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Added safeguard against failure in __del__.
author Guido van Rossum <guido@python.org>
date Wed, 27 Nov 1996 19:39:58 +0000
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1 \section{Built-in Functions}
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2
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3 The Python interpreter has a number of functions built into it that
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4 are always available. They are listed here in alphabetical order.
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5
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6
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7 \renewcommand{\indexsubitem}{(built-in function)}
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8 \begin{funcdesc}{abs}{x}
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9 Return the absolute value of a number. The argument may be a plain
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10 or long integer or a floating point number.
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11 \end{funcdesc}
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12
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13 \begin{funcdesc}{apply}{function\, args\optional{, keywords}}
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14 The \var{function} argument must be a callable object (a user-defined or
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15 built-in function or method, or a class object) and the \var{args}
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16 argument must be a tuple. The \var{function} is called with
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17 \var{args} as argument list; the number of arguments is the the length
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18 of the tuple. (This is different from just calling
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19 \code{\var{func}(\var{args})}, since in that case there is always
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20 exactly one argument.)
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21 If the optional \var{keywords} argument is present, it must be a
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22 dictionary whose keys are strings. It specifies keyword arguments to
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23 be added to the end of the the argument list.
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24 \end{funcdesc}
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25
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26 \begin{funcdesc}{chr}{i}
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27 Return a string of one character whose \ASCII{} code is the integer
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28 \var{i}, e.g., \code{chr(97)} returns the string \code{'a'}. This is the
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29 inverse of \code{ord()}. The argument must be in the range [0..255],
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30 inclusive.
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31 \end{funcdesc}
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32
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33 \begin{funcdesc}{cmp}{x\, y}
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34 Compare the two objects \var{x} and \var{y} and return an integer
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35 according to the outcome. The return value is negative if \code{\var{x}
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36 < \var{y}}, zero if \code{\var{x} == \var{y}} and strictly positive if
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37 \code{\var{x} > \var{y}}.
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38 \end{funcdesc}
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39
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40 \begin{funcdesc}{coerce}{x\, y}
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41 Return a tuple consisting of the two numeric arguments converted to
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42 a common type, using the same rules as used by arithmetic
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43 operations.
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44 \end{funcdesc}
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45
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46 \begin{funcdesc}{compile}{string\, filename\, kind}
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47 Compile the \var{string} into a code object. Code objects can be
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48 executed by an \code{exec} statement or evaluated by a call to
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49 \code{eval()}. The \var{filename} argument should
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50 give the file from which the code was read; pass e.g. \code{'<string>'}
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51 if it wasn't read from a file. The \var{kind} argument specifies
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52 what kind of code must be compiled; it can be \code{'exec'} if
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53 \var{string} consists of a sequence of statements, \code{'eval'}
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54 if it consists of a single expression, or \code{'single'} if
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55 it consists of a single interactive statement (in the latter case,
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56 expression statements that evaluate to something else than
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57 \code{None} will printed).
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58 \end{funcdesc}
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59
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60 \begin{funcdesc}{delattr}{object\, name}
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61 This is a relative of \code{setattr}. The arguments are an
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62 object and a string. The string must be the name
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63 of one of the object's attributes. The function deletes
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64 the named attribute, provided the object allows it. For example,
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65 \code{delattr(\var{x}, '\var{foobar}')} is equivalent to
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66 \code{del \var{x}.\var{foobar}}.
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67 \end{funcdesc}
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68
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69 \begin{funcdesc}{dir}{}
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70 Without arguments, return the list of names in the current local
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71 symbol table. With a module, class or class instance object as
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72 argument (or anything else that has a \code{__dict__} attribute),
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73 returns the list of names in that object's attribute dictionary.
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74 The resulting list is sorted. For example:
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75
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76 \bcode\begin{verbatim}
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77 >>> import sys
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78 >>> dir()
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79 ['sys']
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80 >>> dir(sys)
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81 ['argv', 'exit', 'modules', 'path', 'stderr', 'stdin', 'stdout']
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82 >>>
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83 \end{verbatim}\ecode
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84 \end{funcdesc}
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85
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86 \begin{funcdesc}{divmod}{a\, b}
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87 Take two numbers as arguments and return a pair of integers
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88 consisting of their integer quotient and remainder. With mixed
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89 operand types, the rules for binary arithmetic operators apply. For
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90 plain and long integers, the result is the same as
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91 \code{(\var{a} / \var{b}, \var{a} \%{} \var{b})}.
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92 For floating point numbers the result is the same as
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93 \code{(math.floor(\var{a} / \var{b}), \var{a} \%{} \var{b})}.
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94 \end{funcdesc}
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95
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96 \begin{funcdesc}{eval}{expression\optional{\, globals\optional{\, locals}}}
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97 The arguments are a string and two optional dictionaries. The
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98 \var{expression} argument is parsed and evaluated as a Python
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99 expression (technically speaking, a condition list) using the
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100 \var{globals} and \var{locals} dictionaries as global and local name
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101 space. If the \var{locals} dictionary is omitted it defaults to
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102 the \var{globals} dictionary. If both dictionaries are omitted, the
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103 expression is executed in the environment where \code{eval} is
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104 called. The return value is the result of the evaluated expression.
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105 Syntax errors are reported as exceptions. Example:
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106
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107 \bcode\begin{verbatim}
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108 >>> x = 1
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109 >>> print eval('x+1')
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110 2
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111 >>>
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112 \end{verbatim}\ecode
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113
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114 This function can also be used to execute arbitrary code objects
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115 (e.g.\ created by \code{compile()}). In this case pass a code
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116 object instead of a string. The code object must have been compiled
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117 passing \code{'eval'} to the \var{kind} argument.
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118
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119 Hints: dynamic execution of statements is supported by the
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120 \code{exec} statement. Execution of statements from a file is
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121 supported by the \code{execfile()} function. The \code{globals()}
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122 and \code{locals()} functions returns the current global and local
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123 dictionary, respectively, which may be useful
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124 to pass around for use by \code{eval()} or \code{execfile()}.
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125
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126 \end{funcdesc}
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127
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128 \begin{funcdesc}{execfile}{file\optional{\, globals\optional{\, locals}}}
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129 This function is similar to the
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130 \code{exec} statement, but parses a file instead of a string. It is
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131 different from the \code{import} statement in that it does not use
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132 the module administration --- it reads the file unconditionally and
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133 does not create a new module.\footnote{It is used relatively rarely
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134 so does not warrant being made into a statement.}
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135
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136 The arguments are a file name and two optional dictionaries. The
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137 file is parsed and evaluated as a sequence of Python statements
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138 (similarly to a module) using the \var{globals} and \var{locals}
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139 dictionaries as global and local name space. If the \var{locals}
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140 dictionary is omitted it defaults to the \var{globals} dictionary.
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141 If both dictionaries are omitted, the expression is executed in the
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142 environment where \code{execfile()} is called. The return value is
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143 \code{None}.
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144 \end{funcdesc}
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145
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146 \begin{funcdesc}{filter}{function\, list}
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148 \var{function} returns true. If \var{list} is a string or a tuple,
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149 the result also has that type; otherwise it is always a list. If
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150 \var{function} is \code{None}, the identity function is assumed,
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152 removed.
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153 \end{funcdesc}
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154
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155 \begin{funcdesc}{float}{x}
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156 Convert a number to floating point. The argument may be a plain or
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157 long integer or a floating point number.
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158 \end{funcdesc}
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159
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160 \begin{funcdesc}{getattr}{object\, name}
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161 The arguments are an object and a string. The string must be the
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162 name
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163 of one of the object's attributes. The result is the value of that
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165 \code{\var{x}.\var{foobar}}.
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166 \end{funcdesc}
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167
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168 \begin{funcdesc}{globals}{}
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169 Return a dictionary representing the current global symbol table.
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170 This is always the dictionary of the current module (inside a
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172 module from which it is called).
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173 \end{funcdesc}
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174
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175 \begin{funcdesc}{hasattr}{object\, name}
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176 The arguments are an object and a string. The result is 1 if the
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177 string is the name of one of the object's attributes, 0 if not.
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178 (This is implemented by calling \code{getattr(object, name)} and
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179 seeing whether it raises an exception or not.)
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180 \end{funcdesc}
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181
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182 \begin{funcdesc}{hash}{object}
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183 Return the hash value of the object (if it has one). Hash values
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184 are 32-bit integers. They are used to quickly compare dictionary
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185 keys during a dictionary lookup. Numeric values that compare equal
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186 have the same hash value (even if they are of different types, e.g.
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187 1 and 1.0).
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188 \end{funcdesc}
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189
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190 \begin{funcdesc}{hex}{x}
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191 Convert an integer number (of any size) to a hexadecimal string.
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192 The result is a valid Python expression.
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193 \end{funcdesc}
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194
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195 \begin{funcdesc}{id}{object}
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196 Return the `identity' of an object. This is an integer which is
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197 guaranteed to be unique and constant for this object during its
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198 lifetime. (Two objects whose lifetimes are disjunct may have the
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199 same id() value.) (Implementation note: this is the address of the
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200 object.)
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201 \end{funcdesc}
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202
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203 \begin{funcdesc}{input}{\optional{prompt}}
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204 Almost equivalent to \code{eval(raw_input(\var{prompt}))}. Like
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205 \code{raw_input()}, the \var{prompt} argument is optional. The difference
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206 is that a long input expression may be broken over multiple lines using
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207 the backslash convention.
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208 \end{funcdesc}
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209
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210 \begin{funcdesc}{int}{x}
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211 Convert a number to a plain integer. The argument may be a plain or
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212 long integer or a floating point number. Conversion of floating
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213 point numbers to integers is defined by the C semantics; normally
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214 the conversion truncates towards zero.\footnote{This is ugly --- the
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215 language definition should require truncation towards zero.}
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216 \end{funcdesc}
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217
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218 \begin{funcdesc}{len}{s}
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219 Return the length (the number of items) of an object. The argument
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220 may be a sequence (string, tuple or list) or a mapping (dictionary).
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221 \end{funcdesc}
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222
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223 \begin{funcdesc}{locals}{}
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224 Return a dictionary representing the current local symbol table.
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225 Inside a function, modifying this dictionary does not always have the
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226 desired effect.
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227 \end{funcdesc}
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228
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229 \begin{funcdesc}{long}{x}
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230 Convert a number to a long integer. The argument may be a plain or
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231 long integer or a floating point number.
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232 \end{funcdesc}
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233
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234 \begin{funcdesc}{map}{function\, list\, ...}
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235 Apply \var{function} to every item of \var{list} and return a list
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236 of the results. If additional \var{list} arguments are passed,
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237 \var{function} must take that many arguments and is applied to
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238 the items of all lists in parallel; if a list is shorter than another
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239 it is assumed to be extended with \code{None} items. If
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240 \var{function} is \code{None}, the identity function is assumed; if
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241 there are multiple list arguments, \code{map} returns a list
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242 consisting of tuples containing the corresponding items from all lists
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243 (i.e. a kind of transpose operation). The \var{list} arguments may be
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244 any kind of sequence; the result is always a list.
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245 \end{funcdesc}
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246
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247 \begin{funcdesc}{max}{s}
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248 Return the largest item of a non-empty sequence (string, tuple or
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249 list).
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250 \end{funcdesc}
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251
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252 \begin{funcdesc}{min}{s}
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253 Return the smallest item of a non-empty sequence (string, tuple or
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254 list).
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255 \end{funcdesc}
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256
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257 \begin{funcdesc}{oct}{x}
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258 Convert an integer number (of any size) to an octal string. The
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259 result is a valid Python expression.
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260 \end{funcdesc}
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261
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262 \begin{funcdesc}{open}{filename\optional{\, mode\optional{\, bufsize}}}
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263 Return a new file object (described earlier under Built-in Types).
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264 The first two arguments are the same as for \code{stdio}'s
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265 \code{fopen()}: \var{filename} is the file name to be opened,
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266 \var{mode} indicates how the file is to be opened: \code{'r'} for
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267 reading, \code{'w'} for writing (truncating an existing file), and
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269 systems means that {\em all} writes append to the end of the file,
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270 regardless of the current seek position).
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271 Modes \code{'r+'}, \code{'w+'} and
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272 \code{'a+'} open the file for updating, provided the underlying
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273 \code{stdio} library understands this. On systems that differentiate
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274 between binary and text files, \code{'b'} appended to the mode opens
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275 the file in binary mode. If the file cannot be opened, \code{IOError}
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276 is raised.
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277 If \var{mode} is omitted, it defaults to \code{'r'}.
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278 The optional \var{bufsize} argument specifies the file's desired
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279 buffer size: 0 means unbuffered, 1 means line buffered, any other
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280 positive value means use a buffer of (approximately) that size. A
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281 negative \var{bufsize} means to use the system default, which is
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282 usually line buffered for for tty devices and fully buffered for other
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283 files.%
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284 \footnote{Specifying a buffer size currently has no effect on systems
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285 that don't have \code{setvbuf()}. The interface to specify the buffer
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286 size is not done using a method that calls \code{setvbuf()}, because
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287 that may dump core when called after any I/O has been performed, and
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288 there's no reliable way to determine whether this is the case.}
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289 \end{funcdesc}
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290
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291 \begin{funcdesc}{ord}{c}
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292 Return the \ASCII{} value of a string of one character. E.g.,
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293 \code{ord('a')} returns the integer \code{97}. This is the inverse of
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294 \code{chr()}.
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295 \end{funcdesc}
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296
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297 \begin{funcdesc}{pow}{x\, y\optional{\, z}}
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298 Return \var{x} to the power \var{y}; if \var{z} is present, return
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299 \var{x} to the power \var{y}, modulo \var{z} (computed more
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300 efficiently than \code{pow(\var{x}, \var{y}) \% \var{z}}).
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301 The arguments must have
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302 numeric types. With mixed operand types, the rules for binary
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303 arithmetic operators apply. The effective operand type is also the
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304 type of the result; if the result is not expressible in this type, the
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305 function raises an exception; e.g., \code{pow(2, -1)} or \code{pow(2,
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306 35000)} is not allowed.
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307 \end{funcdesc}
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308
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309 \begin{funcdesc}{range}{\optional{start\,} end\optional{\, step}}
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310 This is a versatile function to create lists containing arithmetic
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311 progressions. It is most often used in \code{for} loops. The
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312 arguments must be plain integers. If the \var{step} argument is
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313 omitted, it defaults to \code{1}. If the \var{start} argument is
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314 omitted, it defaults to \code{0}. The full form returns a list of
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315 plain integers \code{[\var{start}, \var{start} + \var{step},
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316 \var{start} + 2 * \var{step}, \ldots]}. If \var{step} is positive,
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317 the last element is the largest \code{\var{start} + \var{i} *
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318 \var{step}} less than \var{end}; if \var{step} is negative, the last
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319 element is the largest \code{\var{start} + \var{i} * \var{step}}
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320 greater than \var{end}. \var{step} must not be zero (or else an
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321 exception is raised). Example:
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322
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323 \bcode\begin{verbatim}
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324 >>> range(10)
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325 [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
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326 >>> range(1, 11)
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327 [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
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328 >>> range(0, 30, 5)
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329 [0, 5, 10, 15, 20, 25]
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330 >>> range(0, 10, 3)
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331 [0, 3, 6, 9]
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332 >>> range(0, -10, -1)
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333 [0, -1, -2, -3, -4, -5, -6, -7, -8, -9]
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334 >>> range(0)
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335 []
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336 >>> range(1, 0)
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337 []
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338 >>>
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339 \end{verbatim}\ecode
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340 \end{funcdesc}
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341
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342 \begin{funcdesc}{raw_input}{\optional{prompt}}
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343 If the \var{prompt} argument is present, it is written to standard output
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344 without a trailing newline. The function then reads a line from input,
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345 converts it to a string (stripping a trailing newline), and returns that.
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346 When \EOF{} is read, \code{EOFError} is raised. Example:
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347
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348 \bcode\begin{verbatim}
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349 >>> s = raw_input('--> ')
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350 --> Monty Python's Flying Circus
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351 >>> s
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352 "Monty Python's Flying Circus"
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353 >>>
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354 \end{verbatim}\ecode
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355 \end{funcdesc}
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356
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357 \begin{funcdesc}{reduce}{function\, list\optional{\, initializer}}
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358 Apply the binary \var{function} to the items of \var{list} so as to
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359 reduce the list to a single value. E.g.,
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360 \code{reduce(lambda x, y: x*y, \var{list}, 1)} returns the product of
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361 the elements of \var{list}. The optional \var{initializer} can be
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362 thought of as being prepended to \var{list} so as to allow reduction
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363 of an empty \var{list}. The \var{list} arguments may be any kind of
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364 sequence.
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365 \end{funcdesc}
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366
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367 \begin{funcdesc}{reload}{module}
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368 Re-parse and re-initialize an already imported \var{module}. The
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369 argument must be a module object, so it must have been successfully
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370 imported before. This is useful if you have edited the module source
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371 file using an external editor and want to try out the new version
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372 without leaving the Python interpreter. The return value is the
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373 module object (i.e.\ the same as the \var{module} argument).
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374
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375 There are a number of caveats:
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376
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377 If a module is syntactically correct but its initialization fails, the
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378 first \code{import} statement for it does not bind its name locally,
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379 but does store a (partially initialized) module object in
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380 \code{sys.modules}. To reload the module you must first
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381 \code{import} it again (this will bind the name to the partially
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382 initialized module object) before you can \code{reload()} it.
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383
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384 When a module is reloaded, its dictionary (containing the module's
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385 global variables) is retained. Redefinitions of names will override
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386 the old definitions, so this is generally not a problem. If the new
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387 version of a module does not define a name that was defined by the old
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388 version, the old definition remains. This feature can be used to the
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389 module's advantage if it maintains a global table or cache of objects
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390 --- with a \code{try} statement it can test for the table's presence
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391 and skip its initialization if desired.
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392
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393 It is legal though generally not very useful to reload built-in or
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394 dynamically loaded modules, except for \code{sys}, \code{__main__} and
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395 \code{__builtin__}. In certain cases, however, extension modules are
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396 not designed to be initialized more than once, and may fail in
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397 arbitrary ways when reloaded.
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398
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399 If a module imports objects from another module using \code{from}
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400 {\ldots} \code{import} {\ldots}, calling \code{reload()} for the other
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401 module does not redefine the objects imported from it --- one way
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402 around this is to re-execute the \code{from} statement, another is to
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403 use \code{import} and qualified names (\var{module}.\var{name})
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404 instead.
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405
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406 If a module instantiates instances of a class, reloading the module
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407 that defines the class does not affect the method definitions of the
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408 instances --- they continue to use the old class definition. The same
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409 is true for derived classes.
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410 \end{funcdesc}
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411
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412 \begin{funcdesc}{repr}{object}
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413 Return a string containing a printable representation of an object.
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414 This is the same value yielded by conversions (reverse quotes).
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415 It is sometimes useful to be able to access this operation as an
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416 ordinary function. For many types, this function makes an attempt
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417 to return a string that would yield an object with the same value
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418 when passed to \code{eval()}.
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419 \end{funcdesc}
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420
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421 \begin{funcdesc}{round}{x\, n}
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422 Return the floating point value \var{x} rounded to \var{n} digits
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423 after the decimal point. If \var{n} is omitted, it defaults to zero.
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424 The result is a floating point number. Values are rounded to the
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425 closest multiple of 10 to the power minus \var{n}; if two multiples
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426 are equally close, rounding is done away from 0 (so e.g.
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427 \code{round(0.5)} is \code{1.0} and \code{round(-0.5)} is \code{-1.0}).
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428 \end{funcdesc}
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429
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430 \begin{funcdesc}{setattr}{object\, name\, value}
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431 This is the counterpart of \code{getattr}. The arguments are an
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432 object, a string and an arbitrary value. The string must be the name
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433 of one of the object's attributes. The function assigns the value to
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434 the attribute, provided the object allows it. For example,
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435 \code{setattr(\var{x}, '\var{foobar}', 123)} is equivalent to
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436 \code{\var{x}.\var{foobar} = 123}.
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437 \end{funcdesc}
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438
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439 \begin{funcdesc}{str}{object}
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440 Return a string containing a nicely printable representation of an
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441 object. For strings, this returns the string itself. The difference
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442 with \code{repr(\var{object})} is that \code{str(\var{object})} does not
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443 always attempt to return a string that is acceptable to \code{eval()};
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444 its goal is to return a printable string.
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445 \end{funcdesc}
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446
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447 \begin{funcdesc}{tuple}{sequence}
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448 Return a tuple whose items are the same and in the same order as
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449 \var{sequence}'s items. If \var{sequence} is alread a tuple, it
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450 is returned unchanged. For instance, \code{tuple('abc')} returns
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451 returns \code{('a', 'b', 'c')} and \code{tuple([1, 2, 3])} returns
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452 \code{(1, 2, 3)}.
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453 \end{funcdesc}
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454
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455 \begin{funcdesc}{type}{object}
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456 Return the type of an \var{object}. The return value is a type
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457 object. The standard module \code{types} defines names for all
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458 built-in types.
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459 \stmodindex{types}
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460 \obindex{type}
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461 For instance:
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462
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463 \bcode\begin{verbatim}
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464 >>> import types
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465 >>> if type(x) == types.StringType: print "It's a string"
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466 \end{verbatim}\ecode
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467 \end{funcdesc}
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468
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469 \begin{funcdesc}{vars}{\optional{object}}
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470 Without arguments, return a dictionary corresponding to the current
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471 local symbol table. With a module, class or class instance object as
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472 argument (or anything else that has a \code{__dict__} attribute),
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473 returns a dictionary corresponding to the object's symbol table.
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474 The returned dictionary should not be modified: the effects on the
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475 corresponding symbol table are undefined.%
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476 \footnote{In the current implementation, local variable bindings
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477 cannot normally be affected this way, but variables retrieved from
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478 other scopes (e.g. modules) can be. This may change.}
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479 \end{funcdesc}
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480
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481 \begin{funcdesc}{xrange}{\optional{start\,} end\optional{\, step}}
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482 This function is very similar to \code{range()}, but returns an
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483 ``xrange object'' instead of a list. This is an opaque sequence type
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484 which yields the same values as the corresponding list, without
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485 actually storing them all simultaneously. The advantage of
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486 \code{xrange()} over \code{range()} is minimal (since \code{xrange()}
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487 still has to create the values when asked for them) except when a very
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488 large range is used on a memory-starved machine (e.g. MS-DOS) or when all
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489 of the range's elements are never used (e.g. when the loop is usually
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490 terminated with \code{break}).
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491 \end{funcdesc}