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author Guido van Rossum <guido@python.org>
date Thu, 17 Jul 1997 16:29:42 +0000
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1 \section{Built-in Module \sectcode{sys}}
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2
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3 \bimodindex{sys}
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4 This module provides access to some variables used or maintained by the
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5 interpreter and to functions that interact strongly with the interpreter.
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6 It is always available.
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7
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8 \renewcommand{\indexsubitem}{(in module sys)}
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9
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10 \begin{datadesc}{argv}
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11 The list of command line arguments passed to a Python script.
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12 \code{sys.argv[0]} is the script name (it is operating system
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13 dependent whether this is a full pathname or not).
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14 If the command was executed using the \samp{-c} command line option
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15 to the interpreter, \code{sys.argv[0]} is set to the string
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16 \code{"-c"}.
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17 If no script name was passed to the Python interpreter,
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18 \code{sys.argv} has zero length.
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19 \end{datadesc}
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20
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21 \begin{datadesc}{builtin_module_names}
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22 A tuple of strings giving the names of all modules that are compiled
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23 into this Python interpreter. (This information is not available in
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24 any other way --- \code{sys.modules.keys()} only lists the imported
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25 modules.)
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26 \end{datadesc}
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27
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28 \begin{datadesc}{exc_type}
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29 \dataline{exc_value}
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30 \dataline{exc_traceback}
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31 These three variables are not always defined; they are set when an
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32 exception handler (an \code{except} clause of a \code{try} statement) is
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33 invoked. Their meaning is: \code{exc_type} gets the exception type of
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34 the exception being handled; \code{exc_value} gets the exception
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35 parameter (its \dfn{associated value} or the second argument to
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36 \code{raise}); \code{exc_traceback} gets a traceback object (see the
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37 Reference Manual) which
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38 encapsulates the call stack at the point where the exception
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39 originally occurred.
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40 \obindex{traceback}
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41 \end{datadesc}
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42
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43 \begin{datadesc}{exec_prefix}
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44 A string giving the site-specific
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45 directory prefix where the platform-dependent Python files are
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46 installed; by default, this is also \code{"/usr/local"}. This can be
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47 set at build time with the \code{--exec-prefix} argument to the
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48 \code{configure} script. Specifically, all configuration files
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49 (e.g. the \code{config.h} header file) are installed in the directory
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50 \code{sys.exec_prefix+"/lib/python\emph{VER}/config"}, and shared library
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51 modules are installed in
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52 \code{sys.exec_prefix+"/lib/python\emph{VER}/sharedmodules"},
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53 where \emph{VER} is equal to \code{sys.version[:3]}.
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54 \end{datadesc}
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55
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56 \begin{funcdesc}{exit}{n}
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57 Exit from Python with numeric exit status \var{n}. This is
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58 implemented by raising the \code{SystemExit} exception, so cleanup
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59 actions specified by \code{finally} clauses of \code{try} statements
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60 are honored, and it is possible to catch the exit attempt at an outer
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61 level.
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62 \end{funcdesc}
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63
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64 \begin{datadesc}{exitfunc}
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65 This value is not actually defined by the module, but can be set by
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66 the user (or by a program) to specify a clean-up action at program
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67 exit. When set, it should be a parameterless function. This function
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68 will be called when the interpreter exits in any way (except when a
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69 fatal error occurs: in that case the interpreter's internal state
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70 cannot be trusted).
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71 \end{datadesc}
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72
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73 \begin{datadesc}{last_type}
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74 \dataline{last_value}
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75 \dataline{last_traceback}
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76 These three variables are not always defined; they are set when an
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77 exception is not handled and the interpreter prints an error message
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78 and a stack traceback. Their intended use is to allow an interactive
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79 user to import a debugger module and engage in post-mortem debugging
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80 without having to re-execute the command that caused the error (which
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81 may be hard to reproduce). The meaning of the variables is the same
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82 as that of \code{exc_type}, \code{exc_value} and \code{exc_tracaback},
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83 respectively.
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84 \end{datadesc}
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85
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86 \begin{datadesc}{modules}
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87 Gives the list of modules that have already been loaded.
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88 This can be manipulated to force reloading of modules and other tricks.
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89 \end{datadesc}
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90
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91 \begin{datadesc}{path}
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92 A list of strings that specifies the search path for modules.
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93 Initialized from the environment variable \code{PYTHONPATH}, or an
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94 installation-dependent default.
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96 The first item of this list, \code{sys.path[0]}, is the
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97 directory containing the script that was used to invoke the Python
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98 interpreter. If the script directory is not available (e.g. if the
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99 interpreter is invoked interactively or if the script is read from
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100 standard input), \code{sys.path[0]} is the empty string, which directs
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101 Python to search modules in the current directory first. Notice that
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102 the script directory is inserted {\em before} the entries inserted as
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103 a result of \code{\$PYTHONPATH}.
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104 \end{datadesc}
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105
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106 \begin{datadesc}{platform}
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107 This string contains a platform identifier, e.g. \code{sunos5} or
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108 \code{linux1}. This can be used to append platform-specific
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109 components to \code{sys.path}, for instance.
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110 \end{datadesc}
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112 \begin{datadesc}{prefix}
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113 A string giving the site-specific directory prefix where the platform
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114 independent Python files are installed; by default, this is the string
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115 \code{"/usr/local"}. This can be set at build time with the
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116 \code{--prefix} argument to the \code{configure} script. The main
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117 collection of Python library modules is installed in the directory
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118 \code{sys.prefix+"/lib/python\emph{VER}"} while the platform
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119 independent header files (all except \code{config.h}) are stored in
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120 \code{sys.prefix+"/include/python\emph{VER}"},
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121 where \emph{VER} is equal to \code{sys.version[:3]}.
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122
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123 \end{datadesc}
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124
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125 \begin{datadesc}{ps1}
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126 \dataline{ps2}
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127 Strings specifying the primary and secondary prompt of the
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128 interpreter. These are only defined if the interpreter is in
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129 interactive mode. Their initial values in this case are
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130 \code{'>>> '} and \code{'... '}.
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131 \end{datadesc}
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132
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133 \begin{funcdesc}{setcheckinterval}{interval}
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134 Set the interpreter's ``check interval''. This integer value
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135 determines how often the interpreter checks for periodic things such
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136 as thread switches and signal handlers. The default is 10, meaning
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137 the check is performed every 10 Python virtual instructions. Setting
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138 it to a larger value may increase performance for programs using
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139 threads. Setting it to a value $\leq 0$ checks every virtual instruction,
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140 maximizing responsiveness as well as overhead.
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141 \end{funcdesc}
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142
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143 \begin{funcdesc}{settrace}{tracefunc}
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144 Set the system's trace function, which allows you to implement a
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145 Python source code debugger in Python. See section ``How It Works''
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146 in the chapter on the Python Debugger.
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147 \end{funcdesc}
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148 \index{trace function}
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149 \index{debugger}
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150
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151 \begin{funcdesc}{setprofile}{profilefunc}
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152 Set the system's profile function, which allows you to implement a
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153 Python source code profiler in Python. See the chapter on the
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154 Python Profiler. The system's profile function
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155 is called similarly to the system's trace function (see
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156 \code{sys.settrace}), but it isn't called for each executed line of
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157 code (only on call and return and when an exception occurs). Also,
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158 its return value is not used, so it can just return \code{None}.
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159 \end{funcdesc}
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160 \index{profile function}
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161 \index{profiler}
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162
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163 \begin{datadesc}{stdin}
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164 \dataline{stdout}
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165 \dataline{stderr}
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166 File objects corresponding to the interpreter's standard input,
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167 output and error streams. \code{sys.stdin} is used for all
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168 interpreter input except for scripts but including calls to
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169 \code{input()} and \code{raw_input()}. \code{sys.stdout} is used
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170 for the output of \code{print} and expression statements and for the
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171 prompts of \code{input()} and \code{raw_input()}. The interpreter's
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172 own prompts and (almost all of) its error messages go to
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173 \code{sys.stderr}. \code{sys.stdout} and \code{sys.stderr} needn't
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174 be built-in file objects: any object is acceptable as long as it has
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175 a \code{write} method that takes a string argument. (Changing these
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176 objects doesn't affect the standard I/O streams of processes
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177 executed by \code{popen()}, \code{system()} or the \code{exec*()}
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178 family of functions in the \code{os} module.)
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179 \stmodindex{os}
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180 \end{datadesc}
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181
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182 \begin{datadesc}{tracebacklimit}
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183 When this variable is set to an integer value, it determines the
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184 maximum number of levels of traceback information printed when an
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185 unhandled exception occurs. The default is 1000. When set to 0 or
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186 less, all traceback information is suppressed and only the exception
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187 type and value are printed.
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188 \end{datadesc}
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189
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190 \begin{datadesc}{version}
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191 A string containing the version number of the Python interpreter.
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192 \end{datadesc}