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System-specific parameters and functions
========================================

Access system-specific parameters and functions.

This module provides access to some variables used or maintained by the
interpreter and to functions that interact strongly with the
interpreter.  It is always available.

`argv'
     The list of command line arguments passed to a Python script.
     `argv[0]' is the script name (it is operating system dependent
     whether this is a full pathname or not).  If the command was
     executed using the `-c' command line option to the interpreter,
     `argv[0]' is set to the string `'-c''.  If no script name was
     passed to the Python interpreter, `argv' has zero length.

`byteorder'
     An indicator of the native byte order.  This will have the value
     `'big'' on big-endian (most-signigicant byte first) platforms, and
     `'little'' on little-endian (least-significant byte first)
     platforms.  _Added in Python version 2.0_

`builtin_module_names'
     A tuple of strings giving the names of all modules that are
     compiled into this Python interpreter.  (This information is not
     available in any other way -- `modules.keys()' only lists the
     imported modules.)

`copyright'
     A string containing the copyright pertaining to the Python
     interpreter.

`dllhandle'
     Integer specifying the handle of the Python DLL.  Availability:
     Windows.

`displayhook(VALUE)'
     If VALUE is not `None', this function prints it to `sys.stdout',
     and saves it in `__builtin__._'.

     `sys.displayhook' is called on the result of evaluating an
     expression entered in an interactive Python session.  The display
     of these values can be customized by assigning another
     one-argument function to `sys.displayhook'.

`excepthook(TYPE, VALUE, TRACEBACK)'
     This function prints out a given traceback and exception to
     `sys.stderr'.

     When an exception is raised and uncaught, the interpreter calls
     `sys.excepthook' with three arguments, the exception class,
     exception instance, and a traceback object.  In an interactive
     session this happens just before control is returned to the
     prompt; in a Python program this happens just before the program
     exits.  The handling of such top-level exceptions can be
     customized by assigning another three-argument function to
     `sys.excepthook'.

`__displayhook__'

`__excepthook__'
     These objects contain the original values of `displayhook' and
     `excepthook' at the start of the program.  They are saved so that
     `displayhook' and `excepthook' can be restored in case they happen
     to get replaced with broken objects.

`exc_info()'
     This function returns a tuple of three values that give information
     about the exception that is currently being handled.  The
     information returned is specific both to the current thread and to
     the current stack frame.  If the current stack frame is not
     handling an exception, the information is taken from the calling
     stack frame, or its caller, and so on until a stack frame is found
     that is handling an exception.  Here, "handling an exception" is
     defined as "executing or having executed an except clause."  For
     any stack frame, only information about the most recently handled
     exception is accessible.

     If no exception is being handled anywhere on the stack, a tuple
     containing three `None' values is returned.  Otherwise, the values
     returned are `(TYPE, VALUE, TRACEBACK)'.  Their meaning is: TYPE
     gets the exception type of the exception being handled (a string
     or class object); VALUE gets the exception parameter (its
     "associated value" or the second argument to `raise', which is
     always a class instance if the exception type is a class object);
     TRACEBACK gets a traceback object (see the Reference Manual) which
     encapsulates the call stack at the point where the exception
     originally occurred.

     *Warning:* assigning the TRACEBACK return value to a local
     variable in a function that is handling an exception will cause a
     circular reference. This will prevent anything referenced by a
     local variable in the same function or by the traceback from being
     garbage collected.  Since most functions don't need access to the
     traceback, the best solution is to use something like `exctype,
     value = sys.exc_info()[:2]' to extract only the exception type and
     value.  If you do need the traceback, make sure to delete it after
     use (best done with a `try' ... `finally' statement) or to call
     `exc_info()' in a function that does not itself handle an
     exception.

`exc_type'

`exc_value'

`exc_traceback'
     _This is deprecated in Python 1.5.  Use `exc_info()' instead._
     Since they are global variables, they are not specific to the
     current thread, so their use is not safe in a multi-threaded
     program.  When no exception is being handled, `exc_type' is set to
     `None' and the other two are undefined.

`exec_prefix'
     A string giving the site-specific directory prefix where the
     platform-dependent Python files are installed; by default, this is
     also `'/usr/local''.  This can be set at build time with the
     `--exec-prefix' argument to the `configure' script.  Specifically,
     all configuration files (e.g. the `config.h' header file) are
     installed in the directory `exec_prefix +
     '/lib/pythonVERSION/config'', and shared library modules are
     installed in `exec_prefix + '/lib/pythonVERSION/lib-dynload'',
     where VERSION is equal to `version[:3]'.

`executable'
     A string giving the name of the executable binary for the Python
     interpreter, on systems where this makes sense.

`exit([arg])'
     Exit from Python.  This is implemented by raising the `SystemExit'
     exception, so cleanup actions specified by finally clauses of
     `try' statements are honored, and it is possible to intercept the
     exit attempt at an outer level.  The optional argument ARG can be
     an integer giving the exit status (defaulting to zero), or another
     type of object.  If it is an integer, zero is considered
     "successful termination" and any nonzero value is considered
     "abnormal termination" by shells and the like.  Most systems
     require it to be in the range 0-127, and produce undefined results
     otherwise.  Some systems have a convention for assigning specific
     meanings to specific exit codes, but these are generally
     underdeveloped; Unix programs generally use 2 for command line
     syntax errors and 1 for all other kind of errors.  If another type
     of object is passed, `None' is equivalent to passing zero, and any
     other object is printed to `sys.stderr' and results in an exit
     code of 1.  In particular, `sys.exit("some error message")' is a
     quick way to exit a program when an error occurs.

`exitfunc'
     This value is not actually defined by the module, but can be set by
     the user (or by a program) to specify a clean-up action at program
     exit.  When set, it should be a parameterless function.  This
     function will be called when the interpreter exits.  Only one
     function may be installed in this way; to allow multiple functions
     which will be called at termination, use the `atexit' module.
     Note: the exit function is not called when the program is killed
     by a signal, when a Python fatal internal error is detected, or
     when `os._exit()' is called.

`getdefaultencoding()'
     Return the name of the current default string encoding used by the
     Unicode implementation.  _Added in Python version 2.0_

`getrefcount(object)'
     Return the reference count of the OBJECT.  The count returned is
     generally one higher than you might expect, because it includes the
     (temporary) reference as an argument to `getrefcount()'.

`getrecursionlimit()'
     Return the current value of the recursion limit, the maximum depth
     of the Python interpreter stack.  This limit prevents infinite
     recursion from causing an overflow of the C stack and crashing
     Python.  It can be set by `setrecursionlimit()'.

`_getframe([depth])'
     Return a frame object from the call stack.  If optional integer
     DEPTH is given, return the frame object that many calls below the
     top of the stack.  If that is deeper than the call stack,
     `ValueError' is raised.  The default for DEPTH is zero, returning
     the frame at the top of the call stack.

     This function should be used for internal and specialized purposes
     only.

`hexversion'
     The version number encoded as a single integer.  This is
     guaranteed to increase with each version, including proper support
     for non-production releases.  For example, to test that the Python
     interpreter is at least version 1.5.2, use:

          if sys.hexversion >= 0x010502F0:
              # use some advanced feature
              ...
          else:
              # use an alternative implementation or warn the user
              ...

     This is called `hexversion' since it only really looks meaningful
     when viewed as the result of passing it to the built-in `hex()'
     function.  The `version_info' value may be used for a more
     human-friendly encoding of the same information.  _Added in Python
     version 1.5.2_

`last_type'

`last_value'

`last_traceback'
     These three variables are not always defined; they are set when an
     exception is not handled and the interpreter prints an error
     message and a stack traceback.  Their intended use is to allow an
     interactive user to import a debugger module and engage in
     post-mortem debugging without having to re-execute the command
     that caused the error.  (Typical use is `import pdb; pdb.pm()' to
     enter the post-mortem debugger; see the chapter "The Python
     Debugger" for more information.)

     The meaning of the variables is the same as that of the return
     values from `exc_info()' above.  (Since there is only one
     interactive thread, thread-safety is not a concern for these
     variables, unlike for `exc_type' etc.)

`maxint'
     The largest positive integer supported by Python's regular integer
     type.  This is at least 2**31-1.  The largest negative integer is
     `-maxint-1' - the asymmetry results from the use of 2's complement
     binary arithmetic.

`modules'
     This is a dictionary that maps module names to modules which have
     already been loaded.  This can be manipulated to force reloading of
     modules and other tricks.  Note that removing a module from this
     dictionary is _not_ the same as calling `reload()'  on the
     corresponding module object.

`path'
     A list of strings that specifies the search path for modules.
     Initialized from the environment variable `PYTHONPATH', or an
     installation-dependent default.

     The first item of this list, `path[0]', is the directory
     containing the script that was used to invoke the Python
     interpreter.  If the script directory is not available (e.g.  if
     the interpreter is invoked interactively or if the script is read
     from standard input), `path[0]' is the empty string, which directs
     Python to search modules in the current directory first.  Notice
     that the script directory is inserted _before_ the entries
     inserted as a result of `PYTHONPATH'.

`platform'
     This string contains a platform identifier, e.g. `'sunos5'' or
     `'linux1''.  This can be used to append platform-specific
     components to `path', for instance.

`prefix'
     A string giving the site-specific directory prefix where the
     platform independent Python files are installed; by default, this
     is the string `'/usr/local''.  This can be set at build time with
     the `--prefix' argument to the `configure' script.  The main
     collection of Python library modules is installed in the directory
     `prefix + '/lib/pythonVERSION'' while the platform independent
     header files (all except `config.h') are stored in `prefix +
     '/include/pythonVERSION'', where VERSION is equal to `version[:3]'.

`ps1'

`ps2'
     Strings specifying the primary and secondary prompt of the
     interpreter.  These are only defined if the interpreter is in
     interactive mode.  Their initial values in this case are `'>`>'>
     '' and `'... ''.  If a non-string object is assigned to either
     variable, its `str()' is re-evaluated each time the interpreter
     prepares to read a new interactive command; this can be used to
     implement a dynamic prompt.

`setcheckinterval(interval)'
     Set the interpreter's "check interval".  This integer value
     determines how often the interpreter checks for periodic things
     such as thread switches and signal handlers.  The default is `10',
     meaning the check is performed every 10 Python virtual
     instructions.  Setting it to a larger value may increase
     performance for programs using threads.  Setting it to a value
     `<=' 0 checks every virtual instruction, maximizing responsiveness
     as well as overhead.

`setdefaultencoding(name)'
     Set the current default string encoding used by the Unicode
     implementation.  If NAME does not match any available encoding,
     `LookupError' is raised.  This function is only intended to be
     used by the `site' module implementation and, where needed, by
     `sitecustomize'.  Once used by the `site' module, it is removed
     from the `sys' module's namespace.  _Added in Python version 2.0_

`setprofile(profilefunc)'
     Set the system's profile function, which allows you to implement a
     Python source code profiler in Python.  See the chapter on the
     Python Profiler.  The system's profile function is called
     similarly to the system's trace function (see `settrace()'), but
     it isn't called for each executed line of code (only on call and
     return and when an exception occurs).  Also, its return value is
     not used, so it can just return `None'.

`setrecursionlimit(limit)'
     Set the maximum depth of the Python interpreter stack to LIMIT.
     This limit prevents infinite recursion from causing an overflow of
     the C stack and crashing Python.

     The highest possible limit is platform-dependent.  A user may need
     to set the limit higher when she has a program that requires deep
     recursion and a platform that supports a higher limit.  This
     should be done with care, because a too-high limit can lead to a
     crash.

`settrace(tracefunc)'
     Set the system's trace function, which allows you to implement a
     Python source code debugger in Python.  See section "How It Works"
     in the chapter on the Python Debugger.

`stdin'

`stdout'

`stderr'
     File objects corresponding to the interpreter's standard input,
     output and error streams.  `stdin' is used for all interpreter
     input except for scripts but including calls to `input()'  and
     `raw_input()' .  `stdout' is used for the output of `print' and
     expression statements and for the prompts of `input()' and
     `raw_input()'.  The interpreter's own prompts and (almost all of)
     its error messages go to `stderr'.  `stdout' and `stderr' needn't
     be built-in file objects: any object is acceptable as long as it
     has a `write()' method that takes a string argument.  (Changing
     these objects doesn't affect the standard I/O streams of processes
     executed by `os.popen()', `os.system()' or the `exec*()' family of
     functions in the `os' module.)

`__stdin__'

`__stdout__'

`__stderr__'
     These objects contain the original values of `stdin', `stderr' and
     `stdout' at the start of the program.  They are used during
     finalization, and could be useful to restore the actual files to
     known working file objects in case they have been overwritten with
     a broken object.

`tracebacklimit'
     When this variable is set to an integer value, it determines the
     maximum number of levels of traceback information printed when an
     unhandled exception occurs.  The default is `1000'.  When set to 0
     or less, all traceback information is suppressed and only the
     exception type and value are printed.

`version'
     A string containing the version number of the Python interpreter
     plus additional information on the build number and compiler used.
     It has a value of the form `'VERSION (#BUILD_NUMBER, BUILD_DATE,
     BUILD_TIME) [COMPILER]''.  The first three characters are used to
     identify the version in the installation directories (where
     appropriate on each platform).  An example:

          >>> import sys
          >>> sys.version
          '1.5.2 (#0 Apr 13 1999, 10:51:12) [MSC 32 bit (Intel)]'

`version_info'
     A tuple containing the five components of the version number:
     MAJOR, MINOR, MICRO, RELEASELEVEL, and SERIAL.  All values except
     RELEASELEVEL are integers; the release level is `'alpha'',
     `'beta'', `'candidate'', or `'final''.  The `version_info' value
     corresponding to the Python version 2.0 is `(2, 0, 0, 'final', 0)'.
     _Added in Python version 2.0_

`winver'
     The version number used to form registry keys on Windows platforms.
     This is stored as string resource 1000 in the Python DLL.  The
     value is normally the first three characters of `version'.  It is
     provided in the `sys' module for informational purposes; modifying
     this value has no effect on the registry keys used by Python.
     Availability: Windows.


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