Doc/libsys.tex

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