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Install the docs (with fewer rules). Add descriptions for the system variables.
author Guido van Rossum <guido@python.org>
date Mon, 10 Apr 2000 19:36:27 +0000
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children 37d9ded5ab91
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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}{builtin_module_names}
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24 A tuple of strings giving the names of all modules that are compiled
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25 into this Python interpreter. (This information is not available in
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26 any other way --- \code{modules.keys()} only lists the imported
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27 modules.)
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28 \end{datadesc}
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29
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30 \begin{datadesc}{copyright}
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31 A string containing the copyright pertaining to the Python interpreter.
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32 \end{datadesc}
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33
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34 \begin{datadesc}{dllhandle}
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35 Integer specifying the handle of the Python DLL.
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36 Availability: Windows.
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37 \end{datadesc}
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38
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39 \begin{funcdesc}{exc_info}{}
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40 This function returns a tuple of three values that give information
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41 about the exception that is currently being handled. The information
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42 returned is specific both to the current thread and to the current
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43 stack frame. If the current stack frame is not handling an exception,
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44 the information is taken from the calling stack frame, or its caller,
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45 and so on until a stack frame is found that is handling an exception.
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46 Here, ``handling an exception'' is defined as ``executing or having
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47 executed an except clause.'' For any stack frame, only
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48 information about the most recently handled exception is accessible.
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49
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50 If no exception is being handled anywhere on the stack, a tuple
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51 containing three \code{None} values is returned. Otherwise, the
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52 values returned are
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53 \code{(\var{type}, \var{value}, \var{traceback})}.
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54 Their meaning is: \var{type} gets the exception type of the exception
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55 being handled (a string or class object); \var{value} gets the
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56 exception parameter (its \dfn{associated value} or the second argument
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57 to \keyword{raise}, which is always a class instance if the exception
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58 type is a class object); \var{traceback} gets a traceback object (see
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59 the Reference Manual) which encapsulates the call stack at the point
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60 where the exception originally occurred.
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61 \obindex{traceback}
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62
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63 \strong{Warning:} assigning the \var{traceback} return value to a
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64 local variable in a function that is handling an exception will cause
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65 a circular reference. This will prevent anything referenced by a local
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66 variable in the same function or by the traceback from being garbage
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67 collected. Since most functions don't need access to the traceback,
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68 the best solution is to use something like
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69 \code{type, value = sys.exc_info()[:2]}
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70 to extract only the exception type and value. If you do need the
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71 traceback, make sure to delete it after use (best done with a
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72 \keyword{try} ... \keyword{finally} statement) or to call
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73 \function{exc_info()} in a function that does not itself handle an
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74 exception.
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75 \end{funcdesc}
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76
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77 \begin{datadesc}{exc_type}
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78 \dataline{exc_value}
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79 \dataline{exc_traceback}
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80 \deprecated {1.5}
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81 {Use \function{exc_info()} instead.}
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82 Since they are global variables, they are not specific to the current
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83 thread, so their use is not safe in a multi-threaded program. When no
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84 exception is being handled, \code{exc_type} is set to \code{None} and
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85 the other two are undefined.
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86 \end{datadesc}
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87
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88 \begin{datadesc}{exec_prefix}
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89 A string giving the site-specific directory prefix where the
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90 platform-dependent Python files are installed; by default, this is
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91 also \code{'/usr/local'}. This can be set at build time with the
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92 \programopt{-}\programopt{-exec-prefix} argument to the
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93 \program{configure} script. Specifically, all configuration files
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94 (e.g. the \file{config.h} header file) are installed in the directory
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95 \code{exec_prefix + '/lib/python\var{version}/config'}, and shared
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96 library modules are installed in \code{exec_prefix +
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97 '/lib/python\var{version}/lib-dynload'}, where \var{version} is equal
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98 to \code{version[:3]}.
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99 \end{datadesc}
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100
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101 \begin{datadesc}{executable}
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102 A string giving the name of the executable binary for the Python
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103 interpreter, on systems where this makes sense.
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104 \end{datadesc}
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105
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106 \begin{funcdesc}{exit}{\optional{arg}}
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107 Exit from Python. This is implemented by raising the
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108 \exception{SystemExit} exception, so cleanup actions specified by
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109 finally clauses of \keyword{try} statements are honored, and it is
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110 possible to intercept the exit attempt at an outer level. The
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111 optional argument \var{arg} can be an integer giving the exit status
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112 (defaulting to zero), or another type of object. If it is an integer,
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113 zero is considered ``successful termination'' and any nonzero value is
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114 considered ``abnormal termination'' by shells and the like. Most
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115 systems require it to be in the range 0-127, and produce undefined
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116 results otherwise. Some systems have a convention for assigning
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117 specific meanings to specific exit codes, but these are generally
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118 underdeveloped; Unix programs generally use 2 for command line syntax
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119 errors and 1 for all other kind of errors. If another type of object
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120 is passed, \code{None} is equivalent to passing zero, and any other
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121 object is printed to \code{sys.stderr} and results in an exit code of
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122 1. In particular, \code{sys.exit("some error message")} is a quick
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123 way to exit a program when an error occurs.
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124 \end{funcdesc}
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125
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126 \begin{datadesc}{exitfunc}
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127 This value is not actually defined by the module, but can be set by
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128 the user (or by a program) to specify a clean-up action at program
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129 exit. When set, it should be a parameterless function. This function
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130 will be called when the interpreter exits. Note: the exit function
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131 is not called when the program is killed by a signal, when a Python
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132 fatal internal error is detected, or when \code{os._exit()} is called.
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133 \end{datadesc}
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134
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135 \begin{funcdesc}{getrefcount}{object}
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136 Return the reference count of the \var{object}. The count returned is
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137 generally one higher than you might expect, because it includes the
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138 (temporary) reference as an argument to \function{getrefcount()}.
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139 \end{funcdesc}
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140
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141 \begin{datadesc}{last_type}
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142 \dataline{last_value}
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143 \dataline{last_traceback}
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144 These three variables are not always defined; they are set when an
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145 exception is not handled and the interpreter prints an error message
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146 and a stack traceback. Their intended use is to allow an interactive
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147 user to import a debugger module and engage in post-mortem debugging
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148 without having to re-execute the command that caused the error.
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149 (Typical use is \samp{import pdb; pdb.pm()} to enter the post-mortem
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150 debugger; see the chapter ``The Python Debugger'' for more
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151 information.)
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152 \refstmodindex{pdb}
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153
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154 The meaning of the variables is the same
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155 as that of the return values from \function{exc_info()} above.
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156 (Since there is only one interactive thread, thread-safety is not a
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157 concern for these variables, unlike for \code{exc_type} etc.)
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158 \end{datadesc}
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159
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160 \begin{datadesc}{maxint}
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161 The largest positive integer supported by Python's regular integer
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162 type. This is at least 2**31-1. The largest negative integer is
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163 \code{-maxint-1} -- the asymmetry results from the use of 2's
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164 complement binary arithmetic.
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165 \end{datadesc}
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166
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167 \begin{datadesc}{modules}
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168 This is a dictionary that maps module names to modules which have
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169 already been loaded. This can be manipulated to force reloading of
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170 modules and other tricks. Note that removing a module from this
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171 dictionary is \emph{not} the same as calling
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172 \function{reload()}\bifuncindex{reload} on the corresponding module
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173 object.
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174 \end{datadesc}
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175
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176 \begin{datadesc}{path}
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177 \indexiii{module}{search}{path}
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178 A list of strings that specifies the search path for modules.
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179 Initialized from the environment variable \envvar{PYTHONPATH}, or an
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180 installation-dependent default.
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181
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182 The first item of this list, \code{path[0]}, is the
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183 directory containing the script that was used to invoke the Python
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184 interpreter. If the script directory is not available (e.g. if the
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185 interpreter is invoked interactively or if the script is read from
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186 standard input), \code{path[0]} is the empty string, which directs
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187 Python to search modules in the current directory first. Notice that
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188 the script directory is inserted \emph{before} the entries inserted as
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189 a result of \envvar{PYTHONPATH}.
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190 \end{datadesc}
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191
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192 \begin{datadesc}{platform}
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193 This string contains a platform identifier, e.g. \code{'sunos5'} or
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194 \code{'linux1'}. This can be used to append platform-specific
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195 components to \code{path}, for instance.
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196 \end{datadesc}
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197
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198 \begin{datadesc}{prefix}
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199 A string giving the site-specific directory prefix where the platform
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200 independent Python files are installed; by default, this is the string
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201 \code{'/usr/local'}. This can be set at build time with the
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202 \programopt{-}\programopt{-prefix} argument to the
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203 \program{configure} script. The main collection of Python library
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204 modules is installed in the directory \code{prefix +
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205 '/lib/python\var{version}'} while the platform independent header
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206 files (all except \file{config.h}) are stored in \code{prefix +
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207 '/include/python\var{version}'}, where \var{version} is equal to
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208 \code{version[:3]}.
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209 \end{datadesc}
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210
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211 \begin{datadesc}{ps1}
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212 \dataline{ps2}
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213 \index{interpreter prompts}
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214 \index{prompts, interpreter}
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215 Strings specifying the primary and secondary prompt of the
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216 interpreter. These are only defined if the interpreter is in
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217 interactive mode. Their initial values in this case are
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218 \code{'>>> '} and \code{'... '}. If a non-string object is assigned
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219 to either variable, its \function{str()} is re-evaluated each time
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220 the interpreter prepares to read a new interactive command; this can
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221 be used to implement a dynamic prompt.
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222 \end{datadesc}
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223
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224 \begin{funcdesc}{setcheckinterval}{interval}
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225 Set the interpreter's ``check interval''. This integer value
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226 determines how often the interpreter checks for periodic things such
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227 as thread switches and signal handlers. The default is \code{10}, meaning
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228 the check is performed every 10 Python virtual instructions. Setting
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229 it to a larger value may increase performance for programs using
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230 threads. Setting it to a value \code{<=} 0 checks every virtual instruction,
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231 maximizing responsiveness as well as overhead.
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232 \end{funcdesc}
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233
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234 \begin{funcdesc}{setprofile}{profilefunc}
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235 Set the system's profile function, which allows you to implement a
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236 Python source code profiler in Python. See the chapter on the
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237 Python Profiler. The system's profile function
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238 is called similarly to the system's trace function (see
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239 \function{settrace()}), but it isn't called for each executed line of
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240 code (only on call and return and when an exception occurs). Also,
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241 its return value is not used, so it can just return \code{None}.
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242 \end{funcdesc}
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243 \index{profile function}
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244 \index{profiler}
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245
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246 \begin{funcdesc}{settrace}{tracefunc}
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247 Set the system's trace function, which allows you to implement a
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248 Python source code debugger in Python. See section ``How It Works''
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249 in the chapter on the Python Debugger.
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250 \end{funcdesc}
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251 \index{trace function}
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252 \index{debugger}
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253
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254 \begin{datadesc}{stdin}
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255 \dataline{stdout}
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256 \dataline{stderr}
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257 File objects corresponding to the interpreter's standard input,
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258 output and error streams. \code{stdin} is used for all
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259 interpreter input except for scripts but including calls to
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260 \function{input()}\bifuncindex{input} and
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261 \function{raw_input()}\bifuncindex{raw_input}. \code{stdout} is used
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262 for the output of \keyword{print} and expression statements and for the
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263 prompts of \function{input()} and \function{raw_input()}. The interpreter's
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264 own prompts and (almost all of) its error messages go to
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265 \code{stderr}. \code{stdout} and \code{stderr} needn't
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266 be built-in file objects: any object is acceptable as long as it has
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267 a \method{write()} method that takes a string argument. (Changing these
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268 objects doesn't affect the standard I/O streams of processes
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269 executed by \function{os.popen()}, \function{os.system()} or the
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270 \function{exec*()} family of functions in the \refmodule{os} module.)
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271 \refstmodindex{os}
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272 \end{datadesc}
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273
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274 \begin{datadesc}{__stdin__}
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275 \dataline{__stdout__}
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276 \dataline{__stderr__}
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277 These objects contain the original values of \code{stdin},
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278 \code{stderr} and \code{stdout} at the start of the program. They are
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279 used during finalization, and could be useful to restore the actual
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280 files to known working file objects in case they have been overwritten
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281 with a broken object.
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282 \end{datadesc}
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283
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284 \begin{datadesc}{tracebacklimit}
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285 When this variable is set to an integer value, it determines the
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286 maximum number of levels of traceback information printed when an
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287 unhandled exception occurs. The default is \code{1000}. When set to
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288 0 or less, all traceback information is suppressed and only the
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289 exception type and value are printed.
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290 \end{datadesc}
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291
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292 \begin{datadesc}{version}
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293 A string containing the version number of the Python interpreter plus
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294 additional information on the build number and compiler used. It has
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295 a value of the form \code{'\var{version} (\#\var{build_number},
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296 \var{build_date}, \var{build_time}) [\var{compiler}]'}. The first
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297 three characters are used to identify the version in the installation
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298 directories (where appropriate on each platform). An example:
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299
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300 \begin{verbatim}
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301 >>> import sys
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302 >>> sys.version
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303 '1.5.2 (#0 Apr 13 1999, 10:51:12) [MSC 32 bit (Intel)]'
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304 \end{verbatim}
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305 \end{datadesc}
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306
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307 \begin{datadesc}{winver}
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308 The version number used to form registry keys on Windows platforms.
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309 This is stored as string resource 1000 in the Python DLL. The value
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310 is normally the first three characters of \constant{version}. It is
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311 provided in the \module{sys} module for informational purposes;
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312 modifying this value has no effect on the registry keys used by
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313 Python.
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314 Availability: Windows.
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315 \end{datadesc}