency or to provide access to operating system primitives such as |
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1597 system calls. The set of such modules is a configuration option; e.g., |
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1598 the {\tt amoeba} module is only provided on systems that somehow support |
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1599 Amoeba primitives. One particular module deserves some attention: {\tt |
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1600 sys}, which is built into every Python interpreter. The variables {\tt |
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1601 sys.ps1} and {\tt sys.ps2} define the strings used as primary and |
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1602 secondary prompts: |
|
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1603 |
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1604 \bcode\begin{verbatim} |
| 112 | 1605 >>> import sys |
| 1606 >>> sys.ps1 | |
| 1607 '>>> ' | |
| 1608 >>> sys.ps2 | |
| 1609 '... ' | |
| 1610 >>> sys.ps1 = 'C> ' | |
| 1611 C> print 'Yuck!' | |
| 1612 Yuck! | |
| 1613 C> | |
|
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1614 \end{verbatim}\ecode |
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1615 % |
| 112 | 1616 These two variables are only defined if the interpreter is in |
| 1617 interactive mode. | |
| 1618 | |
| 1619 The variable | |
| 1620 {\tt sys.path} | |
| 1621 is a list of strings that determine the interpreter's search path for | |
| 1622 modules. | |
| 1623 It is initialized to a default path taken from the environment variable | |
| 1624 {\tt PYTHONPATH}, | |
| 1625 or from a built-in default if | |
| 1626 {\tt PYTHONPATH} | |
| 1627 is not set. | |
| 1628 You can modify it using standard list operations, e.g.: | |
|
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1629 |
|
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1630 \bcode\begin{verbatim} |
| 112 | 1631 >>> import sys |
| 1632 >>> sys.path.append('/ufs/guido/lib/python') | |
| 1633 >>> | |
|
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1634 \end{verbatim}\ecode |
| 112 | 1635 |
|
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1636 \section{The {\tt dir()} function} |
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1637 |
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1638 The built-in function {\tt dir} is used to find out which names a module |
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1639 defines. It returns a sorted list of strings: |
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1640 |
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1641 \bcode\begin{verbatim} |
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1642 >>> import fibo, sys |
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1643 >>> dir(fibo) |
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1644 ['__name__', 'fib', 'fib2'] |
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1645 >>> dir(sys) |
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1646 ['__name__', 'argv', 'builtin_module_names', 'copyright', 'exit', |
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1647 'maxint', 'modules', 'path', 'ps1', 'ps2', 'setprofile', 'settrace', |
|
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1648 'stderr', 'stdin', 'stdout', 'version'] |
|
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1649 >>> |
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1650 \end{verbatim}\ecode |
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1651 % |
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1652 Without arguments, {\tt dir()} lists the names you have defined currently: |
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1653 |
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1654 \bcode\begin{verbatim} |
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1655 >>> a = [1, 2, 3, 4, 5] |
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1656 >>> import fibo, sys |
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1657 >>> fib = fibo.fib |
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1658 >>> dir() |
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1659 ['__name__', 'a', 'fib', 'fibo', 'sys'] |
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1660 >>> |
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1661 \end{verbatim}\ecode |
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1662 % |
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1663 Note that it lists all types of names: variables, modules, functions, etc. |
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1664 |
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1665 {\tt dir()} does not list the names of built-in functions and variables. |
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1666 If you want a list of those, they are defined in the standard module |
|
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1667 {\tt __builtin__}: |
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1668 |
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1669 \bcode\begin{verbatim} |
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1670 >>> import __builtin__ |
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1671 >>> dir(__builtin__) |
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1672 ['AccessError', 'AttributeError', 'ConflictError', 'EOFError', 'IOError', |
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1673 'ImportError', 'IndexError', 'KeyError', 'KeyboardInterrupt', |
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1674 'MemoryError', 'NameError', 'None', 'OverflowError', 'RuntimeError', |
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1675 'SyntaxError', 'SystemError', 'SystemExit', 'TypeError', 'ValueError', |
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1676 'ZeroDivisionError', '__name__', 'abs', 'apply', 'chr', 'cmp', 'coerce', |
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1677 'compile', 'dir', 'divmod', 'eval', 'execfile', 'filter', 'float', |
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1678 'getattr', 'hasattr', 'hash', 'hex', 'id', 'input', 'int', 'len', 'long', |
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1679 'map', 'max', 'min', 'oct', 'open', 'ord', 'pow', 'range', 'raw_input', |
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1680 'reduce', 'reload', 'repr', 'round', 'setattr', 'str', 'type', 'xrange'] |
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1681 >>> |
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1682 \end{verbatim}\ecode |
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1683 |
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1684 |
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1685 \chapter{Output Formatting} |
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1686 |
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1687 So far we've encountered two ways of writing values: {\em expression |
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1688 statements} and the {\tt print} statement. (A third way is using the |
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1689 {\tt write} method of file objects; the standard output file can be |
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1690 referenced as {\tt sys.stdout}. See the Library Reference for more |
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1691 information on this.) |
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1692 |
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1693 Often you'll want more control over the formatting of your output than |
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1694 simply printing space-separated values. The key to nice formatting in |
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1695 Python is to do all the string handling yourself; using string slicing |
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1696 and concatenation operations you can create any lay-out you can imagine. |
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1697 The standard module {\tt string} contains some useful operations for |
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1698 padding strings to a given column width; these will be discussed shortly. |
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1699 Finally, the \code{\%} operator (modulo) with a string left argument |
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1700 interprets this string as a C sprintf format string to be applied to the |
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1701 right argument, and returns the string resulting from this formatting |
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1702 operation. |
| 112 | 1703 |
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1704 One question remains, of course: how do you convert values to strings? |
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1705 Luckily, Python has a way to convert any value to a string: just write |
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1706 the value between reverse quotes (\verb/``/). Some examples: |
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1707 |
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1708 \bcode\begin{verbatim} |
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1709 >>> x = 10 * 3.14 |
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1710 >>> y = 200*200 |
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1711 >>> s = 'The value of x is ' + `x` + ', and y is ' + `y` + '...' |
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1712 >>> print s |
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1713 The value of x is 31.4, and y is 40000... |
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1714 >>> # Reverse quotes work on other types besides numbers: |
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1715 ... p = [x, y] |
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1716 >>> ps = `p` |
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1717 >>> ps |
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1718 '[31.4, 40000]' |
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1719 >>> # Converting a string adds string quotes and backslashes: |
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1720 ... hello = 'hello, world\n' |
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1721 >>> hellos = `hello` |
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1722 >>> print hellos |
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1723 'hello, world\012' |
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1724 >>> # The argument of reverse quotes may be a tuple: |
|
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1725 ... `x, y, ('spam', 'eggs')` |
|
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1726 "(31.4, 40000, ('spam', 'eggs'))" |
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1727 >>> |
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1728 \end{verbatim}\ecode |
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1729 % |
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1730 Here are two ways to write a table of squares and cubes: |
| 112 | 1731 |
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1732 \bcode\begin{verbatim} |
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1733 >>> import string |
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1734 >>> for x in range(1, 11): |
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1735 ... print string.rjust(`x`, 2), string.rjust(`x*x`, 3), |
|
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1736 ... # Note trailing comma on previous line |
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1737 ... print string.rjust(`x*x*x`, 4) |
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1738 ... |
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1739 1 1 1 |
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1740 2 4 8 |
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1741 3 9 27 |
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1742 4 16 64 |
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1743 5 25 125 |
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1744 6 36 216 |
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1745 7 49 343 |
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1746 8 64 512 |
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1747 9 81 729 |
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1748 10 100 1000 |
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1749 >>> for x in range(1,11): |
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1750 ... print '%2d %3d %4d' % (x, x*x, x*x*x) |
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1751 ... |
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1752 1 1 1 |
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1753 2 4 8 |
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1754 3 9 27 |
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1755 4 16 64 |
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1756 5 25 125 |
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1757 6 36 216 |
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1758 7 49 343 |
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1759 8 64 512 |
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1760 9 81 729 |
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1761 10 100 1000 |
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1762 >>> |
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1763 \end{verbatim}\ecode |
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1764 % |
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1765 (Note that one space between each column was added by the way {\tt print} |
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1766 works: it always adds spaces between its arguments.) |
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1767 |
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1768 This example demonstrates the function {\tt string.rjust()}, which |
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1769 right-justifies a string in a field of a given width by padding it with |
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1770 spaces on the left. There are similar functions {\tt string.ljust()} |
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1771 and {\tt string.center()}. These functions do not write anything, they |
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1772 just return a new string. If the input string is too long, they don't |
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1773 truncate it, but return it unchanged; this will mess up your column |
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1774 lay-out but that's usually better than the alternative, which would be |
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1775 lying about a value. (If you really want truncation you can always add |
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1776 a slice operation, as in {\tt string.ljust(x,~n)[0:n]}.) |
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1777 |
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1778 There is another function, {\tt string.zfill}, which pads a numeric |
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1779 string on the left with zeros. It understands about plus and minus |
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1780 signs: |
|
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1781 |
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1782 \bcode\begin{verbatim} |
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1783 >>> string.zfill('12', 5) |
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1784 '00012' |
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1785 >>> string.zfill('-3.14', 7) |
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1786 '-003.14' |
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1787 >>> string.zfill('3.14159265359', 5) |
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1788 '3.14159265359' |
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1789 >>> |
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1790 \end{verbatim}\ecode |
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1791 |
|
929
ff02b948c9a8
Add chapter on classes (mostly from ../misc/CLASSES).
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1792 |
|
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1793 \chapter{Errors and Exceptions} |
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1794 |
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1795 Until now error messages haven't been more than mentioned, but if you |
|
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1796 have tried out the examples you have probably seen some. There are |
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1797 (at least) two distinguishable kinds of errors: {\em syntax\ errors} |
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1798 and {\em exceptions}. |
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1799 |
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1800 \section{Syntax Errors} |
| 112 | 1801 |
| 1802 Syntax errors, also known as parsing errors, are perhaps the most common | |
|
271
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*** empty log message ***
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1803 kind of complaint you get while you are still learning Python: |
|
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1804 |
|
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1805 \bcode\begin{verbatim} |
| 112 | 1806 >>> while 1 print 'Hello world' |
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1807 File "<stdin>", line 1 |
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1808 while 1 print 'Hello world' |
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1809 ^ |
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1810 SyntaxError: invalid syntax |
| 112 | 1811 >>> |
|
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1812 \end{verbatim}\ecode |
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1813 % |
| 112 | 1814 The parser repeats the offending line and displays a little `arrow' |
| 1815 pointing at the earliest point in the line where the error was detected. | |
| 1816 The error is caused by (or at least detected at) the token | |
|
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1817 {\em preceding} |
| 112 | 1818 the arrow: in the example, the error is detected at the keyword |
| 1819 {\tt print}, since a colon ({\tt :}) is missing before it. | |
|
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1820 File name and line number are printed so you know where to look in case |
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1821 the input came from a script. |
| 112 | 1822 |
|
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1823 \section{Exceptions} |
| 112 | 1824 |
|
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1825 Even if a statement or expression is syntactically correct, it may |
|
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1826 cause an error when an attempt is made to execute it. |
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1827 Errors detected during execution are called {\em exceptions} and are |
|
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1828 not unconditionally fatal: you will soon learn how to handle them in |
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1829 Python programs. Most exceptions are not handled by programs, |
|
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1830 however, and result in error messages as shown here: |
|
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1831 |
|
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|
1832 \bcode\small\begin{verbatim} |
| 112 | 1833 >>> 10 * (1/0) |
|
1426
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|
1834 Traceback (innermost last): |
|
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1835 File "<stdin>", line 1 |
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|
1836 ZeroDivisionError: integer division or modulo |
|
2063
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|
1837 >>> 4 + spam*3 |
|
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1838 Traceback (innermost last): |
|
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1839 File "<stdin>", line 1 |
|
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|
1840 NameError: spam |
|
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|
1841 >>> '2' + 2 |
|
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1842 Traceback (innermost last): |
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1843 File "<stdin>", line 1 |
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1844 TypeError: illegal argument type for built-in operation |
| 112 | 1845 >>> |
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1846 \end{verbatim}\normalsize\ecode |
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1847 % |
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1848 The last line of the error message indicates what happened. |
| 112 | 1849 Exceptions come in different types, and the type is printed as part of |
| 1850 the message: the types in the example are | |
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1851 {\tt ZeroDivisionError}, |
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1852 {\tt NameError} |
| 112 | 1853 and |
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1854 {\tt TypeError}. |
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1855 The string printed as the exception type is the name of the built-in |
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1856 name for the exception that occurred. This is true for all built-in |
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1857 exceptions, but need not be true for user-defined exceptions (although |
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1858 it is a useful convention). |
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1859 Standard exception names are built-in identifiers (not reserved |
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1860 keywords). |
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1861 |
| 112 | 1862 The rest of the line is a detail whose interpretation depends on the |
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1863 exception type; its meaning is dependent on the exception type. |
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1864 |
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1865 The preceding part of the error message shows the context where the |
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1866 exception happened, in the form of a stack backtrace. |
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1867 In general it contains a stack backtrace listing source lines; however, |
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1868 it will not display lines read from standard input. |
| 112 | 1869 |
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1870 The Python Library Reference Manual lists the built-in exceptions and |
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1871 their meanings. |
| 112 | 1872 |
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1873 \section{Handling Exceptions} |
| 112 | 1874 |
| 1875 It is possible to write programs that handle selected exceptions. | |
| 1876 Look at the following example, which prints a table of inverses of | |
| 1877 some floating point numbers: | |
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1878 |
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1879 \bcode\begin{verbatim} |
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1880 >>> numbers = [0.3333, 2.5, 0, 10] |
| 112 | 1881 >>> for x in numbers: |
| 1882 ... print x, | |
| 1883 ... try: | |
| 1884 ... print 1.0 / x | |
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1885 ... except ZeroDivisionError: |
| 112 | 1886 ... print '*** has no inverse ***' |
| 1887 ... | |
| 1888 0.3333 3.00030003 | |
| 1889 2.5 0.4 | |
| 1890 0 *** has no inverse *** | |
| 1891 10 0.1 | |
| 1892 >>> | |
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1893 \end{verbatim}\ecode |
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1894 % |
| 112 | 1895 The {\tt try} statement works as follows. |
| 1896 \begin{itemize} | |
| 1897 \item | |
| 1898 First, the | |
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1899 {\em try\ clause} |
| 112 | 1900 (the statement(s) between the {\tt try} and {\tt except} keywords) is |
| 1901 executed. | |
| 1902 \item | |
| 1903 If no exception occurs, the | |
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1904 {\em except\ clause} |
| 112 | 1905 is skipped and execution of the {\tt try} statement is finished. |
| 1906 \item | |
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1907 If an exception occurs during execution of the try clause, |
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1908 the rest of the clause is skipped. Then if |
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1909 its type matches the exception named after the {\tt except} keyword, |
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1910 the rest of the try clause is skipped, the except clause is executed, |
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1911 and then execution continues after the {\tt try} statement. |
| 112 | 1912 \item |
| 1913 If an exception occurs which does not match the exception named in the | |
| 1914 except clause, it is passed on to outer try statements; if no handler is | |
| 1915 found, it is an | |
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1916 {\em unhandled\ exception} |
| 112 | 1917 and execution stops with a message as shown above. |
| 1918 \end{itemize} | |
| 1919 A {\tt try} statement may have more than one except clause, to specify | |
| 1920 handlers for different exceptions. | |
| 1921 At most one handler will be executed. | |
| 1922 Handlers only handle exceptions that occur in the corresponding try | |
| 1923 clause, not in other handlers of the same {\tt try} statement. | |
| 1924 An except clause may name multiple exceptions as a parenthesized list, | |
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1925 e.g.: |
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1926 |
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1927 \bcode\begin{verbatim} |
| 112 | 1928 ... except (RuntimeError, TypeError, NameError): |
| 1929 ... pass | |
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1930 \end{verbatim}\ecode |
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1931 % |
| 112 | 1932 The last except clause may omit the exception name(s), to serve as a |
| 1933 wildcard. | |
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1934 Use this with extreme caution, since it is easy to mask a real |
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1935 programming error in this way! |
| 112 | 1936 |
| 1937 When an exception occurs, it may have an associated value, also known as | |
| 1938 the exceptions's | |
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1939 {\em argument}. |
| 112 | 1940 The presence and type of the argument depend on the exception type. |
| 1941 For exception types which have an argument, the except clause may | |
| 1942 specify a variable after the exception name (or list) to receive the | |
| 1943 argument's value, as follows: | |
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1944 |
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1945 \bcode\begin{verbatim} |
| 112 | 1946 >>> try: |
|
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1947 ... spam() |
| 112 | 1948 ... except NameError, x: |
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1949 ... print 'name', x, 'undefined' |
| 112 | 1950 ... |
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1951 name spam undefined |
| 112 | 1952 >>> |
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1953 \end{verbatim}\ecode |
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1954 % |
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1955 If an exception has an argument, it is printed as the last part |
| 112 | 1956 (`detail') of the message for unhandled exceptions. |
| 1957 | |
| 1958 Exception handlers don't just handle exceptions if they occur | |
| 1959 immediately in the try clause, but also if they occur inside functions | |
| 1960 that are called (even indirectly) in the try clause. | |
| 1961 For example: | |
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1962 |
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1963 \bcode\begin{verbatim} |
| 112 | 1964 >>> def this_fails(): |
| 1965 ... x = 1/0 | |
| 1966 ... | |
| 1967 >>> try: | |
| 1968 ... this_fails() | |
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1969 ... except ZeroDivisionError, detail: |
| 112 | 1970 ... print 'Handling run-time error:', detail |
| 1971 ... | |
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1972 Handling run-time error: integer division or modulo |
| 112 | 1973 >>> |
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1974 \end{verbatim}\ecode |
| 112 | 1975 |
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1976 \section{Raising Exceptions} |
| 112 | 1977 |
| 1978 The {\tt raise} statement allows the programmer to force a specified | |
| 1979 exception to occur. | |
| 1980 For example: | |
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1981 |
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1982 \bcode\begin{verbatim} |
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1983 >>> raise NameError, 'HiThere' |
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1984 Traceback (innermost last): |
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1985 File "<stdin>", line 1 |
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1986 NameError: HiThere |
| 112 | 1987 >>> |
|
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1988 \end{verbatim}\ecode |
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1989 % |
| 112 | 1990 The first argument to {\tt raise} names the exception to be raised. |
| 1991 The optional second argument specifies the exception's argument. | |
| 1992 | |
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1993 \section{User-defined Exceptions} |
| 112 | 1994 |
| 1995 Programs may name their own exceptions by assigning a string to a | |
| 1996 variable. | |
| 1997 For example: | |
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1998 |
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1999 \bcode\begin{verbatim} |
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2000 >>> my_exc = 'my_exc' |
| 112 | 2001 >>> try: |
| 2002 ... raise my_exc, 2*2 | |
| 2003 ... except my_exc, val: | |
|
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2004 ... print 'My exception occurred, value:', val |
| 112 | 2005 ... |
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2006 My exception occurred, value: 4 |
| 112 | 2007 >>> raise my_exc, 1 |
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2008 Traceback (innermost last): |
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2009 File "<stdin>", line 1 |
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2010 my_exc: 1 |
| 112 | 2011 >>> |
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2012 \end{verbatim}\ecode |
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2013 % |
| 112 | 2014 Many standard modules use this to report errors that may occur in |
| 2015 functions they define. | |
| 2016 | |
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2017 \section{Defining Clean-up Actions} |
| 112 | 2018 |
| 2019 The {\tt try} statement has another optional clause which is intended to | |
| 2020 define clean-up actions that must be executed under all circumstances. | |
| 2021 For example: | |
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2022 |
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2023 \bcode\begin{verbatim} |
| 112 | 2024 >>> try: |
| 2025 ... raise KeyboardInterrupt | |
| 2026 ... finally: | |
| 2027 ... print 'Goodbye, world!' | |
| 2028 ... | |
| 2029 Goodbye, world! | |
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2030 Traceback (innermost last): |
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2031 File "<stdin>", line 2 |
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2032 KeyboardInterrupt |
| 112 | 2033 >>> |
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2034 \end{verbatim}\ecode |
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2035 % |
|
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2036 A {\tt finally} clause is executed whether or not an exception has |
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2037 occurred in the {\tt try} clause. When an exception has occurred, it |
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2038 is re-raised after the {\tt finally} clause is executed. The |
|
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2039 {\tt finally} clause is also executed ``on the way out'' when the |
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2040 {\tt try} statement is left via a {\tt break} or {\tt return} |
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2041 statement. |
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2042 |
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2043 A {\tt try} statement must either have one or more {\tt except} |
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2044 clauses or one {\tt finally} clause, but not both. |
| 112 | 2045 |
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|
2046 |
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|
2047 \chapter{Classes} |
|
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|
2048 |
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2049 Python's class mechanism adds classes to the language with a minimum |
|
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|
2050 of new syntax and semantics. It is a mixture of the class mechanisms |
|
1767
907605320a5f
Lots of small corrections by Andrew Kuchling (plus all new rotor docs)
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|
2051 found in \Cpp{} and Modula-3. As is true for modules, classes in Python |
|
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|
2052 do not put an absolute barrier between definition and user, but rather |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2053 rely on the politeness of the user not to ``break into the |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2054 definition.'' The most important features of classes are retained |
|
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|
2055 with full power, however: the class inheritance mechanism allows |
|
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|
2056 multiple base classes, a derived class can override any methods of its |
|
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|
2057 base class(es), a method can call the method of a base class with the |
|
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|
2058 same name. Objects can contain an arbitrary amount of private data. |
|
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|
2059 |
|
1767
907605320a5f
Lots of small corrections by Andrew Kuchling (plus all new rotor docs)
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|
2060 In \Cpp{} terminology, all class members (including the data members) are |
|
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|
2061 {\em public}, and all member functions are {\em virtual}. There are |
|
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2062 no special constructors or destructors. As in Modula-3, there are no |
|
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|
2063 shorthands for referencing the object's members from its methods: the |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2064 method function is declared with an explicit first argument |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2065 representing the object, which is provided implicitly by the call. As |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2066 in Smalltalk, classes themselves are objects, albeit in the wider |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2067 sense of the word: in Python, all data types are objects. This |
|
1767
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Lots of small corrections by Andrew Kuchling (plus all new rotor docs)
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|
2068 provides semantics for importing and renaming. But, just like in \Cpp{} |
|
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|
2069 or Modula-3, built-in types cannot be used as base classes for |
|
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Lots of small corrections by Andrew Kuchling (plus all new rotor docs)
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|
2070 extension by the user. Also, like in \Cpp{} but unlike in Modula-3, most |
|
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|
2071 built-in operators with special syntax (arithmetic operators, |
|
1751
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2072 subscripting etc.) can be redefined for class members. |
|
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|
2073 |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2074 |
|
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|
2075 \section{A word about terminology} |
|
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|
2076 |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2077 Lacking universally accepted terminology to talk about classes, I'll |
|
1767
907605320a5f
Lots of small corrections by Andrew Kuchling (plus all new rotor docs)
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|
2078 make occasional use of Smalltalk and \Cpp{} terms. (I'd use Modula-3 |
|
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|
2079 terms, since its object-oriented semantics are closer to those of |
|
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Lots of small corrections by Andrew Kuchling (plus all new rotor docs)
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|
2080 Python than \Cpp{}, but I expect that few readers have heard of it...) |
|
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|
2081 |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2082 I also have to warn you that there's a terminological pitfall for |
|
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|
2083 object-oriented readers: the word ``object'' in Python does not |
|
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Lots of small corrections by Andrew Kuchling (plus all new rotor docs)
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|
2084 necessarily mean a class instance. Like \Cpp{} and Modula-3, and unlike |
|
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|
2085 Smalltalk, not all types in Python are classes: the basic built-in |
|
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|
2086 types like integers and lists aren't, and even somewhat more exotic |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2087 types like files aren't. However, {\em all} Python types share a little |
|
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|
2088 bit of common semantics that is best described by using the word |
|
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|
2089 object. |
|
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|
2090 |
|
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|
2091 Objects have individuality, and multiple names (in multiple scopes) |
|
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|
2092 can be bound to the same object. This is known as aliasing in other |
|
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|
2093 languages. This is usually not appreciated on a first glance at |
|
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|
2094 Python, and can be safely ignored when dealing with immutable basic |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2095 types (numbers, strings, tuples). However, aliasing has an |
|
1751
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Merge alpha100 branch back to main trunk
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|
2096 (intended!) effect on the semantics of Python code involving mutable |
|
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|
2097 objects such as lists, dictionaries, and most types representing |
|
ff02b948c9a8
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|
2098 entities outside the program (files, windows, etc.). This is usually |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2099 used to the benefit of the program, since aliases behave like pointers |
|
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|
2100 in some respects. For example, passing an object is cheap since only |
|
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|
2101 a pointer is passed by the implementation; and if a function modifies |
|
ff02b948c9a8
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|
2102 an object passed as an argument, the caller will see the change --- this |
|
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|
2103 obviates the need for two different argument passing mechanisms as in |
|
ff02b948c9a8
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|
2104 Pascal. |
|
ff02b948c9a8
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|
2105 |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2106 |
|
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|
2107 \section{Python scopes and name spaces} |
|
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|
2108 |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2109 Before introducing classes, I first have to tell you something about |
|
ff02b948c9a8
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|
2110 Python's scope rules. Class definitions play some neat tricks with |
|
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|
2111 name spaces, and you need to know how scopes and name spaces work to |
|
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|
2112 fully understand what's going on. Incidentally, knowledge about this |
|
ff02b948c9a8
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|
2113 subject is useful for any advanced Python programmer. |
|
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|
2114 |
|
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|
2115 Let's begin with some definitions. |
|
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|
2116 |
|
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|
2117 A {\em name space} is a mapping from names to objects. Most name |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2118 spaces are currently implemented as Python dictionaries, but that's |
|
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|
2119 normally not noticeable in any way (except for performance), and it |
|
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|
2120 may change in the future. Examples of name spaces are: the set of |
|
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|
2121 built-in names (functions such as \verb\abs()\, and built-in exception |
|
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|
2122 names); the global names in a module; and the local names in a |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2123 function invocation. In a sense the set of attributes of an object |
|
1934
f3ae7c17ac08
Moved author and date to boilerplate.tex.
Guido van Rossum <guido@python.org>
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1852
diff
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|
2124 also form a name space. The important thing to know about name |
|
929
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|
2125 spaces is that there is absolutely no relation between names in |
|
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|
2126 different name spaces; for instance, two different modules may both |
|
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|
2127 define a function ``maximize'' without confusion --- users of the |
|
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|
2128 modules must prefix it with the module name. |
|
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|
2129 |
|
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|
2130 By the way, I use the word {\em attribute} for any name following a |
|
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|
2131 dot --- for example, in the expression \verb\z.real\, \verb\real\ is |
|
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|
2132 an attribute of the object \verb\z\. Strictly speaking, references to |
|
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|
2133 names in modules are attribute references: in the expression |
|
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|
2134 \verb\modname.funcname\, \verb\modname\ is a module object and |
|
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|
2135 \verb\funcname\ is an attribute of it. In this case there happens to |
|
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|
2136 be a straightforward mapping between the module's attributes and the |
|
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|
2137 global names defined in the module: they share the same name space!% |
|
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|
2138 \footnote{ |
|
1751
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|
2139 Except for one thing. Module objects have a secret read-only |
|
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|
2140 attribute called {\tt __dict__} which returns the dictionary |
|
ec25aad43724
Merge alpha100 branch back to main trunk
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|
2141 used to implement the module's name space; the name |
|
ec25aad43724
Merge alpha100 branch back to main trunk
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|
2142 {\tt __dict__} is an attribute but not a global name. |
|
ec25aad43724
Merge alpha100 branch back to main trunk
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|
2143 Obviously, using this violates the abstraction of name space |
|
ec25aad43724
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|
2144 implementation, and should be restricted to things like |
|
ec25aad43724
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|
2145 post-mortem debuggers... |
|
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|
2146 } |
|
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|
2147 |
|
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|
2148 Attributes may be read-only or writable. In the latter case, |
|
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|
2149 assignment to attributes is possible. Module attributes are writable: |
|
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|
2150 you can write \verb\modname.the_answer = 42\. Writable attributes may |
|
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|
2151 also be deleted with the del statement, e.g. |
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|
2152 \verb\del modname.the_answer\. |
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2153 |
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|
2154 Name spaces are created at different moments and have different |
|
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|
2155 lifetimes. The name space containing the built-in names is created |
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|
2156 when the Python interpreter starts up, and is never deleted. The |
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|
2157 global name space for a module is created when the module definition |
|
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|
2158 is read in; normally, module name spaces also last until the |
|
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|
2159 interpreter quits. The statements executed by the top-level |
|
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|
2160 invocation of the interpreter, either read from a script file or |
|
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|
2161 interactively, are considered part of a module called \verb\__main__\, |
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2162 so they have their own global name space. (The built-in names |
|
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* lib3.tex (module string): added rindex().
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|
2163 actually also live in a module; this is called \verb\__builtin__\.) |
|
929
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2164 |
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2165 The local name space for a function is created when the function is |
|
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|
2166 called, and deleted when the function returns or raises an exception |
|
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2167 that is not handled within the function. (Actually, forgetting would |
|
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|
2168 be a better way to describe what actually happens.) Of course, |
|
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|
2169 recursive invocations each have their own local name space. |
|
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2170 |
|
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2171 A {\em scope} is a textual region of a Python program where a name space |
|
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2172 is directly accessible. ``Directly accessible'' here means that an |
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|
2173 unqualified reference to a name attempts to find the name in the name |
|
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|
2174 space. |
|
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2175 |
|
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|
2176 Although scopes are determined statically, they are used dynamically. |
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2177 At any time during execution, exactly three nested scopes are in use |
|
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|
2178 (i.e., exactly three name spaces are directly accessible): the |
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|
2179 innermost scope, which is searched first, contains the local names, |
|
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|
2180 the middle scope, searched next, contains the current module's global |
|
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2181 names, and the outermost scope (searched last) is the name space |
|
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|
2182 containing built-in names. |
|
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2183 |
|
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|
2184 Usually, the local scope references the local names of the (textually) |
|
2647
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typos, layout and other small things
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|
2185 current function. Outside of functions, the local scope references |
|
929
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|
2186 the same name space as the global scope: the module's name space. |
|
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|
2187 Class definitions place yet another name space in the local scope. |
|
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2188 |
|
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2189 It is important to realize that scopes are determined textually: the |
|
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|
2190 global scope of a function defined in a module is that module's name |
|
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|
2191 space, no matter from where or by what alias the function is called. |
|
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|
2192 On the other hand, the actual search for names is done dynamically, at |
|
2647
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|
2193 run time --- however, the language definition is evolving towards |
|
929
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|
2194 static name resolution, at ``compile'' time, so don't rely on dynamic |
|
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|
2195 name resolution! (In fact, local variables are already determined |
|
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|
2196 statically.) |
|
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|
2197 |
|
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|
2198 A special quirk of Python is that assignments always go into the |
|
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|
2199 innermost scope. Assignments do not copy data --- they just |
|
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|
2200 bind names to objects. The same is true for deletions: the statement |
|
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|
2201 \verb\del x\ removes the binding of x from the name space referenced by the |
|
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|
2202 local scope. In fact, all operations that introduce new names use the |
|
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|
2203 local scope: in particular, import statements and function definitions |
|
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|
2204 bind the module or function name in the local scope. (The |
|
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|
2205 \verb\global\ statement can be used to indicate that particular |
|
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|
2206 variables live in the global scope.) |
|
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|
2207 |
|
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|
2208 |
|
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|
2209 \section{A first look at classes} |
|
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|
2210 |
|
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|
2211 Classes introduce a little bit of new syntax, three new object types, |
|
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2212 and some new semantics. |
|
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2213 |
|
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|
2214 |
|
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|
2215 \subsection{Class definition syntax} |
|
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2216 |
|
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|
2217 The simplest form of class definition looks like this: |
|
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2218 |
|
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|
2219 \begin{verbatim} |
|
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2220 class ClassName: |
|
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|
2221 <statement-1> |
|
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2222 . |
|
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|
2223 . |
|
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2224 . |
|
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|
2225 <statement-N> |
|
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2226 \end{verbatim} |
|
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2227 |
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2228 Class definitions, like function definitions (\verb\def\ statements) |
|
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2229 must be executed before they have any effect. (You could conceivably |
|
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2230 place a class definition in a branch of an \verb\if\ statement, or |
|
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2231 inside a function.) |
|
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2232 |
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2233 In practice, the statements inside a class definition will usually be |
|
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2234 function definitions, but other statements are allowed, and sometimes |
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2235 useful --- we'll come back to this later. The function definitions |
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2236 inside a class normally have a peculiar form of argument list, |
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2237 dictated by the calling conventions for methods --- again, this is |
|
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|
2238 explained later. |
|
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2239 |
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2240 When a class definition is entered, a new name space is created, and |
|
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2241 used as the local scope --- thus, all assignments to local variables |
|
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2242 go into this new name space. In particular, function definitions bind |
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2243 the name of the new function here. |
|
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2244 |
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2245 When a class definition is left normally (via the end), a {\em class |
|
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2246 object} is created. This is basically a wrapper around the contents |
|
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2247 of the name space created by the class definition; we'll learn more |
|
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|
2248 about class objects in the next section. The original local scope |
|
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2249 (the one in effect just before the class definitions was entered) is |
|
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2250 reinstated, and the class object is bound here to class name given in |
|
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|
2251 the class definition header (ClassName in the example). |
|
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2252 |
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2253 |
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2254 \subsection{Class objects} |
|
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2255 |
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|
2256 Class objects support two kinds of operations: attribute references |
|
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2257 and instantiation. |
|
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2258 |
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|
2259 {\em Attribute references} use the standard syntax used for all |
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2260 attribute references in Python: \verb\obj.name\. Valid attribute |
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2261 names are all the names that were in the class's name space when the |
|
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|
2262 class object was created. So, if the class definition looked like |
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2263 this: |
|
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2264 |
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|
2265 \begin{verbatim} |
|
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2266 class MyClass: |
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2267 i = 12345 |
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2268 def f(x): |
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2269 return 'hello world' |
|
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2270 \end{verbatim} |
|
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2271 |
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2272 then \verb\MyClass.i\ and \verb\MyClass.f\ are valid attribute |
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2273 references, returning an integer and a function object, respectively. |
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2274 Class attributes can also be assigned to, so you can change the |
|
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2275 value of \verb\MyClass.i\ by assignment. |
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2276 |
|
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|
2277 Class {\em instantiation} uses function notation. Just pretend that |
|
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2278 the class object is a parameterless function that returns a new |
|
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2279 instance of the class. For example, (assuming the above class): |
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2280 |
|
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|
2281 \begin{verbatim} |
|
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2282 x = MyClass() |
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2283 \end{verbatim} |
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2284 |
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2285 creates a new {\em instance} of the class and assigns this object to |
|
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|
2286 the local variable \verb\x\. |
|
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2287 |
|
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2288 |
|
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|
2289 \subsection{Instance objects} |
|
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2290 |
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|
2291 Now what can we do with instance objects? The only operations |
|
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2292 understood by instance objects are attribute references. There are |
|
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2293 two kinds of valid attribute names. |
|
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2294 |
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2295 The first I'll call {\em data attributes}. These correspond to |
|
1767
907605320a5f
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|
2296 ``instance variables'' in Smalltalk, and to ``data members'' in \Cpp{}. |
|
929
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|
2297 Data attributes need not be declared; like local variables, they |
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2298 spring into existence when they are first assigned to. For example, |
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2299 if \verb\x\ in the instance of \verb\MyClass\ created above, the |
|
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2300 following piece of code will print the value 16, without leaving a |
|
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|
2301 trace: |
|
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2302 |
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|
2303 \begin{verbatim} |
|
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|
2304 x.counter = 1 |
|
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2305 while x.counter < 10: |
|
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|
2306 x.counter = x.counter * 2 |
|
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2307 print x.counter |
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2308 del x.counter |
|
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|
2309 \end{verbatim} |
|
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2310 |
|
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|
2311 The second kind of attribute references understood by instance objects |
|
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2312 are {\em methods}. A method is a function that ``belongs to'' an |
|
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2313 object. (In Python, the term method is not unique to class instances: |
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2314 other object types can have methods as well, e.g., list objects have |
|
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2315 methods called append, insert, remove, sort, and so on. However, |
|
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2316 below, we'll use the term method exclusively to mean methods of class |
|
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2317 instance objects, unless explicitly stated otherwise.) |
|
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2318 |
|
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2319 Valid method names of an instance object depend on its class. By |
|
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2320 definition, all attributes of a class that are (user-defined) function |
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2321 objects define corresponding methods of its instances. So in our |
|
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2322 example, \verb\x.f\ is a valid method reference, since |
|
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|
2323 \verb\MyClass.f\ is a function, but \verb\x.i\ is not, since |
|
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2324 \verb\MyClass.i\ is not. But \verb\x.f\ is not the |
|
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|
2325 same thing as \verb\MyClass.f\ --- it is a {\em method object}, not a |
|
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|
2326 function object. |
|
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2327 |
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2328 |
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|
2329 \subsection{Method objects} |
|
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2330 |
|
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2331 Usually, a method is called immediately, e.g.: |
|
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2332 |
|
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|
2333 \begin{verbatim} |
|
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2334 x.f() |
|
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2335 \end{verbatim} |
|
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2336 |
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|
2337 In our example, this will return the string \verb\'hello world'\. |
|
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2338 However, it is not necessary to call a method right away: \verb\x.f\ |
|
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2339 is a method object, and can be stored away and called at a later |
|
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2340 moment, for example: |
|
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2341 |
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2342 \begin{verbatim} |
|
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2343 xf = x.f |
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2344 while 1: |
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2345 print xf() |
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2346 \end{verbatim} |
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2347 |
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2348 will continue to print \verb\hello world\ until the end of time. |
|
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2349 |
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2350 What exactly happens when a method is called? You may have noticed |
|
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2351 that \verb\x.f()\ was called without an argument above, even though |
|
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|
2352 the function definition for \verb\f\ specified an argument. What |
|
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2353 happened to the argument? Surely Python raises an exception when a |
|
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2354 function that requires an argument is called without any --- even if |
|
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2355 the argument isn't actually used... |
|
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2356 |
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2357 Actually, you may have guessed the answer: the special thing about |
|
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2358 methods is that the object is passed as the first argument of the |
|
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2359 function. In our example, the call \verb\x.f()\ is exactly equivalent |
|
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2360 to \verb\MyClass.f(x)\. In general, calling a method with a list of |
|
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2361 {\em n} arguments is equivalent to calling the corresponding function |
|
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2362 with an argument list that is created by inserting the method's object |
|
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2363 before the first argument. |
|
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2364 |
|
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|
2365 If you still don't understand how methods work, a look at the |
|
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|
2366 implementation can perhaps clarify matters. When an instance |
|
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2367 attribute is referenced that isn't a data attribute, its class is |
|
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|
2368 searched. If the name denotes a valid class attribute that is a |
|
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|
2369 function object, a method object is created by packing (pointers to) |
|
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|
2370 the instance object and the function object just found together in an |
|
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|
2371 abstract object: this is the method object. When the method object is |
|
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|
2372 called with an argument list, it is unpacked again, a new argument |
|
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|
2373 list is constructed from the instance object and the original argument |
|
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|
2374 list, and the function object is called with this new argument list. |
|
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2375 |
|
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2376 |
|
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|
2377 \section{Random remarks} |
|
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2378 |
|
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|
2379 |
|
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|
2380 [These should perhaps be placed more carefully...] |
|
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2381 |
|
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2382 |
|
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|
2383 Data attributes override method attributes with the same name; to |
|
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|
2384 avoid accidental name conflicts, which may cause hard-to-find bugs in |
|
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|
2385 large programs, it is wise to use some kind of convention that |
|
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|
2386 minimizes the chance of conflicts, e.g., capitalize method names, |
|
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|
2387 prefix data attribute names with a small unique string (perhaps just |
|
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|
2388 an underscore), or use verbs for methods and nouns for data attributes. |
|
929
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2389 |
|
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2390 |
|
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|
2391 Data attributes may be referenced by methods as well as by ordinary |
|
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|
2392 users (``clients'') of an object. In other words, classes are not |
|
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|
2393 usable to implement pure abstract data types. In fact, nothing in |
|
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|
2394 Python makes it possible to enforce data hiding --- it is all based |
|
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|
2395 upon convention. (On the other hand, the Python implementation, |
|
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|
2396 written in C, can completely hide implementation details and control |
|
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|
2397 access to an object if necessary; this can be used by extensions to |
|
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|
2398 Python written in C.) |
|
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|
2399 |
|
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|
2400 |
|
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|
2401 Clients should use data attributes with care --- clients may mess up |
|
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|
2402 invariants maintained by the methods by stamping on their data |
|
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|
2403 attributes. Note that clients may add data attributes of their own to |
|
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|
2404 an instance object without affecting the validity of the methods, as |
|
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|
2405 long as name conflicts are avoided --- again, a naming convention can |
|
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|
2406 save a lot of headaches here. |
|
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|
2407 |
|
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|
2408 |
|
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|
2409 There is no shorthand for referencing data attributes (or other |
|
ff02b948c9a8
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|
2410 methods!) from within methods. I find that this actually increases |
|
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|
2411 the readability of methods: there is no chance of confusing local |
|
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|
2412 variables and instance variables when glancing through a method. |
|
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|
2413 |
|
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|
2414 |
|
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|
2415 Conventionally, the first argument of methods is often called |
|
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|
2416 \verb\self\. This is nothing more than a convention: the name |
|
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|
2417 \verb\self\ has absolutely no special meaning to Python. (Note, |
|
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|
2418 however, that by not following the convention your code may be less |
|
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|
2419 readable by other Python programmers, and it is also conceivable that |
|
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|
2420 a {\em class browser} program be written which relies upon such a |
|
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|
2421 convention.) |
|
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|
2422 |
|
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|
2423 |
|
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|
2424 Any function object that is a class attribute defines a method for |
|
ff02b948c9a8
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|
2425 instances of that class. It is not necessary that the function |
|
ff02b948c9a8
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diff
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|
2426 definition is textually enclosed in the class definition: assigning a |
|
ff02b948c9a8
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diff
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|
2427 function object to a local variable in the class is also ok. For |
|
ff02b948c9a8
Add chapter on classes (mostly from ../misc/CLASSES).
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|
2428 example: |
|
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|
2429 |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2430 \begin{verbatim} |
|
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|
2431 # Function defined outside the class |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2432 def f1(self, x, y): |
|
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|
2433 return min(x, x+y) |
|
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|
2434 |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2435 class C: |
|
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|
2436 f = f1 |
|
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|
2437 def g(self): |
|
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|
2438 return 'hello world' |
|
ff02b948c9a8
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diff
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|
2439 h = g |
|
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|
2440 \end{verbatim} |
|
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|
2441 |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2442 Now \verb\f\, \verb\g\ and \verb\h\ are all attributes of class |
|
ff02b948c9a8
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diff
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|
2443 \verb\C\ that refer to function objects, and consequently they are all |
|
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|
2444 methods of instances of \verb\C\ --- \verb\h\ being exactly equivalent |
|
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diff
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|
2445 to \verb\g\. Note that this practice usually only serves to confuse |
|
ff02b948c9a8
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|
2446 the reader of a program. |
|
ff02b948c9a8
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|
2447 |
|
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|
2448 |
|
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|
2449 Methods may call other methods by using method attributes of the |
|
ff02b948c9a8
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|
2450 \verb\self\ argument, e.g.: |
|
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|
2451 |
|
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|
2452 \begin{verbatim} |
|
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|
2453 class Bag: |
|
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diff
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|
2454 def empty(self): |
|
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diff
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|
2455 self.data = [] |
|
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|
2456 def add(self, x): |
|
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diff
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|
2457 self.data.append(x) |
|
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|
2458 def addtwice(self, x): |
|
952
7d8e51e580d1
Fixed botched lay-out in class example
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933
diff
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|
2459 self.add(x) |
|
7d8e51e580d1
Fixed botched lay-out in class example
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933
diff
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|
2460 self.add(x) |
|
929
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|
2461 \end{verbatim} |
|
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|
2462 |
|
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|
2463 |
|
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|
2464 The instantiation operation (``calling'' a class object) creates an |
|
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|
2465 empty object. Many classes like to create objects in a known initial |
|
1935
d9a3e30b2e47
Removed description of class.init() method.
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1934
diff
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|
2466 state. Therefore a class may define a special method named |
|
d9a3e30b2e47
Removed description of class.init() method.
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1934
diff
changeset
|
2467 \verb\__init__\, like this: |
|
1751
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diff
changeset
|
2468 |
|
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Merge alpha100 branch back to main trunk
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changeset
|
2469 \begin{verbatim} |
|
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changeset
|
2470 def __init__(self): |
|
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Merge alpha100 branch back to main trunk
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diff
changeset
|
2471 self.empty() |
|
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diff
changeset
|
2472 \end{verbatim} |
|
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diff
changeset
|
2473 |
|
ec25aad43724
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diff
changeset
|
2474 When a class defines an \verb\__init__\ method, class instantiation |
|
ec25aad43724
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diff
changeset
|
2475 automatically invokes \verb\__init__\ for the newly-created class |
|
ec25aad43724
Merge alpha100 branch back to main trunk
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diff
changeset
|
2476 instance. So in the \verb\Bag\ example, a new and initialized instance |
|
ec25aad43724
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diff
changeset
|
2477 can be obtained by: |
|
ec25aad43724
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diff
changeset
|
2478 |
|
ec25aad43724
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diff
changeset
|
2479 \begin{verbatim} |
|
ec25aad43724
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|
2480 x = Bag() |
|
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|
2481 \end{verbatim} |
|
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|
2482 |
|
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|
2483 Of course, the \verb\__init__\ method may have arguments for greater |
|
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|
2484 flexibility. In that case, arguments given to the class instantiation |
|
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|
2485 operator are passed on to \verb\__init__\. For example, |
|
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|
2486 |
|
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|
2487 \bcode\begin{verbatim} |
|
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|
2488 >>> class Complex: |
|
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|
2489 ... def __init__(self, realpart, imagpart): |
|
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|
2490 ... self.r = realpart |
|
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|
2491 ... self.i = imagpart |
|
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|
2492 ... |
|
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|
2493 >>> x = Complex(3.0,-4.5) |
|
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|
2494 >>> x.r, x.i |
|
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diff
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|
2495 (3.0, -4.5) |
|
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changeset
|
2496 >>> |
|
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|
2497 \end{verbatim}\ecode |
|
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|
2498 % |
|
929
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|
2499 Methods may reference global names in the same way as ordinary |
|
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|
2500 functions. The global scope associated with a method is the module |
|
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|
2501 containing the class definition. (The class itself is never used as a |
|
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|
2502 global scope!) While one rarely encounters a good reason for using |
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|
2503 global data in a method, there are many legitimate uses of the global |
|
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|
2504 scope: for one thing, functions and modules imported into the global |
|
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|
2505 scope can be used by methods, as well as functions and classes defined |
|
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|
2506 in it. Usually, the class containing the method is itself defined in |
|
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|
2507 this global scope, and in the next section we'll find some good |
|
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|
2508 reasons why a method would want to reference its own class! |
|
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|
2509 |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2510 |
|
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|
2511 \section{Inheritance} |
|
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2512 |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2513 Of course, a language feature would not be worthy of the name ``class'' |
|
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|
2514 without supporting inheritance. The syntax for a derived class |
|
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|
2515 definition looks as follows: |
|
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|
2516 |
|
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|
2517 \begin{verbatim} |
|
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|
2518 class DerivedClassName(BaseClassName): |
|
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|
2519 <statement-1> |
|
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|
2520 . |
|
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|
2521 . |
|
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|
2522 . |
|
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|
2523 <statement-N> |
|
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|
2524 \end{verbatim} |
|
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|
2525 |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2526 The name \verb\BaseClassName\ must be defined in a scope containing |
|
ff02b948c9a8
Add chapter on classes (mostly from ../misc/CLASSES).
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diff
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|
2527 the derived class definition. Instead of a base class name, an |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2528 expression is also allowed. This is useful when the base class is |
|
ff02b948c9a8
Add chapter on classes (mostly from ../misc/CLASSES).
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|
2529 defined in another module, e.g., |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2530 |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2531 \begin{verbatim} |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2532 class DerivedClassName(modname.BaseClassName): |
|
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|
2533 \end{verbatim} |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2534 |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2535 Execution of a derived class definition proceeds the same as for a |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2536 base class. When the class object is constructed, the base class is |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2537 remembered. This is used for resolving attribute references: if a |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2538 requested attribute is not found in the class, it is searched in the |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2539 base class. This rule is applied recursively if the base class itself |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2540 is derived from some other class. |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2541 |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2542 There's nothing special about instantiation of derived classes: |
|
ff02b948c9a8
Add chapter on classes (mostly from ../misc/CLASSES).
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|
2543 \verb\DerivedClassName()\ creates a new instance of the class. Method |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2544 references are resolved as follows: the corresponding class attribute |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2545 is searched, descending down the chain of base classes if necessary, |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2546 and the method reference is valid if this yields a function object. |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2547 |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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diff
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|
2548 Derived classes may override methods of their base classes. Because |
|
ff02b948c9a8
Add chapter on classes (mostly from ../misc/CLASSES).
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diff
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|
2549 methods have no special privileges when calling other methods of the |
|
ff02b948c9a8
Add chapter on classes (mostly from ../misc/CLASSES).
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diff
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|
2550 same object, a method of a base class that calls another method |
|
ff02b948c9a8
Add chapter on classes (mostly from ../misc/CLASSES).
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808
diff
changeset
|
2551 defined in the same base class, may in fact end up calling a method of |
|
1767
907605320a5f
Lots of small corrections by Andrew Kuchling (plus all new rotor docs)
Guido van Rossum <guido@python.org>
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1751
diff
changeset
|
2552 a derived class that overrides it. (For \Cpp{} programmers: all methods |
|
929
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|
2553 in Python are ``virtual functions''.) |
|
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|
2554 |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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|
2555 An overriding method in a derived class may in fact want to extend |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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diff
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|
2556 rather than simply replace the base class method of the same name. |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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diff
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|
2557 There is a simple way to call the base class method directly: just |
|
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diff
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|
2558 call \verb\BaseClassName.methodname(self, arguments)\. This is |
|
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Add chapter on classes (mostly from ../misc/CLASSES).
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diff
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|
2559 occasionally useful to clients as well. (Note that this only works if |
|
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diff
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|
2560 the base class is defined or imported directly in the global scope.) |
|
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|
2561 |
|
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|
2562 |
|
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|
2563 \subsection{Multiple inheritance} |
|
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|
2564 |
|
1751
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Merge alpha100 branch back to main trunk
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|
2565 Python supports a limited form of multiple inheritance as well. A |
|
929
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|
2566 class definition with multiple base classes looks as follows: |
|
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|
2567 |
|
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|
2568 \begin{verbatim} |
|
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|
2569 class DerivedClassName(Base1, Base2, Base3): |
|
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|
2570 <statement-1> |
|
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|
2571 . |
|
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|
2572 . |
|
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|
2573 . |
|
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|
2574 <statement-N> |
|
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|
2575 \end{verbatim} |
|
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2576 |
|
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|
2577 The only rule necessary to explain the semantics is the resolution |
|
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|
2578 rule used for class attribute references. This is depth-first, |
|
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|
2579 left-to-right. Thus, if an attribute is not found in |
|
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|
2580 \verb\DerivedClassName\, it is searched in \verb\Base1\, then |
|
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|
2581 (recursively) in the base classes of \verb\Base1\, and only if it is |
|
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|
2582 not found there, it is searched in \verb\Base2\, and so on. |
|
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|
2583 |
|
1100
ff1899dcda05
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diff
changeset
|
2584 (To some people breadth first---searching \verb\Base2\ and |
|
ff1899dcda05
Incorporated Jan-Hein's changes and texinfo conversion.
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1040
diff
changeset
|
2585 \verb\Base3\ before the base classes of \verb\Base1\---looks more |
|
929
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|
2586 natural. However, this would require you to know whether a particular |
|
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|
2587 attribute of \verb\Base1\ is actually defined in \verb\Base1\ or in |
|
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|
2588 one of its base classes before you can figure out the consequences of |
|
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|
2589 a name conflict with an attribute of \verb\Base2\. The depth-first |
|
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diff
changeset
|
2590 rule makes no differences between direct and inherited attributes of |
|
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|
2591 \verb\Base1\.) |
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2592 |
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|
2593 It is clear that indiscriminate use of multiple inheritance is a |
|
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|
2594 maintenance nightmare, given the reliance in Python on conventions to |
|
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|
2595 avoid accidental name conflicts. A well-known problem with multiple |
|
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2596 inheritance is a class derived from two classes that happen to have a |
|
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|
2597 common base class. While it is easy enough to figure out what happens |
|
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|
2598 in this case (the instance will have a single copy of ``instance |
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|
2599 variables'' or data attributes used by the common base class), it is |
|
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|
2600 not clear that these semantics are in any way useful. |
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2601 |
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2602 |
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|
2603 \section{Odds and ends} |
|
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2604 |
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|
2605 Sometimes it is useful to have a data type similar to the Pascal |
|
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|
2606 ``record'' or C ``struct'', bundling together a couple of named data |
|
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|
2607 items. An empty class definition will do nicely, e.g.: |
|
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2608 |
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|
2609 \begin{verbatim} |
|
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|
2610 class Employee: |
|
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2611 pass |
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2612 |
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|
2613 john = Employee() # Create an empty employee record |
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2614 |
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2615 # Fill the fields of the record |
|
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|
2616 john.name = 'John Doe' |
|
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|
2617 john.dept = 'computer lab' |
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|
2618 john.salary = 1000 |
|
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2619 \end{verbatim} |
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2620 |
|
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2621 |
|
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|
2622 A piece of Python code that expects a particular abstract data type |
|
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2623 can often be passed a class that emulates the methods of that data |
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2624 type instead. For instance, if you have a function that formats some |
|
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|
2625 data from a file object, you can define a class with methods |
|
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|
2626 \verb\read()\ and \verb\readline()\ that gets the data from a string |
|
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|
2627 buffer instead, and pass it as an argument. (Unfortunately, this |
|
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|
2628 technique has its limitations: a class can't define operations that |
|
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|
2629 are accessed by special syntax such as sequence subscripting or |
|
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|
2630 arithmetic operators, and assigning such a ``pseudo-file'' to |
|
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|
2631 \verb\sys.stdin\ will not cause the interpreter to read further input |
|
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|
2632 from it.) |
|
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2633 |
|
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|
2634 |
|
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|
2635 Instance method objects have attributes, too: \verb\m.im_self\ is the |
|
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|
2636 object of which the method is an instance, and \verb\m.im_func\ is the |
|
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|
2637 function object corresponding to the method. |
|
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|
2638 |
|
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|
2639 |
|
4491
6a75b3cf023c
Done with the "New in 1.4" chapter.
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|
2640 \chapter{Recent Additions as of Release 1.1} |
|
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2641 |
|
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|
2642 Python is an evolving language. Since this tutorial was last |
|
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|
2643 thoroughly revised, several new features have been added to the |
|
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|
2644 language. While ideally I should revise the tutorial to incorporate |
|
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|
2645 them in the mainline of the text, lack of time currently requires me |
|
1934
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Moved author and date to boilerplate.tex.
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|
2646 to take a more modest approach. In this chapter I will briefly list the |
|
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|
2647 most important improvements to the language and how you can use them |
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|
2648 to your benefit. |
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|
2649 |
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|
2650 \section{The Last Printed Expression} |
|
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|
2651 |
|
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|
2652 In interactive mode, the last printed expression is assigned to the |
|
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|
2653 variable \code{_}. This means that when you are using Python as a |
|
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|
2654 desk calculator, it is somewhat easier to continue calculations, for |
|
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|
2655 example: |
|
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|
2656 |
|
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|
2657 \begin{verbatim} |
|
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|
2658 >>> tax = 17.5 / 100 |
|
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|
2659 >>> price = 3.50 |
|
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|
2660 >>> price * tax |
|
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|
2661 0.6125 |
|
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|
2662 >>> price + _ |
|
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|
2663 4.1125 |
|
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|
2664 >>> round(_, 2) |
|
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|
2665 4.11 |
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|
2666 >>> |
|
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|
2667 \end{verbatim} |
|
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2668 |
|
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|
2669 For reasons too embarrassing to explain, this variable is implemented |
|
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|
2670 as a built-in (living in the module \code{__builtin__}), so it should |
|
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2671 be treated as read-only by the user. I.e. don't explicitly assign a |
|
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|
2672 value to it --- you would create an independent local variable with |
|
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|
2673 the same name masking the built-in variable with its magic behavior. |
|
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|
2674 |
|
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2675 \section{String Literals} |
|
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2676 |
|
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2677 \subsection{Double Quotes} |
|
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2678 |
|
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|
2679 Python can now also use double quotes to surround string literals, |
|
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|
2680 e.g. \verb\"this doesn't hurt a bit"\. There is no semantic |
|
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|
2681 difference between strings surrounded by single or double quotes. |
|
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2682 |
|
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2683 \subsection{Continuation Of String Literals} |
|
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2684 |
|
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|
2685 String literals can span multiple lines by escaping newlines with |
|
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|
2686 backslashes, e.g. |
|
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2687 |
|
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|
2688 \begin{verbatim} |
|
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|
2689 hello = "This is a rather long string containing\n\ |
|
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|
2690 several lines of text just as you would do in C.\n\ |
|
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|
2691 Note that whitespace at the beginning of the line is\ |
|
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|
2692 significant.\n" |
|
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|
2693 print hello |
|
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|
2694 \end{verbatim} |
|
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2695 |
|
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|
2696 which would print the following: |
|
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|
2697 \begin{verbatim} |
|
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2698 This is a rather long string containing |
|
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|
2699 several lines of text just as you would do in C. |
|
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2700 Note that whitespace at the beginning of the line is significant. |
|
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2701 \end{verbatim} |
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2702 |
|
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2703 \subsection{Triple-quoted strings} |
|
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2704 |
|
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2705 In some cases, when you need to include really long strings (e.g. |
|
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2706 containing several paragraphs of informational text), it is annoying |
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|
2707 that you have to terminate each line with \verb@\n\@, especially if |
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|
2708 you would like to reformat the text occasionally with a powerful text |
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|
2709 editor like Emacs. For such situations, ``triple-quoted'' strings can |
|
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|
2710 be used, e.g. |
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|
2711 |
|
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|
2712 \begin{verbatim} |
|
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|
2713 hello = """ |
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2714 |
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|
2715 This string is bounded by triple double quotes (3 times "). |
|
2344
279d51a18ae4
added chapter on new things in 1.2
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|
2716 Unescaped newlines in the string are retained, though \ |
|
1751
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|
2717 it is still possible\nto use all normal escape sequences. |
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2718 |
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|
2719 Whitespace at the beginning of a line is |
|
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|
2720 significant. If you need to include three opening quotes |
|
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|
2721 you have to escape at least one of them, e.g. \""". |
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2722 |
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|
2723 This string ends in a newline. |
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|
2724 """ |
|
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|
2725 \end{verbatim} |
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2726 |
|
2344
279d51a18ae4
added chapter on new things in 1.2
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changeset
|
2727 Triple-quoted strings can be surrounded by three single quotes as |
|
279d51a18ae4
added chapter on new things in 1.2
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|
2728 well, again without semantic difference. |
|
1751
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2729 |
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|
2730 \subsection{String Literal Juxtaposition} |
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2731 |
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|
2732 One final twist: you can juxtapose multiple string literals. Two or |
|
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|
2733 more adjacent string literals (but not arbitrary expressions!) |
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|
2734 separated only by whitespace will be concatenated (without intervening |
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2735 whitespace) into a single string object at compile time. This makes |
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|
2736 it possible to continue a long string on the next line without |
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|
2737 sacrificing indentation or performance, unlike the use of the string |
|
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|
2738 concatenation operator \verb\+\ or the continuation of the literal |
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|
2739 itself on the next line (since leading whitespace is significant |
|
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|
2740 inside all types of string literals). Note that this feature, like |
|
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|
2741 all string features except triple-quoted strings, is borrowed from |
|
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|
2742 Standard C. |
|
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2743 |
|
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|
2744 \section{The Formatting Operator} |
|
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2745 |
|
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|
2746 \subsection{Basic Usage} |
|
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2747 |
|
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|
2748 The chapter on output formatting is really out of date: there is now |
|
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|
2749 an almost complete interface to C-style printf formats. This is done |
|
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|
2750 by overloading the modulo operator (\verb\%\) for a left operand |
|
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|
2751 which is a string, e.g. |
|
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2752 |
|
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|
2753 \begin{verbatim} |
|
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|
2754 >>> import math |
|
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|
2755 >>> print 'The value of PI is approximately %5.3f.' % math.pi |
|
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|
2756 The value of PI is approximately 3.142. |
|
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|
2757 >>> |
|
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|
2758 \end{verbatim} |
|
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|
2759 |
|
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|
2760 If there is more than one format in the string you pass a tuple as |
|
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|
2761 right operand, e.g. |
|
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|
2762 |
|
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|
2763 \begin{verbatim} |
|
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|
2764 >>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 8637678} |
|
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|
2765 >>> for name, phone in table.items(): |
|
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|
2766 ... print '%-10s ==> %10d' % (name, phone) |
|
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|
2767 ... |
|
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|
2768 Jack ==> 4098 |
|
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|
2769 Dcab ==> 8637678 |
|
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|
2770 Sjoerd ==> 4127 |
|
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|
2771 >>> |
|
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|
2772 \end{verbatim} |
|
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|
2773 |
|
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|
2774 Most formats work exactly as in C and require that you pass the proper |
|
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|
2775 type (however, if you don't you get an exception, not a core dump). |
|
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|
2776 The \verb\%s\ format is more relaxed: if the corresponding argument is |
|
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|
2777 not a string object, it is converted to string using the \verb\str()\ |
|
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changeset
|
2778 built-in function. Using \verb\*\ to pass the width or precision in |
|
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|
2779 as a separate (integer) argument is supported. The C formats |
|
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Merge alpha100 branch back to main trunk
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|
2780 \verb\%n\ and \verb\%p\ are not supported. |
|
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Merge alpha100 branch back to main trunk
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|
2781 |
|
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changeset
|
2782 \subsection{Referencing Variables By Name} |
|
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|
2783 |
|
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changeset
|
2784 If you have a really long format string that you don't want to split |
|
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|
2785 up, it would be nice if you could reference the variables to be |
|
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|
2786 formatted by name instead of by position. This can be done by using |
|
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|
2787 an extension of C formats using the form \verb\%(name)format\, e.g. |
|
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|
2788 |
|
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|
2789 \begin{verbatim} |
|
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|
2790 >>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 8637678} |
|
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|
2791 >>> print 'Jack: %(Jack)d; Sjoerd: %(Sjoerd)d; Dcab: %(Dcab)d' % table |
|
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|
2792 Jack: 4098; Sjoerd: 4127; Dcab: 8637678 |
|
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|
2793 >>> |
|
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|
2794 \end{verbatim} |
|
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|
2795 |
|
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changeset
|
2796 This is particularly useful in combination with the new built-in |
|
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changeset
|
2797 \verb\vars()\ function, which returns a dictionary containing all |
|
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|
2798 local variables. |
|
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changeset
|
2799 |
|
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|
2800 \section{Optional Function Arguments} |
|
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|
2801 |
|
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changeset
|
2802 It is now possible to define functions with a variable number of |
|
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|
2803 arguments. There are two forms, which can be combined. |
|
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|
2804 |
|
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|
2805 \subsection{Default Argument Values} |
|
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|
2806 |
|
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changeset
|
2807 The most useful form is to specify a default value for one or more |
|
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|
2808 arguments. This creates a function that can be called with fewer |
|
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|
2809 arguments than it is defined, e.g. |
|
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|
2810 |
|
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changeset
|
2811 \begin{verbatim} |
|
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|
2812 def ask_ok(prompt, retries = 4, complaint = 'Yes or no, please!'): |
|
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|
2813 while 1: |
|
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|
2814 ok = raw_input(prompt) |
|
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|
2815 if ok in ('y', 'ye', 'yes'): return 1 |
|
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|
2816 if ok in ('n', 'no', 'nop', 'nope'): return 0 |
|
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|
2817 retries = retries - 1 |
|
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|
2818 if retries < 0: raise IOError, 'refusenik user' |
|
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|
2819 print complaint |
|
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|
2820 \end{verbatim} |
|
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|
2821 |
|
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changeset
|
2822 This function can be called either like this: |
|
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changeset
|
2823 \verb\ask_ok('Do you really want to quit?')\ or like this: |
|
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|
2824 \verb\ask_ok('OK to overwrite the file?', 2)\. |
|
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|
2825 |
|
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|
2826 The default values are evaluated at the point of function definition |
|
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2827 in the {\em defining} scope, so that e.g. |
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2828 |
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|
2829 \begin{verbatim} |
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2830 i = 5 |
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2831 def f(arg = i): print arg |
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2832 i = 6 |
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2833 f() |
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2834 \end{verbatim} |
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2835 |
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2836 will print \verb\5\. |
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2837 |
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2838 \subsection{Arbitrary Argument Lists} |
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2839 |
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2840 It is also possible to specify that a function can be called with an |
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2841 arbitrary number of arguments. These arguments will be wrapped up in |
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2842 a tuple. Before the variable number of arguments, zero or more normal |
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2843 arguments may occur, e.g. |
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2844 |
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2845 \begin{verbatim} |
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2846 def fprintf(file, format, *args): |
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2847 file.write(format % args) |
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2848 \end{verbatim} |
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2849 |
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2850 This feature may be combined with the previous, e.g. |
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2851 |
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2852 \begin{verbatim} |
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2853 def but_is_it_useful(required, optional = None, *remains): |
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2854 print "I don't know" |
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2855 \end{verbatim} |
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2856 |
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2857 \section{Lambda And Functional Programming Tools} |
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2858 |
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2859 \subsection{Lambda Forms} |
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2860 |
|
1767
907605320a5f
Lots of small corrections by Andrew Kuchling (plus all new rotor docs)
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2861 By popular demand, a few features commonly found in functional |
|
1751
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2862 programming languages and Lisp have been added to Python. With the |
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2863 \verb\lambda\ keyword, small anonymous functions can be created. |
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2864 Here's a function that returns the sum of its two arguments: |
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2865 \verb\lambda a, b: a+b\. Lambda forms can be used wherever function |
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2866 objects are required. They are syntactically restricted to a single |
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2867 expression. Semantically, they are just syntactic sugar for a normal |
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2868 function definition. Like nested function definitions, lambda forms |
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2869 cannot reference variables from the containing scope, but this can be |
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2870 overcome through the judicious use of default argument values, e.g. |
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2871 |
|
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|
2872 \begin{verbatim} |
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2873 def make_incrementor(n): |
|
1852
09de723b2596
* Doc/tut.tex (subsection{Lambda Forms}): fix typo in lambda
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|
2874 return lambda x, incr=n: x+incr |
|
1751
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2875 \end{verbatim} |
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2876 |
|
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2877 \subsection{Map, Reduce and Filter} |
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2878 |
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|
2879 Three new built-in functions on sequences are good candidate to pass |
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|
2880 lambda forms. |
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2881 |
|
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2882 \subsubsection{Map.} |
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2883 |
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2884 \verb\map(function, sequence)\ calls \verb\function(item)\ for each of |
|
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2885 the sequence's items and returns a list of the return values. For |
|
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|
2886 example, to compute some cubes: |
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2887 |
|
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|
2888 \begin{verbatim} |
|
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2889 >>> map(lambda x: x*x*x, range(1, 11)) |
|
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|
2890 [1, 8, 27, 64, 125, 216, 343, 512, 729, 1000] |
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|
2891 >>> |
|
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|
2892 \end{verbatim} |
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|
2893 |
|
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|
2894 More than one sequence may be passed; the function must then have as |
|
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|
2895 many arguments as there are sequences and is called with the |
|
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|
2896 corresponding item from each sequence (or \verb\None\ if some sequence |
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|
2897 is shorter than another). If \verb\None\ is passed for the function, |
|
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|
2898 a function returning its argument(s) is substituted. |
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|
2899 |
|
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|
2900 Combining these two special cases, we see that |
|
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|
2901 \verb\map(None, list1, list2)\ is a convenient way of turning a pair |
|
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|
2902 of lists into a list of pairs. For example: |
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|
2903 |
|
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|
2904 \begin{verbatim} |
|
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|
2905 >>> seq = range(8) |
|
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|
2906 >>> map(None, seq, map(lambda x: x*x, seq)) |
|
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|
2907 [(0, 0), (1, 1), (2, 4), (3, 9), (4, 16), (5, 25), (6, 36), (7, 49)] |
|
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|
2908 >>> |
|
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|
2909 \end{verbatim} |
|
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|
2910 |
|
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|
2911 \subsubsection{Filter.} |
|
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|
2912 |
|
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|
2913 \verb\filter(function, sequence)\ returns a sequence (of the same |
|
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|
2914 type, if possible) consisting of those items from the sequence for |
|
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2915 which \verb\function(item)\ is true. For example, to compute some |
|
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|
2916 primes: |
|
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2917 |
|
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|
2918 \begin{verbatim} |
|
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2919 >>> filter(lambda x: x%2 != 0 and x%3 != 0, range(2, 25)) |
|
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|
2920 [5, 7, 11, 13, 17, 19, 23] |
|
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|
2921 >>> |
|
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|
2922 \end{verbatim} |
|
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|
2923 |
|
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|
2924 \subsubsection{Reduce.} |
|
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|
2925 |
|
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|
2926 \verb\reduce(function, sequence)\ returns a single value constructed |
|
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|
2927 by calling the (binary) function on the first two items of the |
|
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2928 sequence, then on the result and the next item, and so on. For |
|
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|
2929 example, to compute the sum of the numbers 1 through 10: |
|
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|
2930 |
|
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|
2931 \begin{verbatim} |
|
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2932 >>> reduce(lambda x, y: x+y, range(1, 11)) |
|
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|
2933 55 |
|
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|
2934 >>> |
|
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|
2935 \end{verbatim} |
|
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|
2936 |
|
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|
2937 If there's only one item in the sequence, its value is returned; if |
|
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|
2938 the sequence is empty, an exception is raised. |
|
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2939 |
|
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|
2940 A third argument can be passed to indicate the starting value. In this |
|
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|
2941 case the starting value is returned for an empty sequence, and the |
|
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|
2942 function is first applied to the starting value and the first sequence |
|
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2943 item, then to the result and the next item, and so on. For example, |
|
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2944 |
|
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|
2945 \begin{verbatim} |
|
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|
2946 >>> def sum(seq): |
|
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2947 ... return reduce(lambda x, y: x+y, seq, 0) |
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2948 ... |
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|
2949 >>> sum(range(1, 11)) |
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2950 55 |
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|
2951 >>> sum([]) |
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|
2952 0 |
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|
2953 >>> |
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|
2954 \end{verbatim} |
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2955 |
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2956 \section{Continuation Lines Without Backslashes} |
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2957 |
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|
2958 While the general mechanism for continuation of a source line on the |
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2959 next physical line remains to place a backslash on the end of the |
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2960 line, expressions inside matched parentheses (or square brackets, or |
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|
2961 curly braces) can now also be continued without using a backslash. |
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|
2962 This is particularly useful for calls to functions with many |
|
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2963 arguments, and for initializations of large tables. |
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2964 |
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|
2965 For example: |
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2966 |
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|
2967 \begin{verbatim} |
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2968 month_names = ['Januari', 'Februari', 'Maart', |
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2969 'April', 'Mei', 'Juni', |
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2970 'Juli', 'Augustus', 'September', |
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2971 'Oktober', 'November', 'December'] |
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2972 \end{verbatim} |
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2973 |
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2974 and |
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2975 |
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2976 \begin{verbatim} |
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2977 CopyInternalHyperLinks(self.context.hyperlinks, |
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2978 copy.context.hyperlinks, |
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|
2979 uidremap) |
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|
2980 \end{verbatim} |
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2981 |
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2982 \section{Regular Expressions} |
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2983 |
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2984 While C's printf-style output formats, transformed into Python, are |
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2985 adequate for most output formatting jobs, C's scanf-style input |
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2986 formats are not very powerful. Instead of scanf-style input, Python |
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|
2987 offers Emacs-style regular expressions as a powerful input and |
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|
2988 scanning mechanism. Read the corresponding section in the Library |
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2989 Reference for a full description. |
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2990 |
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|
2991 \section{Generalized Dictionaries} |
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2992 |
|
2453
cdd3a721b40e
a few typographical changes (e.g. -- => ---) and lots of new stuff in the WWW chapter
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2993 The keys of dictionaries are no longer restricted to strings --- they |
|
2344
279d51a18ae4
added chapter on new things in 1.2
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2994 can be any immutable basic type including strings, numbers, tuples, or |
|
279d51a18ae4
added chapter on new things in 1.2
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|
2995 (certain) class instances. (Lists and dictionaries are not acceptable |
|
279d51a18ae4
added chapter on new things in 1.2
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|
2996 as dictionary keys, in order to avoid problems when the object used as |
|
279d51a18ae4
added chapter on new things in 1.2
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|
2997 a key is modified.) |
|
1751
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2998 |
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2999 Dictionaries have two new methods: \verb\d.values()\ returns a list of |
|
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3000 the dictionary's values, and \verb\d.items()\ returns a list of the |
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|
3001 dictionary's (key, value) pairs. Like \verb\d.keys()\, these |
|
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3002 operations are slow for large dictionaries. Examples: |
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3003 |
|
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|
3004 \begin{verbatim} |
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3005 >>> d = {100: 'honderd', 1000: 'duizend', 10: 'tien'} |
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|
3006 >>> d.keys() |
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|
3007 [100, 10, 1000] |
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|
3008 >>> d.values() |
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|
3009 ['honderd', 'tien', 'duizend'] |
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|
3010 >>> d.items() |
|
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|
3011 [(100, 'honderd'), (10, 'tien'), (1000, 'duizend')] |
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|
3012 >>> |
|
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|
3013 \end{verbatim} |
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|
3014 |
|
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|
3015 \section{Miscellaneous New Built-in Functions} |
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|
3016 |
|
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|
3017 The function \verb\vars()\ returns a dictionary containing the current |
|
1934
f3ae7c17ac08
Moved author and date to boilerplate.tex.
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|
3018 local variables. With a module argument, it returns that module's |
|
1751
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|
3019 global variables. The old function \verb\dir(x)\ returns |
|
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|
3020 \verb\vars(x).keys()\. |
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|
3021 |
|
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|
3022 The function \verb\round(x)\ returns a floating point number rounded |
|
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|
3023 to the nearest integer (but still expressed as a floating point |
|
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|
3024 number). E.g. \verb\round(3.4) == 3.0\ and \verb\round(3.5) == 4.0\. |
|
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|
3025 With a second argument it rounds to the specified number of digits, |
|
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|
3026 e.g. \verb\round(math.pi, 4) == 3.1416\ or even |
|
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|
3027 \verb\round(123.4, -2) == 100.0\. |
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|
3028 |
|
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|
3029 The function \verb\hash(x)\ returns a hash value for an object. |
|
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|
3030 All object types acceptable as dictionary keys have a hash value (and |
|
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|
3031 it is this hash value that the dictionary implementation uses). |
|
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|
3032 |
|
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|
3033 The function \verb\id(x)\ return a unique identifier for an object. |
|
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|
3034 For two objects x and y, \verb\id(x) == id(y)\ if and only if |
|
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|
3035 \verb\x is y\. (In fact the object's address is used.) |
|
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3036 |
|
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|
3037 The function \verb\hasattr(x, name)\ returns whether an object has an |
|
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|
3038 attribute with the given name (a string value). The function |
|
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|
3039 \verb\getattr(x, name)\ returns the object's attribute with the given |
|
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|
3040 name. The function \verb\setattr(x, name, value)\ assigns a value to |
|
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|
3041 an object's attribute with the given name. These three functions are |
|
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|
3042 useful if the attribute names are not known beforehand. Note that |
|
2063
5ed065943526
spam -> foo (etc.) in examples
Guido van Rossum <guido@python.org>
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changeset
|
3043 \verb\getattr(x, 'spam')\ is equivalent to \verb\x.spam\, and |
|
5ed065943526
spam -> foo (etc.) in examples
Guido van Rossum <guido@python.org>
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|
3044 \verb\setattr(x, 'spam', y)\ is equivalent to \verb\x.spam = y\. By |
|
1751
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|
3045 definition, \verb\hasattr(x, name)\ returns true if and only if |
|
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|
3046 \verb\getattr(x, name)\ returns without raising an exception. |
|
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|
3047 |
|
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|
3048 \section{Else Clause For Try Statement} |
|
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|
3049 |
|
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|
3050 The \verb\try...except\ statement now has an optional \verb\else\ |
|
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|
3051 clause, which must follow all \verb\except\ clauses. It is useful to |
|
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|
3052 place code that must be executed if the \verb\try\ clause does not |
|
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|
3053 raise an exception. For example: |
|
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|
3054 |
|
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|
3055 \begin{verbatim} |
|
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|
3056 for arg in sys.argv: |
|
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|
3057 try: |
|
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|
3058 f = open(arg, 'r') |
|
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|
3059 except IOError: |
|
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|
3060 print 'cannot open', arg |
|
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|
3061 else: |
|
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|
3062 print arg, 'has', len(f.readlines()), 'lines' |
|
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|
3063 f.close() |
|
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|
3064 \end{verbatim} |
|
929
ff02b948c9a8
Add chapter on classes (mostly from ../misc/CLASSES).
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3065 |
|
1935
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3066 |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3067 \section{New Class Features in Release 1.1} |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3068 |
| 2013 | 3069 Some changes have been made to classes: the operator overloading |
|
1935
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3070 mechanism is more flexible, providing more support for non-numeric use |
|
1937
320baeab787a
Add simpler __getattr__ example and document __call__
Guido van Rossum <guido@python.org>
parents:
1935
diff
changeset
|
3071 of operators (including calling an object as if it were a function), |
|
320baeab787a
Add simpler __getattr__ example and document __call__
Guido van Rossum <guido@python.org>
parents:
1935
diff
changeset
|
3072 and it is possible to trap attribute accesses. |
|
1935
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3073 |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3074 \subsection{New Operator Overloading} |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3075 |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3076 It is no longer necessary to coerce both sides of an operator to the |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3077 same class or type. A class may still provide a \code{__coerce__} |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3078 method, but this method may return objects of different types or |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3079 classes if it feels like it. If no \code{__coerce__} is defined, any |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3080 argument type or class is acceptable. |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3081 |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3082 In order to make it possible to implement binary operators where the |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3083 right-hand side is a class instance but the left-hand side is not, |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3084 without using coercions, right-hand versions of all binary operators |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3085 may be defined. These have an `r' prepended to their name, |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3086 e.g. \code{__radd__}. |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3087 |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3088 For example, here's a very simple class for representing times. Times |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3089 are initialized from a number of seconds (like time.time()). Times |
| 2553 | 3090 are printed like this: \code{Wed Mar 15 12:28:48 1995}. Subtracting |
|
1935
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3091 two Times gives their difference in seconds. Adding or subtracting a |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3092 Time and a number gives a new Time. You can't add two times, nor can |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3093 you subtract a Time from a number. |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3094 |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3095 \begin{verbatim} |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3096 import time |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3097 |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3098 class Time: |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3099 def __init__(self, seconds): |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3100 self.seconds = seconds |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3101 def __repr__(self): |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3102 return time.ctime(self.seconds) |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3103 def __add__(self, x): |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3104 return Time(self.seconds + x) |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3105 __radd__ = __add__ # support for x+t |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3106 def __sub__(self, x): |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3107 if hasattr(x, 'seconds'): # test if x could be a Time |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3108 return self.seconds - x.seconds |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3109 else: |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3110 return self.seconds - x |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3111 |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3112 now = Time(time.time()) |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3113 tomorrow = 24*3600 + now |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3114 yesterday = now - today |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3115 print tomorrow - yesterday # prints 172800 |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3116 \end{verbatim} |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3117 |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3118 \subsection{Trapping Attribute Access} |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3119 |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3120 You can define three new ``magic'' methods in a class now: |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3121 \code{__getattr__(self, name)}, \code{__setattr__(self, name, value)} |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3122 and \code{__delattr__(self, name)}. |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3123 |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3124 The \code{__getattr__} method is called when an attribute access fails, |
|
2453
cdd3a721b40e
a few typographical changes (e.g. -- => ---) and lots of new stuff in the WWW chapter
Guido van Rossum <guido@python.org>
parents:
2352
diff
changeset
|
3125 i.e. when an attribute access would otherwise raise AttributeError --- |
|
1935
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3126 this is {\em after} the instance's dictionary and its class hierarchy |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3127 have been searched for the named attribute. Note that if this method |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3128 attempts to access any undefined instance attribute it will be called |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3129 recursively! |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3130 |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3131 The \code{__setattr__} and \code{__delattr__} methods are called when |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3132 assignment to, respectively deletion of an attribute are attempted. |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3133 They are called {\em instead} of the normal action (which is to insert |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3134 or delete the attribute in the instance dictionary). If either of |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3135 these methods most set or delete any attribute, they can only do so by |
|
2453
cdd3a721b40e
a few typographical changes (e.g. -- => ---) and lots of new stuff in the WWW chapter
Guido van Rossum <guido@python.org>
parents:
2352
diff
changeset
|
3136 using the instance dictionary directly --- \code{self.__dict__} --- else |
|
1935
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3137 they would be called recursively. |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3138 |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3139 For example, here's a near-universal ``Wrapper'' class that passes all |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3140 its attribute accesses to another object. Note how the |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3141 \code{__init__} method inserts the wrapped object in |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3142 \code{self.__dict__} in order to avoid endless recursion |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3143 (\code{__setattr__} would call \code{__getattr__} which would call |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3144 itself recursively). |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3145 |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3146 \begin{verbatim} |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3147 class Wrapper: |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3148 def __init__(self, wrapped): |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3149 self.__dict__['wrapped'] = wrapped |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3150 def __getattr__(self, name): |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3151 return getattr(self.wrapped, name) |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3152 def __setattr__(self, name, value): |
| 2013 | 3153 setattr(self.wrapped, name, value) |
|
1935
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3154 def __delattr__(self, name): |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3155 delattr(self.wrapped, name) |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3156 |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3157 import sys |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3158 f = Wrapper(sys.stdout) |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3159 f.write('hello world\n') # prints 'hello world' |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3160 \end{verbatim} |
|
d9a3e30b2e47
Removed description of class.init() method.
Guido van Rossum <guido@python.org>
parents:
1934
diff
changeset
|
3161 |
|
1937
320baeab787a
Add simpler __getattr__ example and document __call__
Guido van Rossum <guido@python.org>
parents:
1935
diff
changeset
|
3162 A simpler example of \code{__getattr__} is an attribute that is |
|
320baeab787a
Add simpler __getattr__ example and document __call__
Guido van Rossum <guido@python.org>
parents:
1935
diff
changeset
|
3163 computed each time (or the first time) it it accessed. For instance: |
|
320baeab787a
Add simpler __getattr__ example and document __call__
Guido van Rossum <guido@python.org>
parents:
1935
diff
changeset
|
3164 |
|
320baeab787a
Add simpler __getattr__ example and document __call__
Guido van Rossum <guido@python.org>
parents:
1935
diff
changeset
|
3165 \begin{verbatim} |
|
320baeab787a
Add simpler __getattr__ example and document __call__
Guido van Rossum <guido@python.org>
parents:
1935
diff
changeset
|
3166 from math import pi |
|
320baeab787a
Add simpler __getattr__ example and document __call__
Guido van Rossum <guido@python.org>
parents:
1935
diff
changeset
|
3167 |
|
320baeab787a
Add simpler __getattr__ example and document __call__
Guido van Rossum <guido@python.org>
parents:
1935
diff
changeset
|
3168 class Circle: |
|
320baeab787a
Add simpler __getattr__ example and document __call__
Guido van Rossum <guido@python.org>
parents:
1935
diff
changeset
|
3169 def __init__(self, radius): |
|
320baeab787a
Add simpler __getattr__ example and document __call__
Guido van Rossum <guido@python.org>
parents:
1935
diff
changeset
|
3170 self.radius = radius |
|
320baeab787a
Add simpler __getattr__ example and document __call__
Guido van Rossum <guido@python.org>
parents:
1935
diff
changeset
|
3171 def __getattr__(self, name): |
|
320baeab787a
Add simpler __getattr__ example and document __call__
Guido van Rossum <guido@python.org>
parents:
1935
diff
changeset
|
3172 if name == 'circumference': |
|
320baeab787a
Add simpler __getattr__ example and document __call__
Guido van Rossum <guido@python.org>
parents:
1935
diff
changeset
|
3173 return 2 * pi * self.radius |
|
320baeab787a
Add simpler __getattr__ example and document __call__
Guido van Rossum <guido@python.org>
parents:
1935
diff
changeset
|
3174 if name == 'diameter': |
|
320baeab787a
Add simpler __getattr__ example and document __call__
Guido van Rossum <guido@python.org>
parents:
1935
diff
changeset
|
3175 return 2 * self.radius |
|
320baeab787a
Add simpler __getattr__ example and document __call__
Guido van Rossum <guido@python.org>
parents:
1935
diff
changeset
|
3176 if name == 'area': |
|
320baeab787a
Add simpler __getattr__ example and document __call__
Guido van Rossum <guido@python.org>
parents:
1935
diff
changeset
|
3177 return pi * pow(self.radius, 2) |
|
320baeab787a
Add simpler __getattr__ example and document __call__
Guido van Rossum <guido@python.org>
parents:
1935
diff
changeset
|
3178 raise AttributeError, name |
|
320baeab787a
Add simpler __getattr__ example and document __call__
Guido van Rossum <guido@python.org>
parents:
1935
diff
changeset
|
3179 \end{verbatim} |
|
320baeab787a
Add simpler __getattr__ example and document __call__
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diff
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|
3180 |
|
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Add simpler __getattr__ example and document __call__
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diff
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|
3181 \subsection{Calling a Class Instance} |
|
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Add simpler __getattr__ example and document __call__
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diff
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|
3182 |
|
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Add simpler __getattr__ example and document __call__
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1935
diff
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|
3183 If a class defines a method \code{__call__} it is possible to call its |
|
320baeab787a
Add simpler __getattr__ example and document __call__
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diff
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|
3184 instances as if they were functions. For example: |
|
320baeab787a
Add simpler __getattr__ example and document __call__
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1935
diff
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|
3185 |
|
320baeab787a
Add simpler __getattr__ example and document __call__
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1935
diff
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|
3186 \begin{verbatim} |
|
320baeab787a
Add simpler __getattr__ example and document __call__
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diff
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|
3187 class PresetSomeArguments: |
|
320baeab787a
Add simpler __getattr__ example and document __call__
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|
3188 def __init__(self, func, *args): |
|
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Add simpler __getattr__ example and document __call__
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|
3189 self.func, self.args = func, args |
|
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Add simpler __getattr__ example and document __call__
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diff
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|
3190 def __call__(self, *args): |
|
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Add simpler __getattr__ example and document __call__
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|
3191 return apply(self.func, self.args + args) |
|
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Add simpler __getattr__ example and document __call__
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diff
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|
3192 |
|
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Add simpler __getattr__ example and document __call__
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diff
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|
3193 f = PresetSomeArguments(pow, 2) # f(i) computes powers of 2 |
|
320baeab787a
Add simpler __getattr__ example and document __call__
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|
3194 for i in range(10): print f(i), # prints 1 2 4 8 16 32 64 128 256 512 |
|
320baeab787a
Add simpler __getattr__ example and document __call__
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diff
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|
3195 print # append newline |
|
320baeab787a
Add simpler __getattr__ example and document __call__
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diff
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|
3196 \end{verbatim} |
|
320baeab787a
Add simpler __getattr__ example and document __call__
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|
3197 |
|
2344
279d51a18ae4
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|
3198 |
|
279d51a18ae4
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|
3199 \chapter{New in Release 1.2} |
|
279d51a18ae4
added chapter on new things in 1.2
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|
3200 |
|
279d51a18ae4
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|
3201 |
|
279d51a18ae4
added chapter on new things in 1.2
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|
3202 This chapter describes even more recent additions to the Python |
|
279d51a18ae4
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|
3203 language and library. |
|
279d51a18ae4
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|
3204 |
|
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|
3205 |
|
279d51a18ae4
added chapter on new things in 1.2
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|
3206 \section{New Class Features} |
|
279d51a18ae4
added chapter on new things in 1.2
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|
3207 |
|
279d51a18ae4
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|
3208 The semantics of \code{__coerce__} have been changed to be more |
|
279d51a18ae4
added chapter on new things in 1.2
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|
3209 reasonable. As an example, the new standard module \code{Complex} |
|
279d51a18ae4
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|
3210 implements fairly complete complex numbers using this. Additional |
|
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added chapter on new things in 1.2
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|
3211 examples of classes with and without \code{__coerce__} methods can be |
|
279d51a18ae4
added chapter on new things in 1.2
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|
3212 found in the \code{Demo/classes} subdirectory, modules \code{Rat} and |
|
279d51a18ae4
added chapter on new things in 1.2
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|
3213 \code{Dates}. |
|
279d51a18ae4
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diff
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|
3214 |
|
279d51a18ae4
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|
3215 If a class defines no \code{__coerce__} method, this is equivalent to |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3216 the following definition: |
|
279d51a18ae4
added chapter on new things in 1.2
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|
3217 |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
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|
3218 \begin{verbatim} |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
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|
3219 def __coerce__(self, other): return self, other |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
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|
3220 \end{verbatim} |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3221 |
|
279d51a18ae4
added chapter on new things in 1.2
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2063
diff
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|
3222 If \code{__coerce__} coerces itself to an object of a different type, |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3223 the operation is carried out using that type --- in release 1.1, this |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
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|
3224 would cause an error. |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3225 |
|
279d51a18ae4
added chapter on new things in 1.2
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2063
diff
changeset
|
3226 Comparisons involving class instances now invoke \code{__coerce__} |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
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|
3227 exactly as if \code{cmp(x, y)} were a binary operator like \code{+} |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
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|
3228 (except if \code{x} and \code{y} are the same object). |
|
279d51a18ae4
added chapter on new things in 1.2
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2063
diff
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|
3229 |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
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|
3230 \section{Unix Signal Handling} |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
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|
3231 |
|
3659
825bd8c11b1c
Note that EOF is ^Z on DOS. Spell Unix as \UNIX in some more places.
Guido van Rossum <guido@python.org>
parents:
3210
diff
changeset
|
3232 On {\UNIX}, Python now supports signal handling. The module |
|
2344
279d51a18ae4
added chapter on new things in 1.2
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diff
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|
3233 \code{signal} exports functions \code{signal}, \code{pause} and |
|
3659
825bd8c11b1c
Note that EOF is ^Z on DOS. Spell Unix as \UNIX in some more places.
Guido van Rossum <guido@python.org>
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3210
diff
changeset
|
3234 \code{alarm}, which act similar to their {\UNIX} counterparts. The |
|
2344
279d51a18ae4
added chapter on new things in 1.2
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|
3235 module also exports the conventional names for the various signal |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
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|
3236 classes (also usable with \code{os.kill()}) and \code{SIG_IGN} and |
|
279d51a18ae4
added chapter on new things in 1.2
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|
3237 \code{SIG_DFL}. See the section on \code{signal} in the Library |
|
279d51a18ae4
added chapter on new things in 1.2
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|
3238 Reference Manual for more information. |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
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|
3239 |
|
279d51a18ae4
added chapter on new things in 1.2
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|
3240 \section{Exceptions Can Be Classes} |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
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|
3241 |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3242 User-defined exceptions are no longer limited to being string objects |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
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|
3243 --- they can be identified by classes as well. Using this mechanism it |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
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|
3244 is possible to create extensible hierarchies of exceptions. |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
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|
3245 |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3246 There are two new valid (semantic) forms for the raise statement: |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3247 |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3248 \begin{verbatim} |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3249 raise Class, instance |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3250 |
|
279d51a18ae4
added chapter on new things in 1.2
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2063
diff
changeset
|
3251 raise instance |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3252 \end{verbatim} |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3253 |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3254 In the first form, \code{instance} must be an instance of \code{Class} |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
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|
3255 or of a class derived from it. The second form is a shorthand for |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3256 |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3257 \begin{verbatim} |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3258 raise instance.__class__, instance |
|
279d51a18ae4
added chapter on new things in 1.2
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|
3259 \end{verbatim} |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3260 |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3261 An except clause may list classes as well as string objects. A class |
|
279d51a18ae4
added chapter on new things in 1.2
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|
3262 in an except clause is compatible with an exception if it is the same |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3263 class or a base class thereof (but not the other way around --- an |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
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|
3264 except clause listing a derived class is not compatible with a base |
|
279d51a18ae4
added chapter on new things in 1.2
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|
3265 class). For example, the following code will print B, C, D in that |
|
279d51a18ae4
added chapter on new things in 1.2
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changeset
|
3266 order: |
|
279d51a18ae4
added chapter on new things in 1.2
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changeset
|
3267 |
|
279d51a18ae4
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diff
changeset
|
3268 \begin{verbatim} |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3269 class B: |
|
279d51a18ae4
added chapter on new things in 1.2
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|
3270 pass |
|
279d51a18ae4
added chapter on new things in 1.2
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|
3271 class C(B): |
|
279d51a18ae4
added chapter on new things in 1.2
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|
3272 pass |
|
279d51a18ae4
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|
3273 class D(C): |
|
279d51a18ae4
added chapter on new things in 1.2
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|
3274 pass |
|
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|
3275 |
|
279d51a18ae4
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|
3276 for c in [B, C, D]: |
|
279d51a18ae4
added chapter on new things in 1.2
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changeset
|
3277 try: |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3278 raise c() |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3279 except D: |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
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|
3280 print "D" |
|
279d51a18ae4
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diff
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|
3281 except C: |
|
279d51a18ae4
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diff
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|
3282 print "C" |
|
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diff
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|
3283 except B: |
|
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|
3284 print "B" |
|
279d51a18ae4
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|
3285 \end{verbatim} |
|
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|
3286 |
|
279d51a18ae4
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changeset
|
3287 Note that if the except clauses were reversed (with ``\code{except B}'' |
|
279d51a18ae4
added chapter on new things in 1.2
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|
3288 first), it would have printed B, B, B --- the first matching except |
|
279d51a18ae4
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|
3289 clause is triggered. |
|
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|
3290 |
|
279d51a18ae4
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changeset
|
3291 When an error message is printed for an unhandled exception which is a |
|
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|
3292 class, the class name is printed, then a colon and a space, and |
|
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|
3293 finally the instance converted to a string using the built-in function |
|
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|
3294 \code{str()}. |
|
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|
3295 |
|
279d51a18ae4
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changeset
|
3296 In this release, the built-in exceptions are still strings. |
|
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|
3297 |
|
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|
3298 |
|
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|
3299 \section{Object Persistency and Object Copying} |
|
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|
3300 |
|
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|
3301 Two new modules, \code{pickle} and \code{shelve}, support storage and |
|
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|
3302 retrieval of (almost) arbitrary Python objects on disk, using the |
|
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|
3303 \code{dbm} package. A third module, \code{copy}, provides flexible |
|
2352
26b1b15fc8ec
no need to reference doc strings for documented modules
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|
3304 object copying operations. More information on these modules is |
|
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no need to reference doc strings for documented modules
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|
3305 provided in the Library Reference Manual. |
|
2344
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|
3306 |
|
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|
3307 \subsection{Persistent Objects} |
|
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|
3308 |
|
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|
3309 The module \code{pickle} provides a general framework for objects to |
|
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|
3310 disassemble themselves into a stream of bytes and to reassemble such a |
|
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|
3311 stream back into an object. It copes with reference sharing, |
|
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|
3312 recursive objects and instances of user-defined classes, but not |
|
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|
3313 (directly) with objects that have ``magical'' links into the operating |
|
279d51a18ae4
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|
3314 system such as open files, sockets or windows. |
|
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|
3315 |
|
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|
3316 The \code{pickle} module defines a simple protocol whereby |
|
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|
3317 user-defined classes can control how they are disassembled and |
|
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|
3318 assembled. The method \code{__getinitargs__()}, if defined, returns |
|
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|
3319 the argument list for the constructor to be used at assembly time (by |
|
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|
3320 default the constructor is called without arguments). The methods |
|
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|
3321 \code{__getstate__()} and \code{__setstate__()} are used to pass |
|
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|
3322 additional state from disassembly to assembly; by default the |
|
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|
3323 instance's \code{__dict__} is passed and restored. |
|
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|
3324 |
|
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|
3325 Note that \code{pickle} does not open or close any files --- it can be |
|
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|
3326 used equally well for moving objects around on a network or store them |
|
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|
3327 in a database. For ease of debugging, and the inevitable occasional |
|
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|
3328 manual patch-up, the constructed byte streams consist of printable |
|
2550
0f6ff45bd816
replace ASCII by macro call
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diff
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|
3329 \ASCII{} characters only (though it's not designed to be pretty). |
|
2344
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|
3330 |
|
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|
3331 The module \code{shelve} provides a simple model for storing objects |
|
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|
3332 on files. The operation \code{shelve.open(filename)} returns a |
|
279d51a18ae4
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|
3333 ``shelf'', which is a simple persistent database with a |
|
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|
3334 dictionary-like interface. Database keys are strings, objects stored |
|
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added chapter on new things in 1.2
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|
3335 in the database can be anything that \code{pickle} will handle. |
|
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|
3336 |
|
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|
3337 \subsection{Copying Objects} |
|
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diff
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|
3338 |
|
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|
3339 The module \code{copy} exports two functions: \code{copy()} and |
|
279d51a18ae4
added chapter on new things in 1.2
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|
3340 \code{deepcopy()}. The \code{copy()} function returns a ``shallow'' |
|
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added chapter on new things in 1.2
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|
3341 copy of an object; \code{deepcopy()} returns a ``deep'' copy. The |
|
279d51a18ae4
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|
3342 difference between shallow and deep copying is only relevant for |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
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|
3343 compound objects (objects that contain other objects, like lists or |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
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|
3344 class instances): |
|
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added chapter on new things in 1.2
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|
3345 |
|
279d51a18ae4
added chapter on new things in 1.2
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|
3346 \begin{itemize} |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
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|
3347 |
|
279d51a18ae4
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|
3348 \item |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
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|
3349 A shallow copy constructs a new compound object and then (to the |
|
279d51a18ae4
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|
3350 extent possible) inserts {\em the same objects} into in that the |
|
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|
3351 original contains. |
|
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|
3352 |
|
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added chapter on new things in 1.2
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|
3353 \item |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
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|
3354 A deep copy constructs a new compound object and then, recursively, |
|
279d51a18ae4
added chapter on new things in 1.2
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|
3355 inserts {\em copies} into it of the objects found in the original. |
|
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|
3356 |
|
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|
3357 \end{itemize} |
|
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added chapter on new things in 1.2
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|
3358 |
|
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|
3359 Both functions have the same restrictions and use the same protocols |
|
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|
3360 as \code{pickle} --- user-defined classes can control how they are |
|
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|
3361 copied by providing methods named \code{__getinitargs__()}, |
|
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|
3362 \code{__getstate__()} and \code{__setstate__()}. |
|
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|
3363 |
|
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|
3364 |
|
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|
3365 \section{Documentation Strings} |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
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|
3366 |
|
279d51a18ae4
added chapter on new things in 1.2
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|
3367 A variety of objects now have a new attribute, \code{__doc__}, which |
|
279d51a18ae4
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|
3368 is supposed to contain a documentation string (if no documentation is |
|
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|
3369 present, the attribute is \code{None}). New syntax, compatible with |
|
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|
3370 the old interpreter, allows for convenient initialization of the |
|
279d51a18ae4
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|
3371 \code{__doc__} attribute of modules, classes and functions by placing |
|
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|
3372 a string literal by itself as the first statement in the suite. It |
|
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|
3373 must be a literal --- an expression yielding a string object is not |
|
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|
3374 accepted as a documentation string, since future tools may need to |
|
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|
3375 derive documentation from source by parsing. |
|
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|
3376 |
|
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|
3377 Here is a hypothetical, amply documented module called \code{Spam}: |
|
279d51a18ae4
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|
3378 |
|
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|
3379 \begin{verbatim} |
|
279d51a18ae4
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|
3380 """Spam operations. |
|
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|
3381 |
|
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|
3382 This module exports two classes, a function and an exception: |
|
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|
3383 |
|
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|
3384 class Spam: full Spam functionality --- three can sizes |
|
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|
3385 class SpamLight: limited Spam functionality --- only one can size |
|
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|
3386 |
|
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|
3387 def open(filename): open a file and return a corresponding Spam or |
|
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|
3388 SpamLight object |
|
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|
3389 |
|
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|
3390 GoneOff: exception raised for errors; should never happen |
|
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|
3391 |
|
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|
3392 Note that it is always possible to convert a SpamLight object to a |
|
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|
3393 Spam object by a simple method call, but that the reverse operation is |
|
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|
3394 generally costly and may fail for a number of reasons. |
|
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|
3395 """ |
|
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|
3396 |
|
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|
3397 class SpamLight: |
|
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|
3398 """Limited spam functionality. |
|
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|
3399 |
|
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|
3400 Supports a single can size, no flavor, and only hard disks. |
|
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|
3401 """ |
|
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|
3402 |
|
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|
3403 def __init__(self, size=12): |
|
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|
3404 """Construct a new SpamLight instance. |
|
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|
3405 |
|
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|
3406 Argument is the can size. |
|
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|
3407 """ |
|
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|
3408 # etc. |
|
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|
3409 |
|
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|
3410 # etc. |
|
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|
3411 |
|
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|
3412 class Spam(SpamLight): |
|
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|
3413 """Full spam functionality. |
|
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|
3414 |
|
279d51a18ae4
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diff
changeset
|
3415 Supports three can sizes, two flavor varieties, and all floppy |
|
279d51a18ae4
added chapter on new things in 1.2
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parents:
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diff
changeset
|
3416 disk formats still supported by current hardware. |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3417 """ |
|
279d51a18ae4
added chapter on new things in 1.2
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parents:
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diff
changeset
|
3418 |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3419 def __init__(self, size1=8, size2=12, size3=20): |
|
279d51a18ae4
added chapter on new things in 1.2
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parents:
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diff
changeset
|
3420 """Construct a new Spam instance. |
|
279d51a18ae4
added chapter on new things in 1.2
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parents:
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diff
changeset
|
3421 |
|
279d51a18ae4
added chapter on new things in 1.2
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parents:
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diff
changeset
|
3422 Arguments are up to three can sizes. |
|
279d51a18ae4
added chapter on new things in 1.2
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parents:
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diff
changeset
|
3423 """ |
|
279d51a18ae4
added chapter on new things in 1.2
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parents:
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diff
changeset
|
3424 # etc. |
|
279d51a18ae4
added chapter on new things in 1.2
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parents:
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diff
changeset
|
3425 |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
2063
diff
changeset
|
3426 # etc. |
|
279d51a18ae4
added chapter on new things in 1.2
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parents:
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diff
changeset
|
3427 |
|
279d51a18ae4
added chapter on new things in 1.2
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parents:
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diff
changeset
|
3428 def open(filename = "/dev/null"): |
|
279d51a18ae4
added chapter on new things in 1.2
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parents:
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diff
changeset
|
3429 """Open a can of Spam. |
|
279d51a18ae4
added chapter on new things in 1.2
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parents:
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diff
changeset
|
3430 |
|
279d51a18ae4
added chapter on new things in 1.2
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parents:
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diff
changeset
|
3431 Argument must be an existing file. |
|
279d51a18ae4
added chapter on new things in 1.2
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parents:
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diff
changeset
|
3432 """ |
|
279d51a18ae4
added chapter on new things in 1.2
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parents:
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diff
changeset
|
3433 # etc. |
|
279d51a18ae4
added chapter on new things in 1.2
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parents:
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diff
changeset
|
3434 |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3435 class GoneOff: |
|
279d51a18ae4
added chapter on new things in 1.2
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parents:
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diff
changeset
|
3436 """Class used for Spam exceptions. |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3437 |
|
279d51a18ae4
added chapter on new things in 1.2
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parents:
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diff
changeset
|
3438 There shouldn't be any. |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3439 """ |
|
279d51a18ae4
added chapter on new things in 1.2
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parents:
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diff
changeset
|
3440 pass |
|
279d51a18ae4
added chapter on new things in 1.2
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parents:
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diff
changeset
|
3441 \end{verbatim} |
|
279d51a18ae4
added chapter on new things in 1.2
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parents:
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diff
changeset
|
3442 |
|
279d51a18ae4
added chapter on new things in 1.2
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parents:
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diff
changeset
|
3443 After executing ``\code{import Spam}'', the following expressions |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3444 return the various documentation strings from the module: |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3445 |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3446 \begin{verbatim} |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
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diff
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|
3447 Spam.__doc__ |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
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|
3448 Spam.SpamLight.__doc__ |
|
279d51a18ae4
added chapter on new things in 1.2
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parents:
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diff
changeset
|
3449 Spam.SpamLight.__init__.__doc__ |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
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|
3450 Spam.Spam.__doc__ |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3451 Spam.Spam.__init__.__doc__ |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
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|
3452 Spam.open.__doc__ |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3453 Spam.GoneOff.__doc__ |
|
279d51a18ae4
added chapter on new things in 1.2
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parents:
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diff
changeset
|
3454 \end{verbatim} |
|
279d51a18ae4
added chapter on new things in 1.2
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parents:
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diff
changeset
|
3455 |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3456 There are emerging conventions about the content and formatting of |
|
279d51a18ae4
added chapter on new things in 1.2
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parents:
2063
diff
changeset
|
3457 documentation strings. |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3458 |
|
279d51a18ae4
added chapter on new things in 1.2
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parents:
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diff
changeset
|
3459 The first line should always be a short, concise summary of the |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3460 object's purpose. For brevity, it should not explicitly state the |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3461 object's name or type, since these are available by other means |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3462 (except if the name happens to be a verb describing a function's |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3463 operation). This line should begin with a capital letter and end with |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
2063
diff
changeset
|
3464 a period. |
|
279d51a18ae4
added chapter on new things in 1.2
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parents:
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diff
changeset
|
3465 |
|
279d51a18ae4
added chapter on new things in 1.2
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parents:
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diff
changeset
|
3466 If there are more lines in the documentation string, the second line |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
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diff
changeset
|
3467 should be blank, visually separating the summary from the rest of the |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3468 description. The following lines should be one of more of paragraphs |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3469 describing the objects calling conventions, its side effects, etc. |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3470 |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3471 Some people like to copy the Emacs convention of using UPPER CASE for |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3472 function parameters --- this often saves a few words or lines. |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3473 |
|
279d51a18ae4
added chapter on new things in 1.2
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parents:
2063
diff
changeset
|
3474 The Python parser does not strip indentation from multi-line string |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3475 literals in Python, so tools that process documentation have to strip |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3476 indentation. This is done using the following convention. The first |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
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diff
changeset
|
3477 non-blank line {\em after} the first line of the string determines the |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3478 amount of indentation for the entire documentation string. (We can't |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3479 use the first line since it is generally adjacent to the string's |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
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diff
changeset
|
3480 opening quotes so its indentation is not apparent in the string |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
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diff
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|
3481 literal.) Whitespace ``equivalent'' to this indentation is then |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3482 stripped from the start of all lines of the string. Lines that are |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3483 indented less should not occur, but if they occur all their leading |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3484 whitespace should be stripped. Equivalence of whitespace should be |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3485 tested after expansion of tabs (to 8 spaces, normally). |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3486 |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3487 In this release, few of the built-in or standard functions and modules |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
2063
diff
changeset
|
3488 have documentation strings. |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
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diff
changeset
|
3489 |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3490 |
|
279d51a18ae4
added chapter on new things in 1.2
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parents:
2063
diff
changeset
|
3491 \section{Customizing Import and Built-Ins} |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
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diff
changeset
|
3492 |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3493 In preparation for a ``restricted execution mode'' which will be |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
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diff
changeset
|
3494 usable to run code received from an untrusted source (such as a WWW |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3495 server or client), the mechanism by which modules are imported has |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
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diff
changeset
|
3496 been redesigned. It is now possible to provide your own function |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
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diff
changeset
|
3497 \code{__import__} which is called whenever an \code{import} statement |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3498 is executed. There's a built-in function \code{__import__} which |
|
279d51a18ae4
added chapter on new things in 1.2
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changeset
|
3499 provides the default implementation, but more interesting, the various |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
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|
3500 steps it takes are available separately from the new built-in module |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
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diff
changeset
|
3501 \code{imp}. (See the section on \code{imp} in the Library Reference |
|
4452
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
3502 Manual for more information on this module --- it also contains a |
|
2815
1e82dd6dfe2e
added pointer to libimp.tex for __import__ example
Guido van Rossum <guido@python.org>
parents:
2647
diff
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|
3503 complete example of how to write your own \code{__import__} function.) |
|
2344
279d51a18ae4
added chapter on new things in 1.2
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|
3504 |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3505 When you do \code{dir()} in a fresh interactive interpreter you will |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
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|
3506 see another ``secret'' object that's present in every module: |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
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|
3507 \code{__builtins__}. This is either a dictionary or a module |
|
279d51a18ae4
added chapter on new things in 1.2
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|
3508 containing the set of built-in objects used by functions defined in |
|
279d51a18ae4
added chapter on new things in 1.2
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|
3509 current module. Although normally all modules are initialized with a |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
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|
3510 reference to the same dictionary, it is now possible to use a |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
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|
3511 different set of built-ins on a per-module basis. Together with the |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3512 fact that the \code{import} statement uses the \code{__import__} |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
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|
3513 function it finds in the importing modules' dictionary of built-ins, |
|
279d51a18ae4
added chapter on new things in 1.2
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|
3514 this forms the basis for a future restricted execution mode. |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
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|
3515 |
|
279d51a18ae4
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|
3516 |
|
279d51a18ae4
added chapter on new things in 1.2
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|
3517 \section{Python and the World-Wide Web} |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
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|
3518 |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
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|
3519 There is a growing number of modules available for writing WWW tools. |
|
279d51a18ae4
added chapter on new things in 1.2
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diff
changeset
|
3520 The previous release already sported modules \code{gopherlib}, |
|
2352
26b1b15fc8ec
no need to reference doc strings for documented modules
Guido van Rossum <guido@python.org>
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2344
diff
changeset
|
3521 \code{ftplib}, \code{httplib} and \code{urllib} (which unifies the |
|
26b1b15fc8ec
no need to reference doc strings for documented modules
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diff
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|
3522 other three) for accessing data through the commonest WWW protocols. |
|
26b1b15fc8ec
no need to reference doc strings for documented modules
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diff
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|
3523 This release also provides \code{cgi}, to ease the writing of |
|
26b1b15fc8ec
no need to reference doc strings for documented modules
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diff
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|
3524 server-side scripts that use the Common Gateway Interface protocol, |
|
26b1b15fc8ec
no need to reference doc strings for documented modules
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2344
diff
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|
3525 supported by most WWW servers. The module \code{urlparse} provides |
|
26b1b15fc8ec
no need to reference doc strings for documented modules
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2344
diff
changeset
|
3526 precise parsing of a URL string into its components (address scheme, |
|
26b1b15fc8ec
no need to reference doc strings for documented modules
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diff
changeset
|
3527 network location, path, parameters, query, and fragment identifier). |
|
26b1b15fc8ec
no need to reference doc strings for documented modules
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2344
diff
changeset
|
3528 |
|
26b1b15fc8ec
no need to reference doc strings for documented modules
Guido van Rossum <guido@python.org>
parents:
2344
diff
changeset
|
3529 A rudimentary, parser for HTML files is available in the module |
|
26b1b15fc8ec
no need to reference doc strings for documented modules
Guido van Rossum <guido@python.org>
parents:
2344
diff
changeset
|
3530 \code{htmllib}. It currently supports a subset of HTML 1.0 (if you |
|
26b1b15fc8ec
no need to reference doc strings for documented modules
Guido van Rossum <guido@python.org>
parents:
2344
diff
changeset
|
3531 bring it up to date, I'd love to receive your fixes!). Unfortunately |
|
26b1b15fc8ec
no need to reference doc strings for documented modules
Guido van Rossum <guido@python.org>
parents:
2344
diff
changeset
|
3532 Python seems to be too slow for real-time parsing and formatting of |
|
2453
cdd3a721b40e
a few typographical changes (e.g. -- => ---) and lots of new stuff in the WWW chapter
Guido van Rossum <guido@python.org>
parents:
2352
diff
changeset
|
3533 HTML such as required by interactive WWW browsers --- but it's good |
|
cdd3a721b40e
a few typographical changes (e.g. -- => ---) and lots of new stuff in the WWW chapter
Guido van Rossum <guido@python.org>
parents:
2352
diff
changeset
|
3534 enough to write a ``robot'' (an automated WWW browser that searches |
|
cdd3a721b40e
a few typographical changes (e.g. -- => ---) and lots of new stuff in the WWW chapter
Guido van Rossum <guido@python.org>
parents:
2352
diff
changeset
|
3535 the web for information). |
|
2344
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
2063
diff
changeset
|
3536 |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
2063
diff
changeset
|
3537 |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
2063
diff
changeset
|
3538 \section{Miscellaneous} |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
2063
diff
changeset
|
3539 |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
2063
diff
changeset
|
3540 \begin{itemize} |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
2063
diff
changeset
|
3541 |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
2063
diff
changeset
|
3542 \item |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
2063
diff
changeset
|
3543 The \code{socket} module now exports all the needed constants used for |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
2063
diff
changeset
|
3544 socket operations, such as \code{SO_BROADCAST}. |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
2063
diff
changeset
|
3545 |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
2063
diff
changeset
|
3546 \item |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
2063
diff
changeset
|
3547 The functions \code{popen()} and \code{fdopen()} in the \code{os} |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
2063
diff
changeset
|
3548 module now follow the pattern of the built-in function \code{open()}: |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
2063
diff
changeset
|
3549 the default mode argument is \code{'r'} and the optional third |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
2063
diff
changeset
|
3550 argument specifies the buffer size, where \code{0} means unbuffered, |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
2063
diff
changeset
|
3551 \code{1} means line-buffered, and any larger number means the size of |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
2063
diff
changeset
|
3552 the buffer in bytes. |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
2063
diff
changeset
|
3553 |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
2063
diff
changeset
|
3554 \end{itemize} |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
2063
diff
changeset
|
3555 |
|
279d51a18ae4
added chapter on new things in 1.2
Guido van Rossum <guido@python.org>
parents:
2063
diff
changeset
|
3556 |
|
2942
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3557 \chapter{New in Release 1.3} |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3558 |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3559 |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3560 This chapter describes yet more recent additions to the Python |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3561 language and library. |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3562 |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3563 |
|
3164
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3564 \section{Keyword Arguments} |
|
2942
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3565 |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3566 Functions and methods written in Python can now be called using |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3567 keyword arguments of the form \code{\var{keyword} = \var{value}}. For |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3568 instance, the following function: |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3569 |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3570 \begin{verbatim} |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3571 def parrot(voltage, state='a stiff', action='voom', type='Norwegian Blue'): |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3572 print "-- This parrot wouldn't", action, |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3573 print "if you put", voltage, "Volts through it." |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3574 print "-- Lovely plumage, the", type |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3575 print "-- It's", state, "!" |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3576 \end{verbatim} |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3577 |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3578 could be called in any of the following ways: |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3579 |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3580 \begin{verbatim} |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3581 parrot(1000) |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3582 parrot(action = 'VOOOOOM', voltage = 1000000) |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3583 parrot('a thousand', state = 'pushing up the daisies') |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3584 parrot('a million', 'bereft of life', 'jump') |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3585 \end{verbatim} |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3586 |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3587 but the following calls would all be invalid: |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3588 |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3589 \begin{verbatim} |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3590 parrot() # required argument missing |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3591 parrot(voltage=5.0, 'dead') # non-keyword argument following keyword |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3592 parrot(110, voltage=220) # duplicate value for argument |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3593 parrot(actor='John Cleese') # unknown keyword |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3594 \end{verbatim} |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3595 |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3596 In general, an argument list must have the form: zero or more |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3597 positional arguments followed by zero or more keyword arguments, where |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3598 the keywords must be chosen from the formal parameter names. It's not |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3599 important whether a formal parameter has a default value or not. No |
|
4452
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
3600 argument must receive a value more than once --- formal parameter names |
|
2942
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3601 corresponding to positional arguments cannot be used as keywords in |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3602 the same calls. |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3603 |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3604 Note that no special syntax is required to allow a function to be |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3605 called with keyword arguments. The additional costs incurred by |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3606 keyword arguments are only present when a call uses them. |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3607 |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3608 (As far as I know, these rules are exactly the same as used by |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3609 Modula-3, even if they are enforced by totally different means. This |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3610 is intentional.) |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3611 |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3612 When a final formal parameter of the form \code{**\var{name}} is |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3613 present, it receives a dictionary containing all keyword arguments |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3614 whose keyword doesn't correspond to a formal parameter. This may be |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3615 combined with a formal parameter of the form \code{*\var{name}} which |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3616 receives a tuple containing the positional arguments beyond the formal |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3617 parameter list. (\code{*\var{name}} must occur before |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3618 \code{**\var{name}}.) For example, if we define a function like this: |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3619 |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3620 \begin{verbatim} |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3621 def cheeseshop(kind, *arguments, **keywords): |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3622 print "-- Do you have any", kind, '?' |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3623 print "-- I'm sorry, we're all out of", kind |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3624 for arg in arguments: print arg |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3625 print '-'*40 |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3626 for kw in keywords.keys(): print kw, ':', keywords[kw] |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3627 \end{verbatim} |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3628 |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3629 It could be called like this: |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3630 |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3631 \begin{verbatim} |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3632 cheeseshop('Limburger', "It's very runny, sir.", |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3633 "It's really very, VERY runny, sir.", |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3634 client='John Cleese', |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3635 shopkeeper='Michael Palin', |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3636 sketch='Cheese Shop Sketch') |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3637 \end{verbatim} |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3638 |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3639 and of course it would print: |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3640 |
|
3715206397fe
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Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3641 \begin{verbatim} |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3642 -- Do you have any Limburger ? |
|
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chapter on new things in 1.3
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parents:
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diff
changeset
|
3643 -- I'm sorry, we're all out of Limburger |
|
3715206397fe
chapter on new things in 1.3
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parents:
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diff
changeset
|
3644 It's very runny, sir. |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
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diff
changeset
|
3645 It's really very, VERY runny, sir. |
|
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Guido van Rossum <guido@python.org>
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diff
changeset
|
3646 ---------------------------------------- |
|
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Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3647 client : John Cleese |
|
3715206397fe
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Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3648 shopkeeper : Michael Palin |
|
3715206397fe
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Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3649 sketch : Cheese Shop Sketch |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3650 \end{verbatim} |
|
3715206397fe
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Guido van Rossum <guido@python.org>
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diff
changeset
|
3651 |
|
3173
6d34ade2e9d4
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|
3652 Consequences of this change include: |
|
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|
3653 |
|
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chapter on new things in 1.3
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|
3654 \begin{itemize} |
|
3715206397fe
chapter on new things in 1.3
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|
3655 |
|
3715206397fe
chapter on new things in 1.3
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|
3656 \item |
|
3173
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|
3657 The built-in function \code{apply()} now has an optional third |
|
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|
3658 argument, which is a dictionary specifying any keyword arguments to be |
|
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|
3659 passed. For example, |
|
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|
3660 \begin{verbatim} |
|
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|
3661 apply(parrot, (), {'voltage': 20, 'action': 'voomm'}) |
|
6d34ade2e9d4
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|
3662 \end{verbatim} |
|
6d34ade2e9d4
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|
3663 is equivalent to |
|
6d34ade2e9d4
a fe more things: apply 3rd arg, ni, ihooks, rexec
Guido van Rossum <guido@python.org>
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|
3664 \begin{verbatim} |
|
6d34ade2e9d4
a fe more things: apply 3rd arg, ni, ihooks, rexec
Guido van Rossum <guido@python.org>
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|
3665 parrot(voltage=20, action='voomm') |
|
6d34ade2e9d4
a fe more things: apply 3rd arg, ni, ihooks, rexec
Guido van Rossum <guido@python.org>
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|
3666 \end{verbatim} |
|
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|
3667 |
|
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|
3668 \item |
|
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|
3669 There is also a mechanism for functions and methods defined in an |
|
6d34ade2e9d4
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|
3670 extension module (i.e., implemented in C or C++) to receive a |
|
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|
3671 dictionary of their keyword arguments. By default, such functions do |
|
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|
3672 not accept keyword arguments, since the argument names are not |
|
6d34ade2e9d4
a fe more things: apply 3rd arg, ni, ihooks, rexec
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|
3673 available to the interpreter. |
|
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|
3674 |
|
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|
3675 \item |
|
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|
3676 In the effort of implementing keyword arguments, function and |
|
4452
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
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|
3677 especially method calls have been sped up significantly --- for a |
|
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changeset
|
3678 method with ten formal parameters, the call overhead has been cut in |
|
3715206397fe
chapter on new things in 1.3
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changeset
|
3679 half; for a function with one formal parameters, the overhead has been |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
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|
3680 reduced by a third. |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3681 |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3682 \item |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
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changeset
|
3683 The format of \code{.pyc} files has changed (again). |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
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|
3684 |
|
3164
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
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diff
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|
3685 \item |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3686 The \code{access} statement has been disabled. The syntax is still |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3687 recognized but no code is generated for it. (There were some |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
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diff
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|
3688 unpleasant interactions with changes for keyword arguments, and my |
|
95e80b6b8f2e
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parents:
3081
diff
changeset
|
3689 plan is to get rid of \code{access} altogether in favor of a different |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
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|
3690 approach.) |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3691 |
|
2942
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chapter on new things in 1.3
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diff
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|
3692 \end{itemize} |
|
3715206397fe
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Guido van Rossum <guido@python.org>
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changeset
|
3693 |
|
3164
95e80b6b8f2e
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diff
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|
3694 \section{Changes to the WWW and Internet tools} |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
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3081
diff
changeset
|
3695 |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3696 \begin{itemize} |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3697 |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3698 \item |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3699 The \code{htmllib} module has been rewritten in an incompatible |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3700 fashion. The new version is considerably more complete (HTML 2.0 |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3701 except forms, but including all ISO-8859-1 entity definitions), and |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3702 easy to use. Small changes to \code{sgmllib} have also been made, to |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3703 better match the tokenization of HTML as recognized by other web |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3704 tools. |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3705 |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3706 \item |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3707 A new module \code{formatter} has been added, for use with the new |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3708 \code{htmllib} module. |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3709 |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3710 \item |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3711 The \code{urllib}and \code{httplib} modules have been changed somewhat |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3712 to allow overriding unknown URL types and to support authentication. |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3713 They now use \code{mimetools.Message} instead of \code{rfc822.Message} |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3714 to parse headers. The \code{endrequest()} method has been removed |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3715 from the HTTP class since it breaks the interaction with some servers. |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3716 |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3717 \item |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3718 The \code{rfc822.Message} class has been changed to allow a flag to be |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3719 passed in that says that the file is unseekable. |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3720 |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3721 \item |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3722 The \code{ftplib} module has been fixed to be (hopefully) more robust |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3723 on Linux. |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3724 |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3725 \item |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3726 Several new operations that are optionally supported by servers have |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3727 been added to \code{nntplib}: \code{xover}, \code{xgtitle}, |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3728 \code{xpath} and \code{date}. % thanks to Kevan Heydon |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3729 |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3730 \end{itemize} |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3731 |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
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3081
diff
changeset
|
3732 \section{Other Language Changes} |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
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diff
changeset
|
3733 |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3734 \begin{itemize} |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3735 |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3736 \item |
|
95e80b6b8f2e
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diff
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|
3737 The \code{raise} statement now takes an optional argument which |
|
95e80b6b8f2e
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diff
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|
3738 specifies the traceback to be used when printing the exception's stack |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
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diff
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|
3739 trace. This must be a traceback object, such as found in |
|
95e80b6b8f2e
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diff
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|
3740 \code{sys.exc_traceback}. When omitted or given as \code{None}, the |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3741 old behavior (to generate a stack trace entry for the current stack |
|
95e80b6b8f2e
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parents:
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diff
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|
3742 frame) is used. |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
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diff
changeset
|
3743 |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
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diff
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|
3744 \item |
|
95e80b6b8f2e
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diff
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|
3745 The tokenizer is now more tolerant of alien whitespace. Control-L in |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
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|
3746 the leading whitespace of a line resets the column number to zero, |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
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3081
diff
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|
3747 while Control-R just before the end of the line is ignored. |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
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3081
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|
3748 |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
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|
3749 \end{itemize} |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
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diff
changeset
|
3750 |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
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3081
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|
3751 \section{Changes to Built-in Operations} |
|
2942
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|
3752 |
|
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|
3753 \begin{itemize} |
|
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|
3754 |
|
3715206397fe
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|
3755 \item |
|
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|
3756 For file objects, \code{\var{f}.read(0)} and |
|
3715206397fe
chapter on new things in 1.3
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|
3757 \code{\var{f}.readline(0)} now return an empty string rather than |
|
3715206397fe
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|
3758 reading an unlimited number of bytes. For the latter, omit the |
|
3715206397fe
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|
3759 argument altogether or pass a negative value. |
|
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diff
changeset
|
3760 |
|
3715206397fe
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|
3761 \item |
|
3715206397fe
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|
3762 A new system variable, \code{sys.platform}, has been added. It |
|
3715206397fe
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|
3763 specifies the current platform, e.g. \code{sunos5} or \code{linux1}. |
|
3715206397fe
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|
3764 |
|
3715206397fe
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|
3765 \item |
|
3715206397fe
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changeset
|
3766 The built-in functions \code{input()} and \code{raw_input()} now use |
|
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changeset
|
3767 the GNU readline library when it has been configured (formerly, only |
|
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|
3768 interactive input to the interpreter itself was read using GNU |
|
3715206397fe
chapter on new things in 1.3
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|
3769 readline). The GNU readline library provides elaborate line editing |
|
3715206397fe
chapter on new things in 1.3
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|
3770 and history. The Python debugger (\code{pdb}) is the first |
|
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|
3771 beneficiary of this change. |
|
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diff
changeset
|
3772 |
|
3715206397fe
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Guido van Rossum <guido@python.org>
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changeset
|
3773 \item |
|
3715206397fe
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changeset
|
3774 Two new built-in functions, \code{globals()} and \code{locals()}, |
|
3715206397fe
chapter on new things in 1.3
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|
3775 provide access to dictionaries containming current global and local |
|
3715206397fe
chapter on new things in 1.3
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|
3776 variables, respectively. (These augment rather than replace |
|
3715206397fe
chapter on new things in 1.3
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|
3777 \code{vars()}, which returns the current local variables when called |
|
3715206397fe
chapter on new things in 1.3
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|
3778 without an argument, and a module's global variables when called with |
|
3715206397fe
chapter on new things in 1.3
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diff
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|
3779 an argument of type module.) |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
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diff
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|
3780 |
|
3715206397fe
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|
3781 \item |
|
3164
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|
3782 The built-in function \code{compile()} now takes a third possible |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3783 value for the kind of code to be compiled: specifying \code{'single'} |
|
95e80b6b8f2e
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parents:
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diff
changeset
|
3784 generates code for a single interactive statement, which prints the |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
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diff
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|
3785 output of expression statements that evaluate to something else than |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
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|
3786 \code{None}. |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
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diff
changeset
|
3787 |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3788 \end{itemize} |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3789 |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
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diff
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|
3790 \section{Library Changes} |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
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diff
changeset
|
3791 |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3792 \begin{itemize} |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
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3081
diff
changeset
|
3793 |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3794 \item |
|
3173
6d34ade2e9d4
a fe more things: apply 3rd arg, ni, ihooks, rexec
Guido van Rossum <guido@python.org>
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3164
diff
changeset
|
3795 There are new module \code{ni} and \code{ihooks} that support |
|
6d34ade2e9d4
a fe more things: apply 3rd arg, ni, ihooks, rexec
Guido van Rossum <guido@python.org>
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3164
diff
changeset
|
3796 importing modules with hierarchical names such as \code{A.B.C}. This |
|
6d34ade2e9d4
a fe more things: apply 3rd arg, ni, ihooks, rexec
Guido van Rossum <guido@python.org>
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|
3797 is enabled by writing \code{import ni; ni.ni()} at the very top of the |
|
6d34ade2e9d4
a fe more things: apply 3rd arg, ni, ihooks, rexec
Guido van Rossum <guido@python.org>
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|
3798 main program. These modules are amply documented in the Python |
|
6d34ade2e9d4
a fe more things: apply 3rd arg, ni, ihooks, rexec
Guido van Rossum <guido@python.org>
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|
3799 source. |
|
6d34ade2e9d4
a fe more things: apply 3rd arg, ni, ihooks, rexec
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|
3800 |
|
6d34ade2e9d4
a fe more things: apply 3rd arg, ni, ihooks, rexec
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|
3801 \item |
|
6d34ade2e9d4
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|
3802 The module \code{rexec} has been rewritten (incompatibly) to define a |
|
6d34ade2e9d4
a fe more things: apply 3rd arg, ni, ihooks, rexec
Guido van Rossum <guido@python.org>
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|
3803 class and to use \code{ihooks}. |
|
6d34ade2e9d4
a fe more things: apply 3rd arg, ni, ihooks, rexec
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|
3804 |
|
6d34ade2e9d4
a fe more things: apply 3rd arg, ni, ihooks, rexec
Guido van Rossum <guido@python.org>
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|
3805 \item |
|
3164
95e80b6b8f2e
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diff
changeset
|
3806 The \code{string.split()} and \code{string.splitfields()} functions |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3807 are now the same function (the presence or absence of the second |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3808 argument determines which operation is invoked); similar for |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3809 \code{string.join()} and \code{string.joinfields()}. |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3810 |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3811 \item |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3812 The \code{Tkinter} module and its helper \code{Dialog} have been |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3813 revamped to use keyword arguments. Tk 4.0 is now the standard. A new |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3814 module \code{FileDialog} has been added which implements standard file |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3815 selection dialogs. |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3816 |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3817 \item |
|
2942
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|
3818 The optional built-in modules \code{dbm} and \code{gdbm} are more |
|
3715206397fe
chapter on new things in 1.3
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changeset
|
3819 coordinated --- their \code{open()} functions now take the same values |
|
3715206397fe
chapter on new things in 1.3
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changeset
|
3820 for their \var{flag} argument, and the \var{flag} and \var{mode} |
|
3715206397fe
chapter on new things in 1.3
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changeset
|
3821 argument have default values (to open the database for reading only, |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3822 and to create the database with mode \code{0666} minuse the umask, |
|
3164
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3823 respectively). The memory leaks have finally been fixed. |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3824 |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3825 \item |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3826 A new dbm-like module, \code{bsddb}, has been added, which uses the |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3827 BSD DB package's hash method. % thanks to David Ely |
|
2942
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diff
changeset
|
3828 |
|
3715206397fe
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diff
changeset
|
3829 \item |
|
3164
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3830 A portable (though slow) dbm-clone, implemented in Python, has been |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3831 added for systems where none of the above is provided. It is aptly |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3832 dubbed \code{dumbdbm}. |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3833 |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3834 \item |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3835 The module \code{anydbm} provides a unified interface to \code{bsddb}, |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3836 \code{gdbm}, \code{dbm}, and \code{dumbdbm}, choosing the first one |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3837 available. |
|
2942
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
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diff
changeset
|
3838 |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
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diff
changeset
|
3839 \item |
|
3164
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3840 A new extension module, \code{binascii}, provides a variety of |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3841 operations for conversion of text-encoded binary data. |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3842 |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3843 \item |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3844 There are three new or rewritten companion modules implemented in |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3845 Python that can encode and decode the most common such formats: |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3846 \code{uu} (uuencode), \code{base64} and \code{binhex}. |
|
2942
3715206397fe
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|
3847 |
|
3715206397fe
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|
3848 \item |
|
3164
95e80b6b8f2e
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3081
diff
changeset
|
3849 A module to handle the MIME encoding quoted-printable has also been |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3850 added: \code{quopri}. |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
parents:
3081
diff
changeset
|
3851 |
|
3204
d5e76a95b566
mention Fred Drake's parser module
Guido van Rossum <guido@python.org>
parents:
3176
diff
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|
3852 \item |
|
d5e76a95b566
mention Fred Drake's parser module
Guido van Rossum <guido@python.org>
parents:
3176
diff
changeset
|
3853 The parser module (which provides an interface to the Python parser's |
|
d5e76a95b566
mention Fred Drake's parser module
Guido van Rossum <guido@python.org>
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3176
diff
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|
3854 abstract syntax trees) has been rewritten (incompatibly) by Fred |
|
d5e76a95b566
mention Fred Drake's parser module
Guido van Rossum <guido@python.org>
parents:
3176
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changeset
|
3855 Drake. It now lets you change the parse tree and compile the result! |
|
d5e76a95b566
mention Fred Drake's parser module
Guido van Rossum <guido@python.org>
parents:
3176
diff
changeset
|
3856 |
|
3210
cdebee4a3e3e
mention syslog upgrade
Guido van Rossum <guido@python.org>
parents:
3204
diff
changeset
|
3857 \item |
|
cdebee4a3e3e
mention syslog upgrade
Guido van Rossum <guido@python.org>
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3204
diff
changeset
|
3858 The \code{syslog} module has been upgraded and documented. |
|
cdebee4a3e3e
mention syslog upgrade
Guido van Rossum <guido@python.org>
parents:
3204
diff
changeset
|
3859 % thanks to Steve Clift |
|
cdebee4a3e3e
mention syslog upgrade
Guido van Rossum <guido@python.org>
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changeset
|
3860 |
|
3164
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
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diff
changeset
|
3861 \end{itemize} |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
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diff
changeset
|
3862 |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
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|
3863 \section{Other Changes} |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
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diff
changeset
|
3864 |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
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3081
diff
changeset
|
3865 \begin{itemize} |
|
2942
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|
3866 |
|
3715206397fe
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|
3867 \item |
|
3715206397fe
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changeset
|
3868 The dynamic module loader recognizes the fact that different filenames |
|
3715206397fe
chapter on new things in 1.3
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diff
changeset
|
3869 point to the same shared library and loads the library only once, so |
|
3715206397fe
chapter on new things in 1.3
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diff
changeset
|
3870 you can have a single shared library that defines multiple modules. |
|
3715206397fe
chapter on new things in 1.3
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diff
changeset
|
3871 (SunOS / SVR4 style shared libraries only.) |
|
3715206397fe
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Guido van Rossum <guido@python.org>
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diff
changeset
|
3872 |
|
3715206397fe
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diff
changeset
|
3873 \item |
|
3715206397fe
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changeset
|
3874 Jim Fulton's ``abstract object interface'' has been incorporated into |
|
3715206397fe
chapter on new things in 1.3
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diff
changeset
|
3875 the run-time API. For more detailes, read the files |
|
3715206397fe
chapter on new things in 1.3
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changeset
|
3876 \code{Include/abstract.h} and \code{Objects/abstract.c}. |
|
3715206397fe
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|
3877 |
|
3715206397fe
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changeset
|
3878 \item |
|
3164
95e80b6b8f2e
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changeset
|
3879 The Macintosh version is much more robust now. |
|
95e80b6b8f2e
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Guido van Rossum <guido@python.org>
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3081
diff
changeset
|
3880 |
|
95e80b6b8f2e
describe more 1.3 changes
Guido van Rossum <guido@python.org>
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diff
changeset
|
3881 \item |
|
2942
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changeset
|
3882 Numerous things I have forgotten or that are so obscure no-one will |
|
3715206397fe
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changeset
|
3883 notice them anyway :-) |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3884 |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3885 \end{itemize} |
|
3715206397fe
chapter on new things in 1.3
Guido van Rossum <guido@python.org>
parents:
2815
diff
changeset
|
3886 |
|
4118
a86407b3c3ac
Version number; started chapter on what's new in 1.4
Guido van Rossum <guido@python.org>
parents:
3659
diff
changeset
|
3887 |
|
a86407b3c3ac
Version number; started chapter on what's new in 1.4
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3888 \chapter{New in Release 1.4} |
|
a86407b3c3ac
Version number; started chapter on what's new in 1.4
Guido van Rossum <guido@python.org>
parents:
3659
diff
changeset
|
3889 |
|
a86407b3c3ac
Version number; started chapter on what's new in 1.4
Guido van Rossum <guido@python.org>
parents:
3659
diff
changeset
|
3890 |
|
4379
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3891 This chapter describes the major additions to the Python language and |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3892 library in version 1.4. Many minor changes are not listed here; |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3893 it is recommended to read the file \code{Misc/NEWS} in the Python |
|
4452
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Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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changeset
|
3894 source distribution for a complete listing of changes. In particular, |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3895 changes that only affect C programmers or the build and installation |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
3896 process are not described in this chapter (the new installation |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3897 lay-out is explained below under \code{sys.prefix} though). |
|
4118
a86407b3c3ac
Version number; started chapter on what's new in 1.4
Guido van Rossum <guido@python.org>
parents:
3659
diff
changeset
|
3898 |
|
4491
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3899 \section{Language Changes} |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3900 |
|
4118
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Version number; started chapter on what's new in 1.4
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3901 \begin{itemize} |
|
a86407b3c3ac
Version number; started chapter on what's new in 1.4
Guido van Rossum <guido@python.org>
parents:
3659
diff
changeset
|
3902 |
|
a86407b3c3ac
Version number; started chapter on what's new in 1.4
Guido van Rossum <guido@python.org>
parents:
3659
diff
changeset
|
3903 \item |
|
a86407b3c3ac
Version number; started chapter on what's new in 1.4
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3904 Power operator. \code{x**y} is equivalent to \code{pow(x, y)}. |
|
4379
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3905 This operator binds more tightly than \code{*}, \code{/} or \code{\%}, |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3906 and binds from right to left when repeated or combined with unary |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3907 operators. For example, \code{x**y**z} is equivalent to |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3908 \code{x**(y**z)}, and \code{-x**y} is \code{-(x**y)}. |
|
4118
a86407b3c3ac
Version number; started chapter on what's new in 1.4
Guido van Rossum <guido@python.org>
parents:
3659
diff
changeset
|
3909 |
|
a86407b3c3ac
Version number; started chapter on what's new in 1.4
Guido van Rossum <guido@python.org>
parents:
3659
diff
changeset
|
3910 \item |
|
4452
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
3911 Complex numbers. Imaginary literals are writen with a \code{'j'} |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
3912 suffix (\code{'J'} is allowed as well.) Complex numbers with a nonzero |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
3913 real component are written as \code{(\var{real}+\var{imag}j)}. You |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
3914 can also use the new built-in function \code{complex()} which takes |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
3915 one or two arguments: \code{complex(x)} is equivalent to \code{x + |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
3916 0j}, and \code{complex(x, y)} is \code{x + y*0j}. For example, |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
3917 \code{1j**2} yields \code{complex(-1.0)} (which is another way of |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
3918 saying ``the real value 1.0 represented as a complex number.'' |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
3919 |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
3920 Complex numbers are always represented as two floating point numbers, |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
3921 the real and imaginary part. |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
3922 To extract these parts from a complex number \code{z}, |
|
4379
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3923 use \code{z.real} and \code{z.imag}. The conversion functions to |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
4118
diff
changeset
|
3924 floating point and integer (\code{float()}, \code{int()} and |
|
4452
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
3925 \code{long()}) don't work for complex numbers --- there is no one |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
3926 correct way to convert a complex number to a real number. Use |
|
4379
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
4118
diff
changeset
|
3927 \code{abs(z)} to get its magnitude (as a float) or \code{z.real} to |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
4118
diff
changeset
|
3928 get its real part. |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
4118
diff
changeset
|
3929 |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
4118
diff
changeset
|
3930 Module \code{cmath} provides versions of all math functions that take |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
4118
diff
changeset
|
3931 complex arguments and return complex results. (Module \code{math} |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
4118
diff
changeset
|
3932 only supports real numbers, so that \code{math.sqrt(-1)} still raises |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
4118
diff
changeset
|
3933 a \code{ValueError} exception. Numerical experts agree that this is |
|
4452
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
3934 the way it should be.) |
|
4118
a86407b3c3ac
Version number; started chapter on what's new in 1.4
Guido van Rossum <guido@python.org>
parents:
3659
diff
changeset
|
3935 |
|
a86407b3c3ac
Version number; started chapter on what's new in 1.4
Guido van Rossum <guido@python.org>
parents:
3659
diff
changeset
|
3936 \item |
|
a86407b3c3ac
Version number; started chapter on what's new in 1.4
Guido van Rossum <guido@python.org>
parents:
3659
diff
changeset
|
3937 New indexing syntax. It is now possible to use a tuple as an indexing |
|
a86407b3c3ac
Version number; started chapter on what's new in 1.4
Guido van Rossum <guido@python.org>
parents:
3659
diff
changeset
|
3938 expression for a mapping object without parenthesizing it, |
|
4452
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
3939 e.g. \code{x[1, 2, 3]} is equivalent to \code{x[(1, 2, 3)]}. |
|
4118
a86407b3c3ac
Version number; started chapter on what's new in 1.4
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3940 |
|
a86407b3c3ac
Version number; started chapter on what's new in 1.4
Guido van Rossum <guido@python.org>
parents:
3659
diff
changeset
|
3941 \item |
|
a86407b3c3ac
Version number; started chapter on what's new in 1.4
Guido van Rossum <guido@python.org>
parents:
3659
diff
changeset
|
3942 New slicing syntax. In support of the Numerical Python extension |
|
4379
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
4118
diff
changeset
|
3943 (distributed independently), slice indices of the form |
|
4118
a86407b3c3ac
Version number; started chapter on what's new in 1.4
Guido van Rossum <guido@python.org>
parents:
3659
diff
changeset
|
3944 \code{x[lo:hi:stride]} are possible, multiple slice indices separated by |
|
4513
847445aeab2d
Change last Ellipses to Ellipsis :-(
Guido van Rossum <guido@python.org>
parents:
4509
diff
changeset
|
3945 commas are allowed, and an index position may be replaced by an ellipsis, |
|
4118
a86407b3c3ac
Version number; started chapter on what's new in 1.4
Guido van Rossum <guido@python.org>
parents:
3659
diff
changeset
|
3946 as follows: \code{x[a, ..., z]}. There's also a new built-in function |
|
a86407b3c3ac
Version number; started chapter on what's new in 1.4
Guido van Rossum <guido@python.org>
parents:
3659
diff
changeset
|
3947 \code{slice(lo, hi, stride)} and a new built-in object |
|
4513
847445aeab2d
Change last Ellipses to Ellipsis :-(
Guido van Rossum <guido@python.org>
parents:
4509
diff
changeset
|
3948 \code{Ellipsis}, which yield the same effect without using special |
|
4118
a86407b3c3ac
Version number; started chapter on what's new in 1.4
Guido van Rossum <guido@python.org>
parents:
3659
diff
changeset
|
3949 syntax. None of the standard sequence types support indexing with |
|
4452
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
3950 slice objects or ellipses yet. |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
3951 |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
3952 Note that when this new slicing syntax is used, the mapping interface |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
3953 will be used, not the sequence interface. In particular, when a |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
3954 user-defined class instance is sliced using this new slicing syntax, |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
3955 its \code{__getitem__} method is invoked --- the |
|
4379
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
4118
diff
changeset
|
3956 \code{__getslice__} method is only invoked when a single old-style |
|
4452
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
3957 slice is used, i.e. \code{x[lo:hi]}, with possible omission of |
|
4379
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3958 \code{lo} and/or \code{hi}. Some examples: |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3959 |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
4118
diff
changeset
|
3960 \begin{verbatim} |
|
4452
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
3961 x[0:10:2] -> slice(0, 10, 2) |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3962 x[:2:] -> slice(None, 2, None) |
|
4491
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
3963 x[::-1] -> slice(None, None, -1) |
|
4452
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3964 x[::] -> slice(None, None, None) |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3965 x[1, 2:3] -> (1, slice(2, 3, None)) |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3966 x[1:2, 3:4] -> (slice(1, 2, None), slice(3, 4, None)) |
|
4513
847445aeab2d
Change last Ellipses to Ellipsis :-(
Guido van Rossum <guido@python.org>
parents:
4509
diff
changeset
|
3967 x[1:2, ..., 3:4] -> (slice(1, 2, None), Ellipsis, |
|
4452
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3968 slice(3, 4, None)) |
|
4379
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
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parents:
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changeset
|
3969 \end{verbatim} |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
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changeset
|
3970 |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
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parents:
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diff
changeset
|
3971 For more help with this you are referred to the matrix-sig. |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
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changeset
|
3972 |
|
4118
a86407b3c3ac
Version number; started chapter on what's new in 1.4
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parents:
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diff
changeset
|
3973 \item |
|
4379
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
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parents:
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diff
changeset
|
3974 The \code{access} statement is now truly gone; \code{access} is no |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3975 longer a reserved word. This saves a few cycles here and there. |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3976 |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
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parents:
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diff
changeset
|
3977 \item |
|
4491
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
3978 Private variables through name mangling. |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
3979 There is now limited support for class-private |
|
4452
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
3980 identifiers. Any identifier of the form \code{__spam} (at least two |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
3981 leading underscores, at most one trailing underscore) is now textually |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
3982 replaced with \code{_classname__spam}, where \code{classname} is the |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
3983 current class name with leading underscore(s) stripped. This mangling |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
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parents:
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diff
changeset
|
3984 is done without regard of the syntactic position of the identifier, so |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
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parents:
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diff
changeset
|
3985 it can be used to define class-private instance and class variables, |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3986 methods, as well as globals, and even to store instance variables |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3987 private to this class on instances of {\em other} classes. Truncation |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3988 may occur when the mangled name would be longer than 255 characters. |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3989 Outside classes, or when the class name consists of only underscores, |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3990 no mangling occurs. |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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changeset
|
3991 |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3992 Name mangling is intended to give classes an easy way to define |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3993 ``private'' instance variables and methods, without having to worry |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3994 about instance variables defined by derived classes, or mucking with |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3995 instance variables by code outside the class. Note that the mangling |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3996 rules are designed mostly to avoid accidents; it still is possible for |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3997 a determined soul to access or modify a variable that is considered |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3998 private. This can even be useful, e.g. for the debugger, and that's |
|
4491
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
3999 one reason why this loophole is not closed. (Buglet: derivation of a |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
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changeset
|
4000 class with the same name as the base class makes use of private |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4001 variables of the base class possible.) |
|
4379
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
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|
4002 |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
4118
diff
changeset
|
4003 Notice that code passed to \code{exec}, \code{eval()} or |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4004 \code{evalfile()} does not consider the classname of the invoking |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
4118
diff
changeset
|
4005 class to be the current class; this is similar to the effect of the |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
4118
diff
changeset
|
4006 \code{global} statement, the effect of which is likewise restricted to |
|
4452
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
4007 code that is byte-compiled together. The same restriction applies to |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4008 \code{getattr()}, \code{setattr()} and \code{delattr()}, as well as |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4009 when referencing \code{__dict__} directly. |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4010 |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
4011 Here's an example of a class that implements its own |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4012 \code{__getattr__} and \code{__setattr__} methods and stores all |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
4013 attributes in a private variable, in a way that works in Python 1.4 as |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4014 well as in previous versions: |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4015 |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4016 \begin{verbatim} |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4017 class VirtualAttributes: |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4018 __vdict = None |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4019 __vdict_name = locals().keys()[0] |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4020 |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4021 def __init__(self): |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4022 self.__dict__[self.__vdict_name] = {} |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4023 |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4024 def __getattr__(self, name): |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4025 return self.__vdict[name] |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4026 |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4027 def __setattr__(self, name, value): |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4028 self.__vdict[name] = value |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4029 \end{verbatim} |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4030 |
|
4491
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4031 {\em Warning: this is an experimental feature.} To avoid all |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4032 potential problems, refrain from using identifiers starting with |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4033 double underscore except for predefined uses like \code{__init__}. To |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4034 use private names while maintaining future compatibility: refrain from |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4035 using the same private name in classes related via subclassing; avoid |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4036 explicit (manual) mangling/unmangling; and assume that at some point |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4037 in the future, leading double underscore will revert to being just a |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4038 naming convention. Discussion on extensive compile-time declarations |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4039 are currently underway, and it is impossible to predict what solution |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4040 will eventually be chosen for private names. Double leading |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4041 underscore is still a candidate, of course --- just not the only one. |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4042 It is placed in the distribution in the belief that it is useful, and |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4043 so that widespread experience with its use can be gained. It will not |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4044 be removed without providing a better solution and a migration path. |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4045 |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4046 \end{itemize} |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4047 |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4048 \section{Run-time Changes} |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4049 |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4050 \begin{itemize} |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4051 |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4052 \item |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4053 New built-in function \code{list()} converts any sequence to a new list. |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4054 Note that when the argument is a list, the return value is a fresh |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4055 copy, similar to what would be returned by \code{a[:]}. |
|
4379
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4056 |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
4118
diff
changeset
|
4057 \item |
|
4452
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
4058 Improved syntax error message. Syntax errors detected by the code |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
4059 generation phase of the Python bytecode compiler now include a line |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
4060 number. The line number is appended in parentheses. It is suppressed |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
4061 if the error occurs in line 1 (this usually happens in interactive |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
4062 use). |
|
4379
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4063 |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
4118
diff
changeset
|
4064 \item |
|
4452
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
4065 Different exception raised. |
|
4379
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
4118
diff
changeset
|
4066 Unrecognized keyword arguments now raise a \code{TypeError} exception |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
4118
diff
changeset
|
4067 rather than \code{KeyError}. |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
4118
diff
changeset
|
4068 |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
4118
diff
changeset
|
4069 \item |
|
4452
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
4070 Exceptions in \code{__del__} methods. When a \code{__del__} method |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
4071 raises an exception, a warning is written to \code{sys.stderr} and the |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
4072 exception is ignored. Formerly, such exceptions were ignored without |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
4073 warning. (Propagating the exception is not an option since it it is |
|
4491
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4074 invoked from an object finalizer, which cannot return any kind of |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4075 status or error.) (Buglet: The new behavior, while needed in order to |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4076 debug failing \code{__del__} methods, is occasionally annoying, |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4077 because if affects the program's standard error stream. It honors |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4078 assignments to \code{sys.stderr}, so it can be redirected from within |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4079 a program if desired.) |
|
4379
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4080 |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
4118
diff
changeset
|
4081 \item |
|
4491
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4082 You can now discover from which file (if any) a module was loaded by |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4083 inspecting its \code{__file__} attribute. This attribute is not |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4084 present for built-in or frozen modules. It points to the shared |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4085 library file for dynamically loaded modules. (Buglet: this may be a |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4086 relative path and is stored in the \code{.pyc} file on compilation. |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
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diff
changeset
|
4087 If you manipulate the current directory with \code{os.chdir()} or move |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
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parents:
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changeset
|
4088 \code{.pyc} files around, the value may be incorrect.) |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
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changeset
|
4089 |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
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diff
changeset
|
4090 \end{itemize} |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
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|
4091 |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
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changeset
|
4092 \section{New or Updated Modules} |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
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diff
changeset
|
4093 |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
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diff
changeset
|
4094 \begin{itemize} |
|
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|
4095 |
|
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A first stab at describing what's new in 1.4. Still many XXX'es left.
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changeset
|
4096 \item |
|
4452
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changeset
|
4097 New built-in module \code{operator}. While undocumented, the concept |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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changeset
|
4098 is real simply: \code{operator.__add__(x, y)} does exactly the same |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
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changeset
|
4099 thing as \code{x+y} (for all types --- built-in, user-defined, |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
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diff
changeset
|
4100 extension-defined). As a convenience, \code{operator.add} does the |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
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changeset
|
4101 same thing, but beware --- you can't use \code{operator.and} and a few |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
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changeset
|
4102 others where the ``natural'' name for an operator is a reserved |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
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diff
changeset
|
4103 keyword. You can add a single trailing underscore in such cases. |
|
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A first stab at describing what's new in 1.4. Still many XXX'es left.
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changeset
|
4104 |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4105 \item |
|
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Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
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|
4106 New built-in module \code{errno}. See the Library Reference Manual. |
|
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3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
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changeset
|
4107 |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4108 \item |
|
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Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
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diff
changeset
|
4109 Rewritten \code{cgi} module. See the Library Reference Manual. |
|
4379
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A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
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changeset
|
4110 |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4111 \item |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
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changeset
|
4112 Improved restricted execution module (\code{rexec}). New module |
|
4452
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
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changeset
|
4113 \code{Bastion}. Both are now documented in a new chapter on |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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changeset
|
4114 restricted execution in the Library Reference Manual. |
|
4379
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A first stab at describing what's new in 1.4. Still many XXX'es left.
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parents:
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changeset
|
4115 |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
4118
diff
changeset
|
4116 \item |
|
4452
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Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4117 New string operations (all described in the Library Reference Manual): |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4118 \code{lstrip()}, \code{rstrip()} (strip only the left/right |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4119 whitespace), \code{capitalize()} (uppercase the first character, |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4120 lowercase the rest), \code{capwords()} (capitalize each word, |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4121 delimited a la \code{string.split()}), \code{translate()} (string |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4122 transliteration -- this existed before but can now also delete |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4123 characters by specifying a third argument), \code{maketrans()} (a |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4124 convenience function for creating translation tables for |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4125 \code{translate()} and \code{regex.compile()}). The string function |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4126 \code{split()} has an optional third argument which specifies the |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4127 maximum number of separators to split; |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4128 e.g. \code{string.split('a=b=c', '=', 1)} yields \code{['a', 'b=c']}. |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4129 (Note that for a long time, \code{split()} and \code{splitfields()} |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4130 are synonyms. |
|
4379
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
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parents:
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diff
changeset
|
4131 |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
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changeset
|
4132 \item |
|
4452
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4133 New regsub operations (see the Library Reference Manual): |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4134 \code{regsub.capwords()} (like \code{string.capwords()} but allows you to |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4135 specify the word delimiter as a regular expression), |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
4136 \code{regsub.splitx()} (like \code{regsub.split()} but returns the |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4137 delimiters as well as the words in the resulting list). The optional |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
4379
diff
changeset
|
4138 \code{maxsep} argument is also supported by \code{regsub.split()}. |
|
4379
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4139 |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4140 \item |
|
4452
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4141 Module files \code{pdb.py} and \code{profile.py} can now be invoked as |
|
4491
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4142 scripts to debug c.q. profile other scripts easily. For example: |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4143 \code{python /usr/local/lib/python1.4/profile.py myscript.py} |
|
4379
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
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parents:
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diff
changeset
|
4144 |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4145 \item |
|
4491
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4146 The \code{os} module now supports the \code{putenv()} function on |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4147 systems where it is provided in the C library (Windows NT and most |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4148 Unix versions). For example, \code{os.putenv('PATH', |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4149 '/bin:/usr/bin')} sets the environment variable \code{PATH} to the |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4150 string \code{'/bin:/usr/bin'}. Such changes to the environment affect |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4151 subprocesses started with \code{os.system()}, \code{os.popen()} or |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4152 \code{os.fork()} and \code{os.execv()}. When \code{putenv()} is |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4153 supported, assignments to items in \code{os.environ} are automatically |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4154 translated into corresponding calls to \code{os.putenv()}; however, |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4155 calls to \code{os.putenv()} don't update \code{os.environ}, so it is |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4156 actually preferable to assign to items of \code{os.environ}. For this |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4157 purpose, the type of \code{os.environ} is changed to a subclass of |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4158 \code{UserDict.UserDict} when \code{os.putenv()} is supported. |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4159 (Buglet: \code{os.execve()} still requires a real dictionary, so it |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4160 won't accept \code{os.environ} as its third argument. However, you |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4161 can now use \code{os.execv()} and it will use your changes to |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4162 \code{os.environ}!.) |
|
4379
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
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parents:
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|
4163 |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4164 \item |
|
4491
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4165 More new functions in the \code{os} module: \code{mkfifo}, |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4166 \code{plock}, \code{remove} (== \code{unlink}), and \code{ftruncate}. |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4167 See the Unix manual (section 2, system calls) for these function. |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4168 More functions are also available under NT. |
|
4379
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
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changeset
|
4169 |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4170 \item |
|
4452
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
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diff
changeset
|
4171 New functions in the fcntl module: \code{lockf()} and \code{flock()} |
|
edab097882d8
Got rid of most XXXes in the News chapter. Bumped the version to 1.4.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4172 (don't ask \code{:-)}). See the Library Reference Manual. |
|
4379
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
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changeset
|
4173 |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4174 \item |
|
4491
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4175 The first item of the module search path, \code{sys.path[0]}, is the |
|
4379
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
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diff
changeset
|
4176 directory containing the script that was used to invoke the Python |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
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diff
changeset
|
4177 interpreter. If the script directory is not available (e.g. if the |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
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diff
changeset
|
4178 interpreter is invoked interactively or if the script is read from |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
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diff
changeset
|
4179 standard input), \code{sys.path[0]} is the empty string, which directs |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
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diff
changeset
|
4180 Python to search modules in the current directory first. Notice that |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
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diff
changeset
|
4181 the script directory is inserted {\em before} the entries inserted as |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
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diff
changeset
|
4182 a result of \code{\$PYTHONPATH}. There is no longer an entry for the |
|
4491
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
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diff
changeset
|
4183 current directory later in the path (unless explicitly set in |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4184 \code{\$PYTHONPATH} or overridden at build time). |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4185 |
|
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Done with the "New in 1.4" chapter.
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parents:
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changeset
|
4186 \end{itemize} |
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6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
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changeset
|
4187 |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4188 \section{Configuration and Installation} |
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6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
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|
4189 |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4190 \begin{itemize} |
|
4379
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
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|
4191 |
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3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4192 \item |
|
4491
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4193 More configuration information is now available to Python programs. |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4194 The variable \code{sys.prefix} gives the site-specific directory |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4195 prefix where the platform independent Python files are installed; by |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4196 default, this is the string \code{"/usr/local"}. This can be set at |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4197 build time with the \code{--prefix} argument to the \code{configure} |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4198 script. The main collection of Python library modules is installed in |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4199 the directory \code{sys.prefix+"/lib/python1.4"} while the platform |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4200 independent header files (all except \code{config.h}) are stored in |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4201 \code{sys.prefix+"/include/python1.4"}. |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4202 |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4203 Similarly, the variable \code{sys.exec_prefix} gives the site-specific |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4204 directory prefix where the platform {\em de}pendent Python files are |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4205 installed; by default, this is also \code{"/usr/local"}. This can be |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4206 set at build time with the \code{--exec-prefix} argument to the |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4207 \code{configure} script. Specifically, all configuration files |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4208 (e.g. the \code{config.h} header file) are installed in the directory |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4209 \code{sys.exec_prefix+"/lib/python1.4/config"}, and shared library |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4210 modules are installed in |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4211 \code{sys.exec_prefix+"/lib/python1.4/sharedmodules"}. |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4212 |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4213 Include files are at \code{sys.prefix+"/include/python1.4"}. |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4214 |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4215 It is not yet decided what the most portable way is to come up with |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4216 the version number used in these pathnames. For compatibility with |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4217 the 1.4beta releases, sys.version[:3] can be used. |
|
4379
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
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diff
changeset
|
4218 |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
4118
diff
changeset
|
4219 On non-Unix systems, these variables are meaningless. |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
4118
diff
changeset
|
4220 |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
4118
diff
changeset
|
4221 \item |
|
4491
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4222 While sites are strongly discouraged from modifying the standard |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4223 Python library (like adding site-specific modules or functions), there |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4224 is now a standard way to invoke site-specific features. The standard |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4225 module \code{site}, when imported, appends two site-specific |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4226 directories to the end of \code{sys.path}: |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4227 \code{\$prefix/lib/site-python} and |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4228 \code{\$exec_prefix/lib/site-python}, where \code{\$prefix} and |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4229 \code{\$exec_prefix} are the directories \code{sys.prefix} and |
|
4379
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
4118
diff
changeset
|
4230 \code{sys.exec_prefix} mentioned above. |
|
3061676a659a
A first stab at describing what's new in 1.4. Still many XXX'es left.
Guido van Rossum <guido@python.org>
parents:
4118
diff
changeset
|
4231 |
|
4491
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4232 After this path manipulation has been performed, an attempt is made to |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4233 import the module \code{sitecustomize}. Any \code{ImportError} |
|
6a75b3cf023c
Done with the "New in 1.4" chapter.
Guido van Rossum <guido@python.org>
parents:
4452
diff
changeset
|
4234 exception raised by this attempt is silently ignored. |
|
4118
a86407b3c3ac
Version number; started chapter on what's new in 1.4
Guido van Rossum <guido@python.org>
parents:
3659
diff
changeset
|
4235 |
|
a86407b3c3ac
Version number; started chapter on what's new in 1.4
Guido van Rossum <guido@python.org>
parents:
3659
diff
changeset
|
4236 \end{itemize} |
|
a86407b3c3ac
Version number; started chapter on what's new in 1.4
Guido van Rossum <guido@python.org>
parents:
3659
diff
changeset
|
4237 |
| 112 | 4238 \end{document} |