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author Guido van Rossum <guido@python.org>
date Thu, 17 Jul 1997 16:29:42 +0000
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1 \section{Built-in Types}
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2
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3 The following sections describe the standard types that are built into
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4 the interpreter. These are the numeric types, sequence types, and
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5 several others, including types themselves. There is no explicit
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6 Boolean type; use integers instead.
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7 \indexii{built-in}{types}
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8 \indexii{Boolean}{type}
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9
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10 Some operations are supported by several object types; in particular,
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11 all objects can be compared, tested for truth value, and converted to
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12 a string (with the \code{`{\rm \ldots}`} notation). The latter conversion is
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13 implicitly used when an object is written by the \code{print} statement.
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14 \stindex{print}
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15
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16 \subsection{Truth Value Testing}
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17
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18 Any object can be tested for truth value, for use in an \code{if} or
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19 \code{while} condition or as operand of the Boolean operations below.
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20 The following values are considered false:
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21 \stindex{if}
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22 \stindex{while}
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23 \indexii{truth}{value}
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24 \indexii{Boolean}{operations}
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25 \index{false}
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26
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27 \begin{itemize}
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28 \renewcommand{\indexsubitem}{(Built-in object)}
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29
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30 \item \code{None}
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31 \ttindex{None}
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32
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33 \item zero of any numeric type, e.g., \code{0}, \code{0L}, \code{0.0}.
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34
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35 \item any empty sequence, e.g., \code{''}, \code{()}, \code{[]}.
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36
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37 \item any empty mapping, e.g., \code{\{\}}.
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38
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39 \item instances of user-defined classes, if the class defines a
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40 \code{__nonzero__()} or \code{__len__()} method, when that
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41 method returns zero.
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42
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43 \end{itemize}
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44
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45 All other values are considered true --- so objects of many types are
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46 always true.
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47 \index{true}
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48
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49 Operations and built-in functions that have a Boolean result always
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50 return \code{0} for false and \code{1} for true, unless otherwise
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51 stated. (Important exception: the Boolean operations \samp{or} and
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52 \samp{and} always return one of their operands.)
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53
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54 \subsection{Boolean Operations}
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55
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56 These are the Boolean operations, ordered by ascending priority:
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57 \indexii{Boolean}{operations}
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58
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59 \begin{tableiii}{|c|l|c|}{code}{Operation}{Result}{Notes}
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60 \lineiii{\var{x} or \var{y}}{if \var{x} is false, then \var{y}, else \var{x}}{(1)}
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61 \hline
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62 \lineiii{\var{x} and \var{y}}{if \var{x} is false, then \var{x}, else \var{y}}{(1)}
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63 \hline
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64 \lineiii{not \var{x}}{if \var{x} is false, then \code{1}, else \code{0}}{(2)}
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65 \end{tableiii}
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66 \opindex{and}
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67 \opindex{or}
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68 \opindex{not}
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69
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70 \noindent
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71 Notes:
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72
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73 \begin{description}
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74
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75 \item[(1)]
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76 These only evaluate their second argument if needed for their outcome.
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77
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78 \item[(2)]
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79 \samp{not} has a lower priority than non-Boolean operators, so e.g.
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80 \code{not a == b} is interpreted as \code{not(a == b)}, and
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81 \code{a == not b} is a syntax error.
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82
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83 \end{description}
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84
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85 \subsection{Comparisons}
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86
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87 Comparison operations are supported by all objects. They all have the
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88 same priority (which is higher than that of the Boolean operations).
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89 Comparisons can be chained arbitrarily, e.g. \code{x < y <= z} is
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90 equivalent to \code{x < y and y <= z}, except that \code{y} is
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91 evaluated only once (but in both cases \code{z} is not evaluated at
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92 all when \code{x < y} is found to be false).
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93 \indexii{chaining}{comparisons}
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94
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95 This table summarizes the comparison operations:
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96
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97 \begin{tableiii}{|c|l|c|}{code}{Operation}{Meaning}{Notes}
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98 \lineiii{<}{strictly less than}{}
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99 \lineiii{<=}{less than or equal}{}
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100 \lineiii{>}{strictly greater than}{}
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101 \lineiii{>=}{greater than or equal}{}
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102 \lineiii{==}{equal}{}
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103 \lineiii{<>}{not equal}{(1)}
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104 \lineiii{!=}{not equal}{(1)}
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105 \lineiii{is}{object identity}{}
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106 \lineiii{is not}{negated object identity}{}
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107 \end{tableiii}
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108 \indexii{operator}{comparison}
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109 \opindex{==} % XXX *All* others have funny characters < ! >
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110 \opindex{is}
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111 \opindex{is not}
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112
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113 \noindent
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114 Notes:
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115
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116 \begin{description}
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117
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118 \item[(1)]
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119 \code{<>} and \code{!=} are alternate spellings for the same operator.
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120 (I couldn't choose between \ABC{} and \C{}! :-)
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121 \indexii{\ABC{}}{language}
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122 \indexii{\C{}}{language}
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123
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124 \end{description}
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125
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126 Objects of different types, except different numeric types, never
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127 compare equal; such objects are ordered consistently but arbitrarily
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128 (so that sorting a heterogeneous array yields a consistent result).
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129 Furthermore, some types (e.g., windows) support only a degenerate
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130 notion of comparison where any two objects of that type are unequal.
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131 Again, such objects are ordered arbitrarily but consistently.
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132 \indexii{types}{numeric}
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133 \indexii{objects}{comparing}
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134
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135 (Implementation note: objects of different types except numbers are
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136 ordered by their type names; objects of the same types that don't
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137 support proper comparison are ordered by their address.)
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138
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139 Two more operations with the same syntactic priority, \code{in} and
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140 \code{not in}, are supported only by sequence types (below).
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141 \opindex{in}
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142 \opindex{not in}
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143
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144 \subsection{Numeric Types}
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145
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146 There are four numeric types: \dfn{plain integers}, \dfn{long integers},
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147 \dfn{floating point numbers}, and \dfn{complex numbers}.
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148 Plain integers (also just called \dfn{integers})
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149 are implemented using \code{long} in \C{}, which gives them at least 32
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150 bits of precision. Long integers have unlimited precision. Floating
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151 point numbers are implemented using \code{double} in \C{}. All bets on
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152 their precision are off unless you happen to know the machine you are
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153 working with.
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154 \indexii{numeric}{types}
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155 \indexii{integer}{types}
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156 \indexii{integer}{type}
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157 \indexiii{long}{integer}{type}
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158 \indexii{floating point}{type}
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159 \indexii{complex number}{type}
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160 \indexii{\C{}}{language}
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161
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162 Complex numbers have a real and imaginary part, which are both
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163 implemented using \code{double} in \C{}. To extract these parts from
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164 a complex number \code{z}, use \code{z.real} and \code{z.imag}.
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165
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166 Numbers are created by numeric literals or as the result of built-in
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167 functions and operators. Unadorned integer literals (including hex
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168 and octal numbers) yield plain integers. Integer literals with an \samp{L}
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170 (\samp{L} is preferred because \code{1l} looks too much like eleven!).
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171 Numeric literals containing a decimal point or an exponent sign yield
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172 floating point numbers. Appending \code{j} or \code{J} to a numeric
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173 literal yields a complex number.
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174 \indexii{numeric}{literals}
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175 \indexii{integer}{literals}
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176 \indexiii{long}{integer}{literals}
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177 \indexii{floating point}{literals}
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178 \indexii{complex number}{literals}
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179 \indexii{hexadecimal}{literals}
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180 \indexii{octal}{literals}
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181
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182 Python fully supports mixed arithmetic: when a binary arithmetic
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183 operator has operands of different numeric types, the operand with the
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184 ``smaller'' type is converted to that of the other, where plain
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185 integer is smaller than long integer is smaller than floating point is
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186 smaller than complex.
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187 Comparisons between numbers of mixed type use the same rule.%
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188 \footnote{As a consequence, the list \code{[1, 2]} is considered equal
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189 to \code{[1.0, 2.0]}, and similar for tuples.}
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190 The functions \code{int()}, \code{long()}, \code{float()},
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191 and \code{complex()} can be used
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192 to coerce numbers to a specific type.
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193 \index{arithmetic}
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194 \bifuncindex{int}
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195 \bifuncindex{long}
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196 \bifuncindex{float}
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197 \bifuncindex{complex}
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198
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199 All numeric types support the following operations, sorted by
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200 ascending priority (operations in the same box have the same
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201 priority; all numeric operations have a higher priority than
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202 comparison operations):
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203
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204 \begin{tableiii}{|c|l|c|}{code}{Operation}{Result}{Notes}
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205 \lineiii{\var{x} + \var{y}}{sum of \var{x} and \var{y}}{}
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206 \lineiii{\var{x} - \var{y}}{difference of \var{x} and \var{y}}{}
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207 \hline
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208 \lineiii{\var{x} * \var{y}}{product of \var{x} and \var{y}}{}
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209 \lineiii{\var{x} / \var{y}}{quotient of \var{x} and \var{y}}{(1)}
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210 \lineiii{\var{x} \%{} \var{y}}{remainder of \code{\var{x} / \var{y}}}{}
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211 \hline
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212 \lineiii{-\var{x}}{\var{x} negated}{}
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213 \lineiii{+\var{x}}{\var{x} unchanged}{}
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214 \hline
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215 \lineiii{abs(\var{x})}{absolute value or magnitude of \var{x}}{}
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216 \lineiii{int(\var{x})}{\var{x} converted to integer}{(2)}
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217 \lineiii{long(\var{x})}{\var{x} converted to long integer}{(2)}
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218 \lineiii{float(\var{x})}{\var{x} converted to floating point}{}
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219 \lineiii{complex(\var{re},\var{im})}{a complex number with real part \var{re}, imaginary part \var{im}. \var{im} defaults to zero.}{}
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220 \lineiii{divmod(\var{x}, \var{y})}{the pair \code{(\var{x} / \var{y}, \var{x} \%{} \var{y})}}{(3)}
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221 \lineiii{pow(\var{x}, \var{y})}{\var{x} to the power \var{y}}{}
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222 \lineiii{\var{x}**\var{y}}{\var{x} to the power \var{y}}{}
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223 \end{tableiii}
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224 \indexiii{operations on}{numeric}{types}
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225
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226 \noindent
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227 Notes:
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228 \begin{description}
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229
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230 \item[(1)]
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231 For (plain or long) integer division, the result is an integer.
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232 The result is always rounded towards minus infinity: 1/2 is 0,
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233 (-1)/2 is -1, 1/(-2) is -1, and (-1)/(-2) is 0.
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234 \indexii{integer}{division}
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235 \indexiii{long}{integer}{division}
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236
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237 \item[(2)]
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238 Conversion from floating point to (long or plain) integer may round or
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239 truncate as in \C{}; see functions \code{floor()} and \code{ceil()} in
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240 module \code{math} for well-defined conversions.
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241 \bifuncindex{floor}
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242 \bifuncindex{ceil}
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243 \indexii{numeric}{conversions}
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244 \stmodindex{math}
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245 \indexii{\C{}}{language}
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246
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247 \item[(3)]
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248 See the section on built-in functions for an exact definition.
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249
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250 \end{description}
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251 % XXXJH exceptions: overflow (when? what operations?) zerodivision
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252
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253 \subsubsection{Bit-string Operations on Integer Types}
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254 \nodename{Bit-string Operations}
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255
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256 Plain and long integer types support additional operations that make
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257 sense only for bit-strings. Negative numbers are treated as their 2's
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258 complement value (for long integers, this assumes a sufficiently large
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259 number of bits that no overflow occurs during the operation).
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260
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261 The priorities of the binary bit-wise operations are all lower than
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262 the numeric operations and higher than the comparisons; the unary
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263 operation \samp{~} has the same priority as the other unary numeric
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264 operations (\samp{+} and \samp{-}).
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265
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266 This table lists the bit-string operations sorted in ascending
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267 priority (operations in the same box have the same priority):
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269 \begin{tableiii}{|c|l|c|}{code}{Operation}{Result}{Notes}
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270 \lineiii{\var{x} | \var{y}}{bitwise \dfn{or} of \var{x} and \var{y}}{}
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271 \hline
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272 \lineiii{\var{x} \^{} \var{y}}{bitwise \dfn{exclusive or} of \var{x} and \var{y}}{}
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273 \hline
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274 \lineiii{\var{x} \&{} \var{y}}{bitwise \dfn{and} of \var{x} and \var{y}}{}
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275 \hline
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276 \lineiii{\var{x} << \var{n}}{\var{x} shifted left by \var{n} bits}{(1), (2)}
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277 \lineiii{\var{x} >> \var{n}}{\var{x} shifted right by \var{n} bits}{(1), (3)}
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278 \hline
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279 \hline
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280 \lineiii{\~\var{x}}{the bits of \var{x} inverted}{}
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281 \end{tableiii}
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282 \indexiii{operations on}{integer}{types}
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283 \indexii{bit-string}{operations}
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284 \indexii{shifting}{operations}
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285 \indexii{masking}{operations}
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286
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287 \noindent
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288 Notes:
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289 \begin{description}
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290 \item[(1)] Negative shift counts are illegal.
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291 \item[(2)] A left shift by \var{n} bits is equivalent to
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292 multiplication by \code{pow(2, \var{n})} without overflow check.
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293 \item[(3)] A right shift by \var{n} bits is equivalent to
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294 division by \code{pow(2, \var{n})} without overflow check.
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295 \end{description}
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296
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297 \subsection{Sequence Types}
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298
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299 There are three sequence types: strings, lists and tuples.
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300
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301 Strings literals are written in single or double quotes:
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302 \code{'xyzzy'}, \code{"frobozz"}. See Chapter 2 of the Python
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303 Reference Manual for more about string literals. Lists are
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304 constructed with square brackets, separating items with commas:
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305 \code{[a, b, c]}. Tuples are constructed by the comma operator (not
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306 within square brackets), with or without enclosing parentheses, but an
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307 empty tuple must have the enclosing parentheses, e.g.,
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308 \code{a, b, c} or \code{()}. A single item tuple must have a trailing
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309 comma, e.g., \code{(d,)}.
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310 \indexii{sequence}{types}
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311 \indexii{string}{type}
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312 \indexii{tuple}{type}
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313 \indexii{list}{type}
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314
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315 Sequence types support the following operations. The \samp{in} and
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316 \samp{not\,in} operations have the same priorities as the comparison
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317 operations. The \samp{+} and \samp{*} operations have the same
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318 priority as the corresponding numeric operations.\footnote{They must
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319 have since the parser can't tell the type of the operands.}
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320
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321 This table lists the sequence operations sorted in ascending priority
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322 (operations in the same box have the same priority). In the table,
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323 \var{s} and \var{t} are sequences of the same type; \var{n}, \var{i}
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324 and \var{j} are integers:
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325
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326 \begin{tableiii}{|c|l|c|}{code}{Operation}{Result}{Notes}
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327 \lineiii{\var{x} in \var{s}}{\code{1} if an item of \var{s} is equal to \var{x}, else \code{0}}{}
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328 \lineiii{\var{x} not in \var{s}}{\code{0} if an item of \var{s} is
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329 equal to \var{x}, else \code{1}}{}
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330 \hline
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331 \lineiii{\var{s} + \var{t}}{the concatenation of \var{s} and \var{t}}{}
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332 \hline
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333 \lineiii{\var{s} * \var{n}{\rm ,} \var{n} * \var{s}}{\var{n} copies of \var{s} concatenated}{}
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334 \hline
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335 \lineiii{\var{s}[\var{i}]}{\var{i}'th item of \var{s}, origin 0}{(1)}
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336 \lineiii{\var{s}[\var{i}:\var{j}]}{slice of \var{s} from \var{i} to \var{j}}{(1), (2)}
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337 \hline
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338 \lineiii{len(\var{s})}{length of \var{s}}{}
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339 \lineiii{min(\var{s})}{smallest item of \var{s}}{}
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340 \lineiii{max(\var{s})}{largest item of \var{s}}{}
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341 \end{tableiii}
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342 \indexiii{operations on}{sequence}{types}
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343 \bifuncindex{len}
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344 \bifuncindex{min}
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345 \bifuncindex{max}
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346 \indexii{concatenation}{operation}
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347 \indexii{repetition}{operation}
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348 \indexii{subscript}{operation}
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349 \indexii{slice}{operation}
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350 \opindex{in}
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351 \opindex{not in}
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352
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353 \noindent
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354 Notes:
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355
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356 \begin{description}
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357
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358 \item[(1)] If \var{i} or \var{j} is negative, the index is relative to
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359 the end of the string, i.e., \code{len(\var{s}) + \var{i}} or
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360 \code{len(\var{s}) + \var{j}} is substituted. But note that \code{-0} is
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361 still \code{0}.
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362
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363 \item[(2)] The slice of \var{s} from \var{i} to \var{j} is defined as
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364 the sequence of items with index \var{k} such that \code{\var{i} <=
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365 \var{k} < \var{j}}. If \var{i} or \var{j} is greater than
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366 \code{len(\var{s})}, use \code{len(\var{s})}. If \var{i} is omitted,
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367 use \code{0}. If \var{j} is omitted, use \code{len(\var{s})}. If
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368 \var{i} is greater than or equal to \var{j}, the slice is empty.
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369
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370 \end{description}
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371
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372 \subsubsection{More String Operations}
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373
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374 String objects have one unique built-in operation: the \code{\%}
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375 operator (modulo) with a string left argument interprets this string
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376 as a C sprintf format string to be applied to the right argument, and
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377 returns the string resulting from this formatting operation.
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378
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379 The right argument should be a tuple with one item for each argument
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380 required by the format string; if the string requires a single
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381 argument, the right argument may also be a single non-tuple object.%
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382 \footnote{A tuple object in this case should be a singleton.}
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383 The following format characters are understood:
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384 \%, c, s, i, d, u, o, x, X, e, E, f, g, G.
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385 Width and precision may be a * to specify that an integer argument
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386 specifies the actual width or precision. The flag characters -, +,
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387 blank, \# and 0 are understood. The size specifiers h, l or L may be
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388 present but are ignored. The \code{\%s} conversion takes any Python
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389 object and converts it to a string using \code{str()} before
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390 formatting it. The ANSI features \code{\%p} and \code{\%n}
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391 are not supported. Since Python strings have an explicit length,
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392 \code{\%s} conversions don't assume that \code{'\e0'} is the end of
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393 the string.
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394
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395 For safety reasons, floating point precisions are clipped to 50;
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396 \code{\%f} conversions for numbers whose absolute value is over 1e25
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397 are replaced by \code{\%g} conversions.%
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398 \footnote{These numbers are fairly arbitrary. They are intended to
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399 avoid printing endless strings of meaningless digits without hampering
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400 correct use and without having to know the exact precision of floating
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401 point values on a particular machine.}
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402 All other errors raise exceptions.
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403
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404 If the right argument is a dictionary (or any kind of mapping), then
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405 the formats in the string must have a parenthesized key into that
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406 dictionary inserted immediately after the \code{\%} character, and
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407 each format formats the corresponding entry from the mapping. E.g.
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408 \bcode\begin{verbatim}
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409 >>> count = 2
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410 >>> language = 'Python'
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411 >>> print '%(language)s has %(count)03d quote types.' % vars()
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412 Python has 002 quote types.
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413 >>>
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414 \end{verbatim}\ecode
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415 In this case no * specifiers may occur in a format (since they
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416 require a sequential parameter list).
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417
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418 Additional string operations are defined in standard module
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419 \code{string} and in built-in module \code{regex}.
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420 \index{string}
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421 \index{regex}
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422
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423 \subsubsection{Mutable Sequence Types}
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424
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425 List objects support additional operations that allow in-place
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426 modification of the object.
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427 These operations would be supported by other mutable sequence types
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428 (when added to the language) as well.
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429 Strings and tuples are immutable sequence types and such objects cannot
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430 be modified once created.
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431 The following operations are defined on mutable sequence types (where
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432 \var{x} is an arbitrary object):
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433 \indexiii{mutable}{sequence}{types}
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434 \indexii{list}{type}
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435
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436 \begin{tableiii}{|c|l|c|}{code}{Operation}{Result}{Notes}
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437 \lineiii{\var{s}[\var{i}] = \var{x}}
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438 {item \var{i} of \var{s} is replaced by \var{x}}{}
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439 \lineiii{\var{s}[\var{i}:\var{j}] = \var{t}}
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440 {slice of \var{s} from \var{i} to \var{j} is replaced by \var{t}}{}
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441 \lineiii{del \var{s}[\var{i}:\var{j}]}
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442 {same as \code{\var{s}[\var{i}:\var{j}] = []}}{}
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443 \lineiii{\var{s}.append(\var{x})}
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444 {same as \code{\var{s}[len(\var{s}):len(\var{s})] = [\var{x}]}}{}
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445 \lineiii{\var{s}.count(\var{x})}
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446 {return number of \var{i}'s for which \code{\var{s}[\var{i}] == \var{x}}}{}
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447 \lineiii{\var{s}.index(\var{x})}
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448 {return smallest \var{i} such that \code{\var{s}[\var{i}] == \var{x}}}{(1)}
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449 \lineiii{\var{s}.insert(\var{i}, \var{x})}
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450 {same as \code{\var{s}[\var{i}:\var{i}] = [\var{x}]}
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451 if \code{\var{i} >= 0}}{}
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452 \lineiii{\var{s}.remove(\var{x})}
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453 {same as \code{del \var{s}[\var{s}.index(\var{x})]}}{(1)}
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454 \lineiii{\var{s}.reverse()}
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455 {reverses the items of \var{s} in place}{(3)}
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456 \lineiii{\var{s}.sort()}
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457 {sort the items of \var{s} in place}{(2), (3)}
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458 \end{tableiii}
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459 \indexiv{operations on}{mutable}{sequence}{types}
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460 \indexiii{operations on}{sequence}{types}
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461 \indexiii{operations on}{list}{type}
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462 \indexii{subscript}{assignment}
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463 \indexii{slice}{assignment}
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464 \stindex{del}
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465 \renewcommand{\indexsubitem}{(list method)}
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466 \ttindex{append}
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467 \ttindex{count}
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468 \ttindex{index}
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469 \ttindex{insert}
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470 \ttindex{remove}
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471 \ttindex{reverse}
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472 \ttindex{sort}
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473
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474 \noindent
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475 Notes:
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476 \begin{description}
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477 \item[(1)] Raises an exception when \var{x} is not found in \var{s}.
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478
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479 \item[(2)] The \code{sort()} method takes an optional argument
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480 specifying a comparison function of two arguments (list items) which
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481 should return \code{-1}, \code{0} or \code{1} depending on whether the
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482 first argument is considered smaller than, equal to, or larger than the
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483 second argument. Note that this slows the sorting process down
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484 considerably; e.g. to sort a list in reverse order it is much faster
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485 to use calls to \code{sort()} and \code{reverse()} than to use
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486 \code{sort()} with a comparison function that reverses the ordering of
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487 the elements.
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488
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489 \item[(3)] The \code{sort()} and \code{reverse()} methods modify the
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490 list in place for economy of space when sorting or reversing a large
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491 list. They don't return the sorted or reversed list to remind you of
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492 this side effect.
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493
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494 \end{description}
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495
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496 \subsection{Mapping Types}
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497
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498 A \dfn{mapping} object maps values of one type (the key type) to
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499 arbitrary objects. Mappings are mutable objects. There is currently
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500 only one standard mapping type, the \dfn{dictionary}. A dictionary's keys are
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501 almost arbitrary values. The only types of values not acceptable as
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502 keys are values containing lists or dictionaries or other mutable
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503 types that are compared by value rather than by object identity.
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504 Numeric types used for keys obey the normal rules for numeric
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505 comparison: if two numbers compare equal (e.g. 1 and 1.0) then they
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506 can be used interchangeably to index the same dictionary entry.
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507
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508 \indexii{mapping}{types}
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509 \indexii{dictionary}{type}
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510
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511 Dictionaries are created by placing a comma-separated list of
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512 \code{\var{key}:\,\var{value}} pairs within braces, for example:
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513 \code{\{'jack':\,4098, 'sjoerd':\,4127\}} or
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514 \code{\{4098:\,'jack', 4127:\,'sjoerd'\}}.
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515
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516 The following operations are defined on mappings (where \var{a} is a
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517 mapping, \var{k} is a key and \var{x} is an arbitrary object):
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518
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519 \begin{tableiii}{|c|l|c|}{code}{Operation}{Result}{Notes}
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520 \lineiii{len(\var{a})}{the number of items in \var{a}}{}
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521 \lineiii{\var{a}[\var{k}]}{the item of \var{a} with key \var{k}}{(1)}
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522 \lineiii{\var{a}[\var{k}] = \var{x}}{set \code{\var{a}[\var{k}]} to \var{x}}{}
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523 \lineiii{del \var{a}[\var{k}]}{remove \code{\var{a}[\var{k}]} from \var{a}}{(1)}
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524 \lineiii{\var{a}.clear()}{remove all items from \code{a}}{}
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525 \lineiii{\var{a}.copy()}{a (shallow) copy of \code{a}}{}
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526 \lineiii{\var{a}.has_key(\var{k})}{\code{1} if \var{a} has a key \var{k}, else \code{0}}{}
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527 \lineiii{\var{a}.items()}{a copy of \var{a}'s list of (key, item) pairs}{(2)}
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528 \lineiii{\var{a}.keys()}{a copy of \var{a}'s list of keys}{(2)}
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529 \lineiii{\var{a}.update(b)}{\code{for k, v in b.items(): a[k] = v}}{(3)}
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530 \lineiii{\var{a}.values()}{a copy of \var{a}'s list of values}{(2)}
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531 \end{tableiii}
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532 \indexiii{operations on}{mapping}{types}
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533 \indexiii{operations on}{dictionary}{type}
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534 \stindex{del}
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535 \bifuncindex{len}
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536 \renewcommand{\indexsubitem}{(dictionary method)}
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537 \ttindex{keys}
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538 \ttindex{has_key}
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539
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540 \noindent
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541 Notes:
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542 \begin{description}
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543 \item[(1)] Raises an exception if \var{k} is not in the map.
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544
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545 \item[(2)] Keys and values are listed in random order.
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546
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547 \item[(3)] \code{b} must be of the same type as \code{a}.
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548 \end{description}
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549
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550 \subsection{Other Built-in Types}
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551
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552 The interpreter supports several other kinds of objects.
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553 Most of these support only one or two operations.
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554
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555 \subsubsection{Modules}
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556
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557 The only special operation on a module is attribute access:
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558 \code{\var{m}.\var{name}}, where \var{m} is a module and \var{name} accesses
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559 a name defined in \var{m}'s symbol table. Module attributes can be
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560 assigned to. (Note that the \code{import} statement is not, strictly
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561 spoken, an operation on a module object; \code{import \var{foo}} does not
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562 require a module object named \var{foo} to exist, rather it requires
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563 an (external) \emph{definition} for a module named \var{foo}
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564 somewhere.)
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565
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566 A special member of every module is \code{__dict__}.
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567 This is the dictionary containing the module's symbol table.
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568 Modifying this dictionary will actually change the module's symbol
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569 table, but direct assignment to the \code{__dict__} attribute is not
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570 possible (i.e., you can write \code{\var{m}.__dict__['a'] = 1}, which
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571 defines \code{\var{m}.a} to be \code{1}, but you can't write \code{\var{m}.__dict__ = \{\}}.
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572
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573 Modules are written like this: \code{<module 'sys'>}.
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574
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575 \subsubsection{Classes and Class Instances}
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576 \nodename{Classes and Instances}
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577
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578 (See Chapters 3 and 7 of the Python Reference Manual for these.)
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579
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580 \subsubsection{Functions}
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581
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582 Function objects are created by function definitions. The only
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583 operation on a function object is to call it:
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584 \code{\var{func}(\var{argument-list})}.
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585
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586 There are really two flavors of function objects: built-in functions
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587 and user-defined functions. Both support the same operation (to call
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588 the function), but the implementation is different, hence the
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589 different object types.
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590
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591 The implementation adds two special read-only attributes:
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592 \code{\var{f}.func_code} is a function's \dfn{code object} (see below) and
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593 \code{\var{f}.func_globals} is the dictionary used as the function's
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594 global name space (this is the same as \code{\var{m}.__dict__} where
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595 \var{m} is the module in which the function \var{f} was defined).
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596
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597 \subsubsection{Methods}
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598 \obindex{method}
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599
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600 Methods are functions that are called using the attribute notation.
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601 There are two flavors: built-in methods (such as \code{append()} on
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602 lists) and class instance methods. Built-in methods are described
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603 with the types that support them.
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604
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605 The implementation adds two special read-only attributes to class
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606 instance methods: \code{\var{m}.im_self} is the object whose method this
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607 is, and \code{\var{m}.im_func} is the function implementing the method.
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608 Calling \code{\var{m}(\var{arg-1}, \var{arg-2}, {\rm \ldots},
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609 \var{arg-n})} is completely equivalent to calling
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610 \code{\var{m}.im_func(\var{m}.im_self, \var{arg-1}, \var{arg-2}, {\rm
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611 \ldots}, \var{arg-n})}.
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612
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613 (See the Python Reference Manual for more info.)
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614
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615 \subsubsection{Code Objects}
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616 \obindex{code}
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617
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618 Code objects are used by the implementation to represent
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619 ``pseudo-compiled'' executable Python code such as a function body.
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620 They differ from function objects because they don't contain a
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621 reference to their global execution environment. Code objects are
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622 returned by the built-in \code{compile()} function and can be
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623 extracted from function objects through their \code{func_code}
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624 attribute.
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625 \bifuncindex{compile}
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626 \ttindex{func_code}
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627
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628 A code object can be executed or evaluated by passing it (instead of a
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629 source string) to the \code{exec} statement or the built-in
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630 \code{eval()} function.
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631 \stindex{exec}
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632 \bifuncindex{eval}
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633
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634 (See the Python Reference Manual for more info.)
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635
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636 \subsubsection{Type Objects}
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637
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638 Type objects represent the various object types. An object's type is
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639 accessed by the built-in function \code{type()}. There are no special
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640 operations on types. The standard module \code{types} defines names
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641 for all standard built-in types.
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642 \bifuncindex{type}
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643 \stmodindex{types}
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644
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645 Types are written like this: \code{<type 'int'>}.
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646
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647 \subsubsection{The Null Object}
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648
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649 This object is returned by functions that don't explicitly return a
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650 value. It supports no special operations. There is exactly one null
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651 object, named \code{None} (a built-in name).
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652
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653 It is written as \code{None}.
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654
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655 \subsubsection{File Objects}
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656
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657 File objects are implemented using \C{}'s \code{stdio} package and can be
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658 created with the built-in function \code{open()} described under
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659 Built-in Functions below. They are also returned by some other
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660 built-in functions and methods, e.g.\ \code{posix.popen()} and
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661 \code{posix.fdopen()} and the \code{makefile()} method of socket
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662 objects.
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663 \bifuncindex{open}
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664
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665 When a file operation fails for an I/O-related reason, the exception
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666 \code{IOError} is raised. This includes situations where the
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667 operation is not defined for some reason, like \code{seek()} on a tty
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668 device or writing a file opened for reading.
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669
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670 Files have the following methods:
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671
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672
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673 \renewcommand{\indexsubitem}{(file method)}
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674
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675 \begin{funcdesc}{close}{}
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676 Close the file. A closed file cannot be read or written anymore.
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677 \end{funcdesc}
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678
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679 \begin{funcdesc}{flush}{}
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680 Flush the internal buffer, like \code{stdio}'s \code{fflush()}.
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681 \end{funcdesc}
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682
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683 \begin{funcdesc}{isatty}{}
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684 Return \code{1} if the file is connected to a tty(-like) device, else
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685 \code{0}.
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686 \end{funcdesc}
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687
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688 \begin{funcdesc}{fileno}{}
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689 Return the integer ``file descriptor'' that is used by the underlying
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690 implementation to request I/O operations from the operating system.
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691 This can be useful for other, lower level interfaces that use file
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692 descriptors, e.g. module \code{fcntl} or \code{os.read} and friends.
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693 \end{funcdesc}
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694
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695 \begin{funcdesc}{read}{\optional{size}}
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696 Read at most \var{size} bytes from the file (less if the read hits
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697 \EOF{} or no more data is immediately available on a pipe, tty or
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698 similar device). If the \var{size} argument is negative or omitted,
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699 read all data until \EOF{} is reached. The bytes are returned as a string
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700 object. An empty string is returned when \EOF{} is encountered
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701 immediately. (For certain files, like ttys, it makes sense to
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702 continue reading after an \EOF{} is hit.)
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703 \end{funcdesc}
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704
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705 \begin{funcdesc}{readline}{\optional{size}}
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706 Read one entire line from the file. A trailing newline character is
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707 kept in the string%
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708 \footnote{The advantage of leaving the newline on is that an empty string
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709 can be returned to mean \EOF{} without being ambiguous. Another
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710 advantage is that (in cases where it might matter, e.g. if you
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711 want to make an exact copy of a file while scanning its lines)
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712 you can tell whether the last line of a file ended in a newline
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713 or not (yes this happens!).}
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714 (but may be absent when a file ends with an
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715 incomplete line). If the \var{size} argument is present and
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716 non-negative, it is a maximum byte count (including the trailing
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diff changeset
717 newline) and an incomplete line may be returned.
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718 An empty string is returned when \EOF{} is hit
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719 immediately. Note: unlike \code{stdio}'s \code{fgets()}, the returned
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720 string contains null characters (\code{'\e 0'}) if they occurred in the
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721 input.
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722 \end{funcdesc}
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723
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724 \begin{funcdesc}{readlines}{\optional{sizehint}}
1471
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725 Read until \EOF{} using \code{readline()} and return a list containing
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726 the lines thus read. If the optional \var{bufferhint} argument is
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diff changeset
727 present, instead of reading up to \EOF{}, whole lines totalling
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728 approximately \var{sizehint} bytes are read.
1471
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729 \end{funcdesc}
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730
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731 \begin{funcdesc}{seek}{offset\, whence}
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732 Set the file's current position, like \code{stdio}'s \code{fseek()}.
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733 The \var{whence} argument is optional and defaults to \code{0}
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734 (absolute file positioning); other values are \code{1} (seek
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735 relative to the current position) and \code{2} (seek relative to the
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736 file's end). There is no return value.
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737 \end{funcdesc}
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738
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739 \begin{funcdesc}{tell}{}
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740 Return the file's current position, like \code{stdio}'s \code{ftell()}.
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741 \end{funcdesc}
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742
3543
22d7f54c5792 Removed some spurios biindex calls.
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743 \begin{funcdesc}{truncate}{\optional{size}}
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744 Truncate the file's size. If the optional size argument present, the
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745 file is truncated to (at most) that size. The size defaults to the
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746 current position. Availability of this function depends on the
4401
d16b8621a3f0 Typos detected by Sjoerd
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diff changeset
747 operating system version (e.g., not all \UNIX{} versions support this
3543
22d7f54c5792 Removed some spurios biindex calls.
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748 operation).
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749 \end{funcdesc}
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750
1471
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751 \begin{funcdesc}{write}{str}
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752 Write a string to the file. There is no return value. Note: due to
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753 buffering, the string may not actually show up in the file until
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754 the \code{flush()} or \code{close()} method is called.
1471
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755 \end{funcdesc}
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756
1683
080811b1d023 documented writelines()
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757 \begin{funcdesc}{writelines}{list}
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758 Write a list of strings to the file. There is no return value.
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759 (The name is intended to match \code{readlines}; \code{writelines}
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760 does not add line separators.)
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761 \end{funcdesc}
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762
5808
dffd22146aab Added fileno(); added readlines(sizehint).
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763 Classes that are trying to simulate a file object should also have a
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764 writable \code{softspace} attribute, which should be initialized to
dffd22146aab Added fileno(); added readlines(sizehint).
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765 zero. (\code{softspace} is used by the \code{print} statement.) This
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766 will be automatic for classes implemented in Python; types implemented
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767 in C will have to provide a writable \code{softspace} attribute.
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768
2576
f47bc5554418 mass changes; fix titles; add examples; correct typos; clarifications;
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769 \subsubsection{Internal Objects}
1471
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770
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771 (See the Python Reference Manual for these.)
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772
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773 \subsection{Special Attributes}
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774
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775 The implementation adds a few special read-only attributes to several
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776 object types, where they are relevant:
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777
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778 \begin{itemize}
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779
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780 \item
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781 \code{\var{x}.__dict__} is a dictionary of some sort used to store an
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782 object's (writable) attributes;
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783
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784 \item
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785 \code{\var{x}.__methods__} lists the methods of many built-in object types,
2609
7ef72873e4d6 restructured library manual accordiung to functional group
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diff changeset
786 e.g., \code{[].__methods__} yields
1471
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787 \code{['append', 'count', 'index', 'insert', 'remove', 'reverse', 'sort']};
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788
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789 \item
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790 \code{\var{x}.__members__} lists data attributes;
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791
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792 \item
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793 \code{\var{x}.__class__} is the class to which a class instance belongs;
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794
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795 \item
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796 \code{\var{x}.__bases__} is the tuple of base classes of a class object.
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797
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798 \end{itemize}