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annotate Doc/lib/libsys.tex @ 25838:8540887bcec6 2.2

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Backport of 2.183: Fix memory leak in add_subclass() found by NealN with valgrind.
author Guido van Rossum <guido@python.org>
date Fri, 18 Oct 2002 13:42:21 +0000
parents 3a4a34fdfa81
children 4c7bb0b4d354
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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 dependent
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15 whether this is a full pathname or not). If the command was
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16 executed using the \programopt{-c} command line option to the
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17 interpreter, \code{argv[0]} is set to the string \code{'-c'}. If no
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18 script name was passed to the Python interpreter, \code{argv} has
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19 zero length.
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20 \end{datadesc}
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21
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22 \begin{datadesc}{byteorder}
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23 An indicator of the native byte order. This will have the value
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24 \code{'big'} on big-endian (most-signigicant byte first) platforms,
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25 and \code{'little'} on little-endian (least-significant byte first)
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26 platforms.
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27 \versionadded{2.0}
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28 \end{datadesc}
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29
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30 \begin{datadesc}{builtin_module_names}
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31 A tuple of strings giving the names of all modules that are compiled
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32 into this Python interpreter. (This information is not available in
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33 any other way --- \code{modules.keys()} only lists the imported
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34 modules.)
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35 \end{datadesc}
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36
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37 \begin{datadesc}{copyright}
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38 A string containing the copyright pertaining to the Python
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39 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}{displayhook}{\var{value}}
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48 If \var{value} is not \code{None}, this function prints it to
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49 \code{sys.stdout}, and saves it in \code{__builtin__._}.
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50
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51 \code{sys.displayhook} is called on the result of evaluating an
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52 expression entered in an interactive Python session. The display of
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53 these values can be customized by assigning another one-argument
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54 function to \code{sys.displayhook}.
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55 \end{funcdesc}
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56
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57 \begin{funcdesc}{excepthook}{\var{type}, \var{value}, \var{traceback}}
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58 This function prints out a given traceback and exception to
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59 \code{sys.stderr}.
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60
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61 When an exception is raised and uncaught, the interpreter calls
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62 \code{sys.excepthook} with three arguments, the exception class,
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63 exception instance, and a traceback object. In an interactive
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64 session this happens just before control is returned to the prompt;
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65 in a Python program this happens just before the program exits. The
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66 handling of such top-level exceptions can be customized by assigning
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67 another three-argument function to \code{sys.excepthook}.
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68 \end{funcdesc}
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69
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70 \begin{datadesc}{__displayhook__}
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71 \dataline{__excepthook__}
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72 These objects contain the original values of \code{displayhook} and
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73 \code{excepthook} at the start of the program. They are saved so
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74 that \code{displayhook} and \code{excepthook} can be restored in
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75 case they happen to get replaced with broken objects.
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76 \end{datadesc}
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77
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78 \begin{funcdesc}{exc_info}{}
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79 This function returns a tuple of three values that give information
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80 about the exception that is currently being handled. The
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81 information returned is specific both to the current thread and to
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82 the current stack frame. If the current stack frame is not handling
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83 an exception, the information is taken from the calling stack frame,
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84 or its caller, and so on until a stack frame is found that is
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85 handling an exception. Here, ``handling an exception'' is defined
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86 as ``executing or having executed an except clause.'' For any stack
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87 frame, only information about the most recently handled exception is
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88 accessible.
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89
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90 If no exception is being handled anywhere on the stack, a tuple
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91 containing three \code{None} values is returned. Otherwise, the
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92 values returned are \code{(\var{type}, \var{value},
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93 \var{traceback})}. Their meaning is: \var{type} gets the exception
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94 type of the exception being handled (a string or class object);
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95 \var{value} gets the exception parameter (its \dfn{associated value}
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96 or the second argument to \keyword{raise}, which is always a class
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97 instance if the exception type is a class object); \var{traceback}
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98 gets a traceback object (see the Reference Manual) which
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99 encapsulates the call stack at the point where the exception
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100 originally occurred. \obindex{traceback}
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101
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102 \warning{Assigning the \var{traceback} return value to a
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103 local variable in a function that is handling an exception will
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104 cause a circular reference. This will prevent anything referenced
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105 by a local variable in the same function or by the traceback from
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106 being garbage collected. Since most functions don't need access to
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107 the traceback, the best solution is to use something like
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108 \code{exctype, value = sys.exc_info()[:2]} to extract only the
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109 exception type and value. If you do need the traceback, make sure
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110 to delete it after use (best done with a \keyword{try}
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111 ... \keyword{finally} statement) or to call \function{exc_info()} in
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112 a function that does not itself handle an exception.} \note{Beginning
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113 with Python 2.2, such cycles are automatically reclaimed when garbage
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114 collection is enabled and they become unreachable, but it remains more
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115 efficient to avoid creating cycles.}
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116 \end{funcdesc}
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117
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118 \begin{datadesc}{exc_type}
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119 \dataline{exc_value}
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120 \dataline{exc_traceback}
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121 \deprecated {1.5}
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122 {Use \function{exc_info()} instead.}
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123 Since they are global variables, they are not specific to the
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124 current thread, so their use is not safe in a multi-threaded
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125 program. When no exception is being handled, \code{exc_type} is set
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126 to \code{None} and the other two are undefined.
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127 \end{datadesc}
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128
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129 \begin{datadesc}{exec_prefix}
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130 A string giving the site-specific directory prefix where the
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131 platform-dependent Python files are installed; by default, this is
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132 also \code{'/usr/local'}. This can be set at build time with the
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133 \longprogramopt{exec-prefix} argument to the \program{configure}
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134 script. Specifically, all configuration files (e.g. the
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135 \file{pyconfig.h} header file) are installed in the directory
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136 \code{exec_prefix + '/lib/python\var{version}/config'}, and shared
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137 library modules are installed in \code{exec_prefix +
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138 '/lib/python\var{version}/lib-dynload'}, where \var{version} is
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139 equal to \code{version[:3]}.
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140 \end{datadesc}
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141
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142 \begin{datadesc}{executable}
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143 A string giving the name of the executable binary for the Python
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144 interpreter, on systems where this makes sense.
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145 \end{datadesc}
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146
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147 \begin{funcdesc}{exit}{\optional{arg}}
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148 Exit from Python. This is implemented by raising the
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149 \exception{SystemExit} exception, so cleanup actions specified by
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150 finally clauses of \keyword{try} statements are honored, and it is
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151 possible to intercept the exit attempt at an outer level. The
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152 optional argument \var{arg} can be an integer giving the exit status
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153 (defaulting to zero), or another type of object. If it is an
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154 integer, zero is considered ``successful termination'' and any
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155 nonzero value is considered ``abnormal termination'' by shells and
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156 the like. Most systems require it to be in the range 0-127, and
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157 produce undefined results otherwise. Some systems have a convention
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158 for assigning specific meanings to specific exit codes, but these
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159 are generally underdeveloped; \UNIX{} programs generally use 2 for
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160 command line syntax errors and 1 for all other kind of errors. If
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161 another type of object is passed, \code{None} is equivalent to
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162 passing zero, and any other object is printed to \code{sys.stderr}
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163 and results in an exit code of 1. In particular,
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164 \code{sys.exit("some error message")} is a quick way to exit a
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165 program when an error occurs.
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166 \end{funcdesc}
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167
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168 \begin{datadesc}{exitfunc}
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169 This value is not actually defined by the module, but can be set by
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170 the user (or by a program) to specify a clean-up action at program
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171 exit. When set, it should be a parameterless function. This
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172 function will be called when the interpreter exits. Only one
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173 function may be installed in this way; to allow multiple functions
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174 which will be called at termination, use the \refmodule{atexit}
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175 module. \note{The exit function is not called when the program is
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176 killed by a signal, when a Python fatal internal error is detected,
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177 or when \code{os._exit()} is called.}
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178 \end{datadesc}
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179
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180 \begin{funcdesc}{getdefaultencoding}{}
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181 Return the name of the current default string encoding used by the
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182 Unicode implementation.
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183 \versionadded{2.0}
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184 \end{funcdesc}
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185
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186 \begin{funcdesc}{getdlopenflags}{}
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187 Return the current value of the flags that are used for
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188 \cfunction{dlopen()} calls. The flag constants are defined in the
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189 \refmodule{dl} and \module{DLFCN} modules.
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190 Availability: \UNIX.
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191 \versionadded{2.2}
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192 \end{funcdesc}
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193
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194 \begin{funcdesc}{getrefcount}{object}
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195 Return the reference count of the \var{object}. The count returned
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196 is generally one higher than you might expect, because it includes
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197 the (temporary) reference as an argument to
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198 \function{getrefcount()}.
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199 \end{funcdesc}
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200
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201 \begin{funcdesc}{getrecursionlimit}{}
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202 Return the current value of the recursion limit, the maximum depth
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203 of the Python interpreter stack. This limit prevents infinite
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204 recursion from causing an overflow of the C stack and crashing
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205 Python. It can be set by \function{setrecursionlimit()}.
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206 \end{funcdesc}
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207
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208 \begin{funcdesc}{_getframe}{\optional{depth}}
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209 Return a frame object from the call stack. If optional integer
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210 \var{depth} is given, return the frame object that many calls below
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211 the top of the stack. If that is deeper than the call stack,
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212 \exception{ValueError} is raised. The default for \var{depth} is
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213 zero, returning the frame at the top of the call stack.
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214
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215 This function should be used for internal and specialized purposes
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216 only.
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217 \end{funcdesc}
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218
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219 \begin{datadesc}{hexversion}
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220 The version number encoded as a single integer. This is guaranteed
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221 to increase with each version, including proper support for
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222 non-production releases. For example, to test that the Python
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223 interpreter is at least version 1.5.2, use:
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224
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225 \begin{verbatim}
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226 if sys.hexversion >= 0x010502F0:
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227 # use some advanced feature
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228 ...
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229 else:
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230 # use an alternative implementation or warn the user
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231 ...
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232 \end{verbatim}
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233
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234 This is called \samp{hexversion} since it only really looks
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235 meaningful when viewed as the result of passing it to the built-in
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236 \function{hex()} function. The \code{version_info} value may be
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237 used for a more human-friendly encoding of the same information.
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238 \versionadded{1.5.2}
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239 \end{datadesc}
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240
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241 \begin{datadesc}{last_type}
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242 \dataline{last_value}
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243 \dataline{last_traceback}
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244 These three variables are not always defined; they are set when an
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245 exception is not handled and the interpreter prints an error message
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246 and a stack traceback. Their intended use is to allow an
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247 interactive user to import a debugger module and engage in
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248 post-mortem debugging without having to re-execute the command that
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249 caused the error. (Typical use is \samp{import pdb; pdb.pm()} to
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250 enter the post-mortem debugger; see chapter \ref{debugger}, ``The
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251 Python Debugger,'' for more information.)
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252
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253 The meaning of the variables is the same as that of the return
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254 values from \function{exc_info()} above. (Since there is only one
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255 interactive thread, thread-safety is not a concern for these
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256 variables, unlike for \code{exc_type} etc.)
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257 \end{datadesc}
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258
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259 \begin{datadesc}{maxint}
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260 The largest positive integer supported by Python's regular integer
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261 type. This is at least 2**31-1. The largest negative integer is
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262 \code{-maxint-1} --- the asymmetry results from the use of 2's
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263 complement binary arithmetic.
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264 \end{datadesc}
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265
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266 \begin{datadesc}{maxunicode}
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267 An integer giving the largest supported code point for a Unicode
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268 character. The value of this depends on the configuration option
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269 that specifies whether Unicode characters are stored as UCS-2 or
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270 UCS-4.
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271 \end{datadesc}
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272
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273 \begin{datadesc}{modules}
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274 This is a dictionary that maps module names to modules which have
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275 already been loaded. This can be manipulated to force reloading of
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276 modules and other tricks. Note that removing a module from this
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277 dictionary is \emph{not} the same as calling
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278 \function{reload()}\bifuncindex{reload} on the corresponding module
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279 object.
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280 \end{datadesc}
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281
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282 \begin{datadesc}{path}
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283 \indexiii{module}{search}{path}
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284 A list of strings that specifies the search path for modules.
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285 Initialized from the environment variable \envvar{PYTHONPATH}, or an
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286 installation-dependent default.
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287
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288 The first item of this list, \code{path[0]}, is the directory
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289 containing the script that was used to invoke the Python
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290 interpreter. If the script directory is not available (e.g. if the
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291 interpreter is invoked interactively or if the script is read from
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292 standard input), \code{path[0]} is the empty string, which directs
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293 Python to search modules in the current directory first. Notice
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294 that the script directory is inserted \emph{before} the entries
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295 inserted as a result of \envvar{PYTHONPATH}.
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296 \end{datadesc}
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297
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298 \begin{datadesc}{platform}
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299 This string contains a platform identifier, e.g. \code{'sunos5'} or
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300 \code{'linux1'}. This can be used to append platform-specific
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301 components to \code{path}, for instance.
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302 \end{datadesc}
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303
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304 \begin{datadesc}{prefix}
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305 A string giving the site-specific directory prefix where the
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306 platform independent Python files are installed; by default, this is
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307 the string \code{'/usr/local'}. This can be set at build time with
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308 the \longprogramopt{prefix} argument to the \program{configure}
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309 script. The main collection of Python library modules is installed
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310 in the directory \code{prefix + '/lib/python\var{version}'} while
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311 the platform independent header files (all except \file{pyconfig.h})
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312 are stored in \code{prefix + '/include/python\var{version}'}, where
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313 \var{version} is equal to \code{version[:3]}.
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314 \end{datadesc}
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315
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316 \begin{datadesc}{ps1}
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317 \dataline{ps2}
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318 \index{interpreter prompts}
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319 \index{prompts, interpreter}
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320 Strings specifying the primary and secondary prompt of the
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321 interpreter. These are only defined if the interpreter is in
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322 interactive mode. Their initial values in this case are
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323 \code{'>\code{>}> '} and \code{'... '}. If a non-string object is
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324 assigned to either variable, its \function{str()} is re-evaluated
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325 each time the interpreter prepares to read a new interactive
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326 command; this can be used to implement a dynamic prompt.
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327 \end{datadesc}
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328
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329 \begin{funcdesc}{setcheckinterval}{interval}
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330 Set the interpreter's ``check interval''. This integer value
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331 determines how often the interpreter checks for periodic things such
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332 as thread switches and signal handlers. The default is \code{10},
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333 meaning the check is performed every 10 Python virtual instructions.
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334 Setting it to a larger value may increase performance for programs
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335 using threads. Setting it to a value \code{<=} 0 checks every
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336 virtual instruction, maximizing responsiveness as well as overhead.
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337 \end{funcdesc}
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338
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339 \begin{funcdesc}{setdefaultencoding}{name}
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340 Set the current default string encoding used by the Unicode
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341 implementation. If \var{name} does not match any available
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342 encoding, \exception{LookupError} is raised. This function is only
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343 intended to be used by the \refmodule{site} module implementation
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344 and, where needed, by \module{sitecustomize}. Once used by the
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345 \refmodule{site} module, it is removed from the \module{sys}
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346 module's namespace.
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347 % Note that \refmodule{site} is not imported if
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348 % the \programopt{-S} option is passed to the interpreter, in which
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349 % case this function will remain available.
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350 \versionadded{2.0}
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351 \end{funcdesc}
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352
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353 \begin{funcdesc}{setdlopenflags}{n}
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354 Set the flags used by the interpreter for \cfunction{dlopen()}
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355 calls, such as when the interpreter loads extension modules. Among
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356 other things, this will enable a lazy resolving of symbols when
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357 importing a module, if called as \code{sys.setdlopenflags(0)}. To
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358 share symbols across extension modules, call as
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359 \code{sys.setdlopenflags(dl.RTLD_NOW | dl.RTLD_GLOBAL)}. Symbolic
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360 names for the flag modules can be either found in the \refmodule{dl}
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361 module, or in the \module{DLFCN} module. If \module{DLFCN} is not
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362 available, it can be generated from \file{/usr/include/dlfcn.h}
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363 using the \program{h2py} script.
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364 Availability: \UNIX.
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365 \versionadded{2.2}
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366 \end{funcdesc}
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367
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368 \begin{funcdesc}{setprofile}{profilefunc}
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369 Set the system's profile function,\index{profile function} which
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370 allows you to implement a Python source code profiler in
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371 Python.\index{profiler} See chapter \ref{profile} for more
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372 information on the Python profiler. The system's profile function
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373 is called similarly to the system's trace function (see
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374 \function{settrace()}), but it isn't called for each executed line
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375 of code (only on call and return, but the return event is reported
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376 even when an exception has been set). The function is
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377 thread-specific, but there is no way for the profiler to know about
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378 context switches between threads, so it does not make sense to use
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379 this in the presence of multiple threads.
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380 Also, its return value is not used, so it can simply return
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381 \code{None}.
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382 \end{funcdesc}
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383
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384 \begin{funcdesc}{setrecursionlimit}{limit}
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385 Set the maximum depth of the Python interpreter stack to
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386 \var{limit}. This limit prevents infinite recursion from causing an
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387 overflow of the C stack and crashing Python.
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388
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389 The highest possible limit is platform-dependent. A user may need
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390 to set the limit higher when she has a program that requires deep
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391 recursion and a platform that supports a higher limit. This should
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392 be done with care, because a too-high limit can lead to a crash.
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393 \end{funcdesc}
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394
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395 \begin{funcdesc}{settrace}{tracefunc}
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396 Set the system's trace function,\index{trace function} which allows
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397 you to implement a Python source code debugger in Python. See
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398 section \ref{debugger-hooks}, ``How It Works,'' in the chapter on
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399 the Python debugger.\index{debugger} The function is
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400 thread-specific; for a debugger to support multiple threads, it must
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401 be registered using \function{settrace()} for each thread being
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402 debugged.
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403 \end{funcdesc}
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404
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405 \begin{datadesc}{stdin}
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406 \dataline{stdout}
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407 \dataline{stderr}
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408 File objects corresponding to the interpreter's standard input,
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409 output and error streams. \code{stdin} is used for all interpreter
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410 input except for scripts but including calls to
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411 \function{input()}\bifuncindex{input} and
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412 \function{raw_input()}\bifuncindex{raw_input}. \code{stdout} is
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413 used for the output of \keyword{print} and expression statements and
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414 for the prompts of \function{input()} and \function{raw_input()}.
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415 The interpreter's own prompts and (almost all of) its error messages
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416 go to \code{stderr}. \code{stdout} and \code{stderr} needn't be
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417 built-in file objects: any object is acceptable as long as it has a
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418 \method{write()} method that takes a string argument. (Changing
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419 these objects doesn't affect the standard I/O streams of processes
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420 executed by \function{os.popen()}, \function{os.system()} or the
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421 \function{exec*()} family of functions in the \refmodule{os}
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422 module.)
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423 \end{datadesc}
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424
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425 \begin{datadesc}{__stdin__}
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426 \dataline{__stdout__}
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427 \dataline{__stderr__}
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428 These objects contain the original values of \code{stdin},
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429 \code{stderr} and \code{stdout} at the start of the program. They
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430 are used during finalization, and could be useful to restore the
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431 actual files to known working file objects in case they have been
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432 overwritten with a broken object.
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433 \end{datadesc}
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434
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435 \begin{datadesc}{tracebacklimit}
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436 When this variable is set to an integer value, it determines the
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437 maximum number of levels of traceback information printed when an
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438 unhandled exception occurs. The default is \code{1000}. When set
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439 to \code{0} or less, all traceback information is suppressed and
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440 only the exception type and value are printed.
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441 \end{datadesc}
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442
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443 \begin{datadesc}{version}
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444 A string containing the version number of the Python interpreter
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445 plus additional information on the build number and compiler used.
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446 It has a value of the form \code{'\var{version}
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447 (\#\var{build_number}, \var{build_date}, \var{build_time})
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448 [\var{compiler}]'}. The first three characters are used to identify
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449 the version in the installation directories (where appropriate on
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450 each platform). An example:
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451
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452 \begin{verbatim}
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453 >>> import sys
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454 >>> sys.version
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455 '1.5.2 (#0 Apr 13 1999, 10:51:12) [MSC 32 bit (Intel)]'
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456 \end{verbatim}
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457 \end{datadesc}
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458
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459 \begin{datadesc}{version_info}
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460 A tuple containing the five components of the version number:
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461 \var{major}, \var{minor}, \var{micro}, \var{releaselevel}, and
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462 \var{serial}. All values except \var{releaselevel} are integers;
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463 the release level is \code{'alpha'}, \code{'beta'},
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464 \code{'candidate'}, or \code{'final'}. The \code{version_info}
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465 value corresponding to the Python version 2.0 is \code{(2, 0, 0,
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466 'final', 0)}.
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467 \versionadded{2.0}
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468 \end{datadesc}
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469
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470 \begin{datadesc}{warnoptions}
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471 This is an implementation detail of the warnings framework; do not
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472 modify this value. Refer to the \refmodule{warnings} module for
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473 more information on the warnings framework.
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474 \end{datadesc}
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475
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476 \begin{datadesc}{winver}
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477 The version number used to form registry keys on Windows platforms.
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478 This is stored as string resource 1000 in the Python DLL. The value
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479 is normally the first three characters of \constant{version}. It is
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480 provided in the \module{sys} module for informational purposes;
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481 modifying this value has no effect on the registry keys used by
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482 Python.
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483 Availability: Windows.
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484 \end{datadesc}