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Remove some debugging prints.
author Greg Ward <gward@python.net>
date Tue, 03 Oct 2000 03:32:37 +0000
parents 97651f28b004
children 788b523d4827
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1 \section{\module{sys} ---
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2 System-specific parameters and functions}
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3
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4 \declaremodule{builtin}{sys}
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5 \modulesynopsis{Access system-specific parameters and functions.}
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6
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7 This module provides access to some variables used or maintained by the
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8 interpreter and to functions that interact strongly with the interpreter.
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9 It is always available.
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10
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11
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12 \begin{datadesc}{argv}
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13 The list of command line arguments passed to a Python script.
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14 \code{argv[0]} is the script name (it is operating system
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15 dependent whether this is a full pathname or not).
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16 If the command was executed using the \programopt{-c} command line
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17 option to the interpreter, \code{argv[0]} is set to the string
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18 \code{'-c'}.
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19 If no script name was passed to the Python interpreter,
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20 \code{argv} has zero length.
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21 \end{datadesc}
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22
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23 \begin{datadesc}{byteorder}
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24 An indicator of the native byte order. This will have the value
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25 \code{'big'} on big-endian (most-signigicant byte first) platforms,
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26 and \code{'little'} on little-endian (least-significant byte first)
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27 platforms.
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28 \versionadded{2.0}
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29 \end{datadesc}
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30
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31 \begin{datadesc}{builtin_module_names}
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32 A tuple of strings giving the names of all modules that are compiled
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33 into this Python interpreter. (This information is not available in
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34 any other way --- \code{modules.keys()} only lists the imported
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35 modules.)
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36 \end{datadesc}
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37
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38 \begin{datadesc}{copyright}
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39 A string containing the copyright pertaining to the Python interpreter.
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40 \end{datadesc}
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41
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42 \begin{datadesc}{dllhandle}
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43 Integer specifying the handle of the Python DLL.
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44 Availability: Windows.
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45 \end{datadesc}
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46
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47 \begin{funcdesc}{exc_info}{}
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48 This function returns a tuple of three values that give information
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49 about the exception that is currently being handled. The information
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50 returned is specific both to the current thread and to the current
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51 stack frame. If the current stack frame is not handling an exception,
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52 the information is taken from the calling stack frame, or its caller,
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53 and so on until a stack frame is found that is handling an exception.
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54 Here, ``handling an exception'' is defined as ``executing or having
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55 executed an except clause.'' For any stack frame, only
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56 information about the most recently handled exception is accessible.
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57
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58 If no exception is being handled anywhere on the stack, a tuple
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59 containing three \code{None} values is returned. Otherwise, the
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60 values returned are
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61 \code{(\var{type}, \var{value}, \var{traceback})}.
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62 Their meaning is: \var{type} gets the exception type of the exception
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63 being handled (a string or class object); \var{value} gets the
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64 exception parameter (its \dfn{associated value} or the second argument
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65 to \keyword{raise}, which is always a class instance if the exception
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66 type is a class object); \var{traceback} gets a traceback object (see
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67 the Reference Manual) which encapsulates the call stack at the point
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68 where the exception originally occurred.
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69 \obindex{traceback}
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70
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71 \strong{Warning:} assigning the \var{traceback} return value to a
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72 local variable in a function that is handling an exception will cause
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73 a circular reference. This will prevent anything referenced by a local
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74 variable in the same function or by the traceback from being garbage
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75 collected. Since most functions don't need access to the traceback,
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76 the best solution is to use something like
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77 \code{type, value = sys.exc_info()[:2]}
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78 to extract only the exception type and value. If you do need the
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79 traceback, make sure to delete it after use (best done with a
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80 \keyword{try} ... \keyword{finally} statement) or to call
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81 \function{exc_info()} in a function that does not itself handle an
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82 exception.
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83 \end{funcdesc}
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84
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85 \begin{datadesc}{exc_type}
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86 \dataline{exc_value}
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87 \dataline{exc_traceback}
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88 \deprecated {1.5}
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89 {Use \function{exc_info()} instead.}
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90 Since they are global variables, they are not specific to the current
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91 thread, so their use is not safe in a multi-threaded program. When no
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92 exception is being handled, \code{exc_type} is set to \code{None} and
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93 the other two are undefined.
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94 \end{datadesc}
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95
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96 \begin{datadesc}{exec_prefix}
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97 A string giving the site-specific directory prefix where the
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98 platform-dependent Python files are installed; by default, this is
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99 also \code{'/usr/local'}. This can be set at build time with the
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100 \longprogramopt{exec-prefix} argument to the
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101 \program{configure} script. Specifically, all configuration files
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102 (e.g. the \file{config.h} header file) are installed in the directory
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103 \code{exec_prefix + '/lib/python\var{version}/config'}, and shared
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104 library modules are installed in \code{exec_prefix +
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105 '/lib/python\var{version}/lib-dynload'}, where \var{version} is equal
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106 to \code{version[:3]}.
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107 \end{datadesc}
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108
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109 \begin{datadesc}{executable}
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110 A string giving the name of the executable binary for the Python
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111 interpreter, on systems where this makes sense.
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112 \end{datadesc}
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113
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114 \begin{funcdesc}{exit}{\optional{arg}}
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115 Exit from Python. This is implemented by raising the
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116 \exception{SystemExit} exception, so cleanup actions specified by
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117 finally clauses of \keyword{try} statements are honored, and it is
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118 possible to intercept the exit attempt at an outer level. The
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119 optional argument \var{arg} can be an integer giving the exit status
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120 (defaulting to zero), or another type of object. If it is an integer,
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121 zero is considered ``successful termination'' and any nonzero value is
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122 considered ``abnormal termination'' by shells and the like. Most
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123 systems require it to be in the range 0-127, and produce undefined
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124 results otherwise. Some systems have a convention for assigning
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125 specific meanings to specific exit codes, but these are generally
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126 underdeveloped; Unix programs generally use 2 for command line syntax
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127 errors and 1 for all other kind of errors. If another type of object
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128 is passed, \code{None} is equivalent to passing zero, and any other
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129 object is printed to \code{sys.stderr} and results in an exit code of
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130 1. In particular, \code{sys.exit("some error message")} is a quick
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131 way to exit a program when an error occurs.
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132 \end{funcdesc}
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133
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134 \begin{datadesc}{exitfunc}
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135 This value is not actually defined by the module, but can be set by
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136 the user (or by a program) to specify a clean-up action at program
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137 exit. When set, it should be a parameterless function. This function
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138 will be called when the interpreter exits. Only one function may be
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139 installed in this way; to allow multiple functions which will be called
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140 at termination, use the \refmodule{atexit} module. Note: the exit function
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141 is not called when the program is killed by a signal, when a Python
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142 fatal internal error is detected, or when \code{os._exit()} is called.
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143 \end{datadesc}
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144
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145 \begin{funcdesc}{getrefcount}{object}
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146 Return the reference count of the \var{object}. The count returned is
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147 generally one higher than you might expect, because it includes the
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148 (temporary) reference as an argument to \function{getrefcount()}.
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149 \end{funcdesc}
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150
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151 \begin{funcdesc}{getrecursionlimit}{}
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152 Return the current value of the recursion limit, the maximum depth of
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153 the Python interpreter stack. This limit prevents infinite recursion
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154 from causing an overflow of the C stack and crashing Python. It can
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155 be set by \function{setrecursionlimit()}.
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156 \end{funcdesc}
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157
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158 \begin{datadesc}{hexversion}
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159 The version number encoded as a single integer. This is guaranteed to
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160 increase with each version, including proper support for
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161 non-production releases. For example, to test that the Python
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162 interpreter is at least version 1.5.2, use:
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163
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164 \begin{verbatim}
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165 if sys.hexversion >= 0x010502F0:
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166 # use some advanced feature
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167 ...
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168 else:
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169 # use an alternative implementation or warn the user
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170 ...
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171 \end{verbatim}
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172
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173 This is called \samp{hexversion} since it only really looks meaningful
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174 when viewed as the result of passing it to the built-in
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175 \function{hex()} function. The \code{version_info} value may be used
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176 for a more human-friendly encoding of the same information.
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177 \versionadded{1.5.2}
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178 \end{datadesc}
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179
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180 \begin{datadesc}{last_type}
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181 \dataline{last_value}
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182 \dataline{last_traceback}
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183 These three variables are not always defined; they are set when an
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184 exception is not handled and the interpreter prints an error message
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185 and a stack traceback. Their intended use is to allow an interactive
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186 user to import a debugger module and engage in post-mortem debugging
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187 without having to re-execute the command that caused the error.
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188 (Typical use is \samp{import pdb; pdb.pm()} to enter the post-mortem
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189 debugger; see the chapter ``The Python Debugger'' for more
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190 information.)
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191 \refstmodindex{pdb}
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192
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193 The meaning of the variables is the same
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194 as that of the return values from \function{exc_info()} above.
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195 (Since there is only one interactive thread, thread-safety is not a
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196 concern for these variables, unlike for \code{exc_type} etc.)
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197 \end{datadesc}
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198
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199 \begin{datadesc}{maxint}
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200 The largest positive integer supported by Python's regular integer
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201 type. This is at least 2**31-1. The largest negative integer is
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202 \code{-maxint-1} -- the asymmetry results from the use of 2's
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203 complement binary arithmetic.
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204 \end{datadesc}
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205
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206 \begin{datadesc}{modules}
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207 This is a dictionary that maps module names to modules which have
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208 already been loaded. This can be manipulated to force reloading of
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209 modules and other tricks. Note that removing a module from this
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210 dictionary is \emph{not} the same as calling
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211 \function{reload()}\bifuncindex{reload} on the corresponding module
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212 object.
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213 \end{datadesc}
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214
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215 \begin{datadesc}{path}
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216 \indexiii{module}{search}{path}
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217 A list of strings that specifies the search path for modules.
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218 Initialized from the environment variable \envvar{PYTHONPATH}, or an
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219 installation-dependent default.
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220
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221 The first item of this list, \code{path[0]}, is the
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222 directory containing the script that was used to invoke the Python
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223 interpreter. If the script directory is not available (e.g. if the
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224 interpreter is invoked interactively or if the script is read from
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225 standard input), \code{path[0]} is the empty string, which directs
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226 Python to search modules in the current directory first. Notice that
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227 the script directory is inserted \emph{before} the entries inserted as
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228 a result of \envvar{PYTHONPATH}.
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229 \end{datadesc}
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230
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231 \begin{datadesc}{platform}
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232 This string contains a platform identifier, e.g. \code{'sunos5'} or
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233 \code{'linux1'}. This can be used to append platform-specific
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234 components to \code{path}, for instance.
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235 \end{datadesc}
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236
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237 \begin{datadesc}{prefix}
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238 A string giving the site-specific directory prefix where the platform
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239 independent Python files are installed; by default, this is the string
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240 \code{'/usr/local'}. This can be set at build time with the
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241 \longprogramopt{prefix} argument to the
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242 \program{configure} script. The main collection of Python library
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243 modules is installed in the directory \code{prefix +
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244 '/lib/python\var{version}'} while the platform independent header
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245 files (all except \file{config.h}) are stored in \code{prefix +
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246 '/include/python\var{version}'}, where \var{version} is equal to
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247 \code{version[:3]}.
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248 \end{datadesc}
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249
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250 \begin{datadesc}{ps1}
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251 \dataline{ps2}
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252 \index{interpreter prompts}
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253 \index{prompts, interpreter}
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254 Strings specifying the primary and secondary prompt of the
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255 interpreter. These are only defined if the interpreter is in
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256 interactive mode. Their initial values in this case are
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257 \code{'>>> '} and \code{'... '}. If a non-string object is assigned
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258 to either variable, its \function{str()} is re-evaluated each time
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259 the interpreter prepares to read a new interactive command; this can
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260 be used to implement a dynamic prompt.
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261 \end{datadesc}
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262
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263 \begin{funcdesc}{setcheckinterval}{interval}
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264 Set the interpreter's ``check interval''. This integer value
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265 determines how often the interpreter checks for periodic things such
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266 as thread switches and signal handlers. The default is \code{10}, meaning
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267 the check is performed every 10 Python virtual instructions. Setting
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268 it to a larger value may increase performance for programs using
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269 threads. Setting it to a value \code{<=} 0 checks every virtual instruction,
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270 maximizing responsiveness as well as overhead.
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271 \end{funcdesc}
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272
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273 \begin{funcdesc}{setprofile}{profilefunc}
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274 Set the system's profile function, which allows you to implement a
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275 Python source code profiler in Python. See the chapter on the
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276 Python Profiler. The system's profile function
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277 is called similarly to the system's trace function (see
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278 \function{settrace()}), but it isn't called for each executed line of
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279 code (only on call and return and when an exception occurs). Also,
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280 its return value is not used, so it can just return \code{None}.
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281 \end{funcdesc}
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282 \index{profile function}
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283 \index{profiler}
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284
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285 \begin{funcdesc}{setrecursionlimit}{limit}
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286 Set the maximum depth of the Python interpreter stack to \var{limit}.
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287 This limit prevents infinite recursion from causing an overflow of the
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288 C stack and crashing Python.
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289
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290 The highest possible limit is platform-dependent. A user may need to
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291 set the limit higher when she has a program that requires deep
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292 recursion and a platform that supports a higher limit. This should be
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293 done with care, because a too-high limit can lead to a crash.
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294 \end{funcdesc}
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295
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296 \begin{funcdesc}{settrace}{tracefunc}
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297 Set the system's trace function, which allows you to implement a
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298 Python source code debugger in Python. See section ``How It Works''
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299 in the chapter on the Python Debugger.
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300 \end{funcdesc}
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301 \index{trace function}
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302 \index{debugger}
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303
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304 \begin{datadesc}{stdin}
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305 \dataline{stdout}
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306 \dataline{stderr}
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307 File objects corresponding to the interpreter's standard input,
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308 output and error streams. \code{stdin} is used for all
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309 interpreter input except for scripts but including calls to
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310 \function{input()}\bifuncindex{input} and
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311 \function{raw_input()}\bifuncindex{raw_input}. \code{stdout} is used
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312 for the output of \keyword{print} and expression statements and for the
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313 prompts of \function{input()} and \function{raw_input()}. The interpreter's
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314 own prompts and (almost all of) its error messages go to
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315 \code{stderr}. \code{stdout} and \code{stderr} needn't
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316 be built-in file objects: any object is acceptable as long as it has
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317 a \method{write()} method that takes a string argument. (Changing these
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318 objects doesn't affect the standard I/O streams of processes
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319 executed by \function{os.popen()}, \function{os.system()} or the
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320 \function{exec*()} family of functions in the \refmodule{os} module.)
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321 \refstmodindex{os}
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322 \end{datadesc}
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323
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324 \begin{datadesc}{__stdin__}
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325 \dataline{__stdout__}
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326 \dataline{__stderr__}
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327 These objects contain the original values of \code{stdin},
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328 \code{stderr} and \code{stdout} at the start of the program. They are
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329 used during finalization, and could be useful to restore the actual
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330 files to known working file objects in case they have been overwritten
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331 with a broken object.
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332 \end{datadesc}
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333
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334 \begin{datadesc}{tracebacklimit}
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335 When this variable is set to an integer value, it determines the
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336 maximum number of levels of traceback information printed when an
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337 unhandled exception occurs. The default is \code{1000}. When set to
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338 0 or less, all traceback information is suppressed and only the
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339 exception type and value are printed.
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340 \end{datadesc}
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341
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342 \begin{datadesc}{version}
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343 A string containing the version number of the Python interpreter plus
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344 additional information on the build number and compiler used. It has
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345 a value of the form \code{'\var{version} (\#\var{build_number},
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346 \var{build_date}, \var{build_time}) [\var{compiler}]'}. The first
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347 three characters are used to identify the version in the installation
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348 directories (where appropriate on each platform). An example:
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349
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350 \begin{verbatim}
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351 >>> import sys
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352 >>> sys.version
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353 '1.5.2 (#0 Apr 13 1999, 10:51:12) [MSC 32 bit (Intel)]'
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354 \end{verbatim}
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355 \end{datadesc}
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356
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357 \begin{datadesc}{version_info}
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358 A tuple containing the five components of the version number:
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359 \var{major}, \var{minor}, \var{micro}, \var{releaselevel}, and
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360 \var{serial}. All values except \var{releaselevel} are integers; the
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361 release level is \code{'alpha'}, \code{'beta'},
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362 \code{'candidate'}, or \code{'final'}. The \code{version_info} value
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363 corresponding to the Python version 2.0 is
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364 \code{(2, 0, 0, 'final', 0)}.
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365 \versionadded{2.0}
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366 \end{datadesc}
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367
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368 \begin{datadesc}{winver}
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369 The version number used to form registry keys on Windows platforms.
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370 This is stored as string resource 1000 in the Python DLL. The value
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371 is normally the first three characters of \constant{version}. It is
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372 provided in the \module{sys} module for informational purposes;
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373 modifying this value has no effect on the registry keys used by
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374 Python.
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375 Availability: Windows.
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376 \end{datadesc}