624 lines
27 KiB
ReStructuredText
624 lines
27 KiB
ReStructuredText
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:mod:`sys` --- System-specific parameters and functions
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=======================================================
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.. module:: sys
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:synopsis: Access system-specific parameters and functions.
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This module provides access to some variables used or maintained by the
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interpreter and to functions that interact strongly with the interpreter. It is
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always available.
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.. data:: argv
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The list of command line arguments passed to a Python script. ``argv[0]`` is the
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script name (it is operating system dependent whether this is a full pathname or
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not). If the command was executed using the :option:`-c` command line option to
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the interpreter, ``argv[0]`` is set to the string ``'-c'``. If no script name
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was passed to the Python interpreter, ``argv[0]`` is the empty string.
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To loop over the standard input, or the list of files given on the
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command line, see the :mod:`fileinput` module.
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.. data:: byteorder
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An indicator of the native byte order. This will have the value ``'big'`` on
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big-endian (most-significant byte first) platforms, and ``'little'`` on
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little-endian (least-significant byte first) platforms.
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.. data:: subversion
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A triple (repo, branch, version) representing the Subversion information of the
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Python interpreter. *repo* is the name of the repository, ``'CPython'``.
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*branch* is a string of one of the forms ``'trunk'``, ``'branches/name'`` or
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``'tags/name'``. *version* is the output of ``svnversion``, if the interpreter
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was built from a Subversion checkout; it contains the revision number (range)
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and possibly a trailing 'M' if there were local modifications. If the tree was
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exported (or svnversion was not available), it is the revision of
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``Include/patchlevel.h`` if the branch is a tag. Otherwise, it is ``None``.
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.. data:: builtin_module_names
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A tuple of strings giving the names of all modules that are compiled into this
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Python interpreter. (This information is not available in any other way ---
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``modules.keys()`` only lists the imported modules.)
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.. data:: copyright
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A string containing the copyright pertaining to the Python interpreter.
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.. function:: _current_frames()
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Return a dictionary mapping each thread's identifier to the topmost stack frame
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currently active in that thread at the time the function is called. Note that
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functions in the :mod:`traceback` module can build the call stack given such a
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frame.
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This is most useful for debugging deadlock: this function does not require the
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deadlocked threads' cooperation, and such threads' call stacks are frozen for as
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long as they remain deadlocked. The frame returned for a non-deadlocked thread
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may bear no relationship to that thread's current activity by the time calling
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code examines the frame.
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This function should be used for internal and specialized purposes only.
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.. data:: dllhandle
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Integer specifying the handle of the Python DLL. Availability: Windows.
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.. function:: displayhook(value)
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If *value* is not ``None``, this function prints it to ``sys.stdout``, and saves
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it in ``builtins._``.
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``sys.displayhook`` is called on the result of evaluating an expression entered
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in an interactive Python session. The display of these values can be customized
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by assigning another one-argument function to ``sys.displayhook``.
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.. function:: excepthook(type, value, traceback)
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This function prints out a given traceback and exception to ``sys.stderr``.
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When an exception is raised and uncaught, the interpreter calls
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``sys.excepthook`` with three arguments, the exception class, exception
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instance, and a traceback object. In an interactive session this happens just
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before control is returned to the prompt; in a Python program this happens just
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before the program exits. The handling of such top-level exceptions can be
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customized by assigning another three-argument function to ``sys.excepthook``.
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.. data:: __displayhook__
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__excepthook__
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These objects contain the original values of ``displayhook`` and ``excepthook``
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at the start of the program. They are saved so that ``displayhook`` and
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``excepthook`` can be restored in case they happen to get replaced with broken
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objects.
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.. function:: exc_info()
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This function returns a tuple of three values that give information about the
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exception that is currently being handled. The information returned is specific
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both to the current thread and to the current stack frame. If the current stack
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frame is not handling an exception, the information is taken from the calling
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stack frame, or its caller, and so on until a stack frame is found that is
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handling an exception. Here, "handling an exception" is defined as "executing
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or having executed an except clause." For any stack frame, only information
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about the most recently handled exception is accessible.
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.. index:: object: traceback
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If no exception is being handled anywhere on the stack, a tuple containing three
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``None`` values is returned. Otherwise, the values returned are ``(type, value,
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traceback)``. Their meaning is: *type* gets the exception type of the exception
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being handled (a class object); *value* gets the exception parameter (its
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:dfn:`associated value` or the second argument to :keyword:`raise`, which is
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always a class instance if the exception type is a class object); *traceback*
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gets a traceback object (see the Reference Manual) which encapsulates the call
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stack at the point where the exception originally occurred.
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.. warning::
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Assigning the *traceback* return value to a local variable in a function that is
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handling an exception will cause a circular reference. This will prevent
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anything referenced by a local variable in the same function or by the traceback
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from being garbage collected. Since most functions don't need access to the
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traceback, the best solution is to use something like ``exctype, value =
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sys.exc_info()[:2]`` to extract only the exception type and value. If you do
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need the traceback, make sure to delete it after use (best done with a
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:keyword:`try` ... :keyword:`finally` statement) or to call :func:`exc_info` in
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a function that does not itself handle an exception.
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.. note::
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Beginning with Python 2.2, such cycles are automatically reclaimed when garbage
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collection is enabled and they become unreachable, but it remains more efficient
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to avoid creating cycles.
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.. data:: exec_prefix
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A string giving the site-specific directory prefix where the platform-dependent
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Python files are installed; by default, this is also ``'/usr/local'``. This can
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be set at build time with the :option:`--exec-prefix` argument to the
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:program:`configure` script. Specifically, all configuration files (e.g. the
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:file:`pyconfig.h` header file) are installed in the directory ``exec_prefix +
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'/lib/pythonversion/config'``, and shared library modules are installed in
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``exec_prefix + '/lib/pythonversion/lib-dynload'``, where *version* is equal to
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``version[:3]``.
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.. data:: executable
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A string giving the name of the executable binary for the Python interpreter, on
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systems where this makes sense.
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.. function:: exit([arg])
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Exit from Python. This is implemented by raising the :exc:`SystemExit`
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exception, so cleanup actions specified by finally clauses of :keyword:`try`
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statements are honored, and it is possible to intercept the exit attempt at an
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outer level. The optional argument *arg* can be an integer giving the exit
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status (defaulting to zero), or another type of object. If it is an integer,
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zero is considered "successful termination" and any nonzero value is considered
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"abnormal termination" by shells and the like. Most systems require it to be in
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the range 0-127, and produce undefined results otherwise. Some systems have a
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convention for assigning specific meanings to specific exit codes, but these are
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generally underdeveloped; Unix programs generally use 2 for command line syntax
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errors and 1 for all other kind of errors. If another type of object is passed,
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``None`` is equivalent to passing zero, and any other object is printed to
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``sys.stderr`` and results in an exit code of 1. In particular,
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``sys.exit("some error message")`` is a quick way to exit a program when an
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error occurs.
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.. data:: float_info
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A dict holding information about the float type. It contains low level
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information about the precision and internal representation. Please study
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your system's :file:`float.h` for more information.
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+---------------------+--------------------------------------------------+
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| key | explanation |
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+=====================+==================================================+
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| :const:`epsilon` | Difference between 1 and the next representable |
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| | floating point number |
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+---------------------+--------------------------------------------------+
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| :const:`dig` | digits (see :file:`float.h`) |
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+---------------------+--------------------------------------------------+
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| :const:`mant_dig` | mantissa digits (see :file:`float.h`) |
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+---------------------+--------------------------------------------------+
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| :const:`max` | maximum representable finite float |
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+---------------------+--------------------------------------------------+
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| :const:`max_exp` | maximum int e such that radix**(e-1) is in the |
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| | range of finite representable floats |
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+---------------------+--------------------------------------------------+
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| :const:`max_10_exp` | maximum int e such that 10**e is in the |
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| | range of finite representable floats |
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+---------------------+--------------------------------------------------+
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| :const:`min` | Minimum positive normalizer float |
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+---------------------+--------------------------------------------------+
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| :const:`min_exp` | minimum int e such that radix**(e-1) is a |
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| | normalized float |
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+---------------------+--------------------------------------------------+
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| :const:`min_10_exp` | minimum int e such that 10**e is a normalized |
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| | float |
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+---------------------+--------------------------------------------------+
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| :const:`radix` | radix of exponent |
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+---------------------+--------------------------------------------------+
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| :const:`rounds` | addition rounds (see :file:`float.h`) |
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+---------------------+--------------------------------------------------+
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.. note::
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The information in the table is simplified.
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.. function:: getcheckinterval()
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Return the interpreter's "check interval"; see :func:`setcheckinterval`.
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.. function:: getdefaultencoding()
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Return the name of the current default string encoding used by the Unicode
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implementation.
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.. function:: getdlopenflags()
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Return the current value of the flags that are used for :cfunc:`dlopen` calls.
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The flag constants are defined in the :mod:`dl` and :mod:`DLFCN` modules.
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Availability: Unix.
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.. function:: getfilesystemencoding()
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Return the name of the encoding used to convert Unicode filenames into system
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file names, or ``None`` if the system default encoding is used. The result value
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depends on the operating system:
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* On Windows 9x, the encoding is "mbcs".
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* On Mac OS X, the encoding is "utf-8".
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* On Unix, the encoding is the user's preference according to the result of
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nl_langinfo(CODESET), or :const:`None` if the ``nl_langinfo(CODESET)`` failed.
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* On Windows NT+, file names are Unicode natively, so no conversion is
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performed. :func:`getfilesystemencoding` still returns ``'mbcs'``, as this is
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the encoding that applications should use when they explicitly want to convert
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Unicode strings to byte strings that are equivalent when used as file names.
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.. function:: getrefcount(object)
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Return the reference count of the *object*. The count returned is generally one
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higher than you might expect, because it includes the (temporary) reference as
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an argument to :func:`getrefcount`.
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.. function:: getrecursionlimit()
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Return the current value of the recursion limit, the maximum depth of the Python
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interpreter stack. This limit prevents infinite recursion from causing an
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overflow of the C stack and crashing Python. It can be set by
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:func:`setrecursionlimit`.
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.. function:: _getframe([depth])
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Return a frame object from the call stack. If optional integer *depth* is
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given, return the frame object that many calls below the top of the stack. If
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that is deeper than the call stack, :exc:`ValueError` is raised. The default
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for *depth* is zero, returning the frame at the top of the call stack.
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This function should be used for internal and specialized purposes only.
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.. function:: getwindowsversion()
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Return a tuple containing five components, describing the Windows version
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currently running. The elements are *major*, *minor*, *build*, *platform*, and
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*text*. *text* contains a string while all other values are integers.
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*platform* may be one of the following values:
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+-----------------------------------------+-----------------------+
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| Constant | Platform |
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+=========================================+=======================+
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| :const:`0 (VER_PLATFORM_WIN32s)` | Win32s on Windows 3.1 |
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+-----------------------------------------+-----------------------+
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| :const:`1 (VER_PLATFORM_WIN32_WINDOWS)` | Windows 95/98/ME |
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+-----------------------------------------+-----------------------+
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| :const:`2 (VER_PLATFORM_WIN32_NT)` | Windows NT/2000/XP |
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+-----------------------------------------+-----------------------+
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| :const:`3 (VER_PLATFORM_WIN32_CE)` | Windows CE |
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+-----------------------------------------+-----------------------+
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This function wraps the Win32 :cfunc:`GetVersionEx` function; see the Microsoft
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documentation for more information about these fields.
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Availability: Windows.
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.. data:: hexversion
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The version number encoded as a single integer. This is guaranteed to increase
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with each version, including proper support for non-production releases. For
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example, to test that the Python interpreter is at least version 1.5.2, use::
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if sys.hexversion >= 0x010502F0:
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# use some advanced feature
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...
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else:
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# use an alternative implementation or warn the user
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...
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This is called ``hexversion`` since it only really looks meaningful when viewed
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as the result of passing it to the built-in :func:`hex` function. The
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``version_info`` value may be used for a more human-friendly encoding of the
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same information.
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.. function:: intern(string)
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Enter *string* in the table of "interned" strings and return the interned string
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-- which is *string* itself or a copy. Interning strings is useful to gain a
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little performance on dictionary lookup -- if the keys in a dictionary are
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interned, and the lookup key is interned, the key comparisons (after hashing)
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can be done by a pointer compare instead of a string compare. Normally, the
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names used in Python programs are automatically interned, and the dictionaries
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used to hold module, class or instance attributes have interned keys.
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Interned strings are not immortal; you must keep a reference to the return
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value of :func:`intern` around to benefit from it.
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.. data:: last_type
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last_value
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last_traceback
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These three variables are not always defined; they are set when an exception is
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not handled and the interpreter prints an error message and a stack traceback.
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Their intended use is to allow an interactive user to import a debugger module
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and engage in post-mortem debugging without having to re-execute the command
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that caused the error. (Typical use is ``import pdb; pdb.pm()`` to enter the
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post-mortem debugger; see chapter :ref:`debugger` for
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more information.)
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The meaning of the variables is the same as that of the return values from
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:func:`exc_info` above. (Since there is only one interactive thread,
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thread-safety is not a concern for these variables, unlike for ``exc_type``
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etc.)
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.. data:: maxint
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The largest positive integer supported by Python's regular integer type. This
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is at least 2\*\*31-1. The largest negative integer is ``-maxint-1`` --- the
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asymmetry results from the use of 2's complement binary arithmetic.
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.. data:: maxunicode
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An integer giving the largest supported code point for a Unicode character. The
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value of this depends on the configuration option that specifies whether Unicode
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characters are stored as UCS-2 or UCS-4.
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.. data:: modules
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This is a dictionary that maps module names to modules which have already been
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loaded. This can be manipulated to force reloading of modules and other tricks.
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.. data:: path
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.. index:: triple: module; search; path
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A list of strings that specifies the search path for modules. Initialized from
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the environment variable :envvar:`PYTHONPATH`, plus an installation-dependent
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default.
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As initialized upon program startup, the first item of this list, ``path[0]``,
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is the directory containing the script that was used to invoke the Python
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interpreter. If the script directory is not available (e.g. if the interpreter
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is invoked interactively or if the script is read from standard input),
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``path[0]`` is the empty string, which directs Python to search modules in the
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current directory first. Notice that the script directory is inserted *before*
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the entries inserted as a result of :envvar:`PYTHONPATH`.
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A program is free to modify this list for its own purposes.
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.. data:: platform
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This string contains a platform identifier, e.g. ``'sunos5'`` or ``'linux1'``.
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This can be used to append platform-specific components to ``path``, for
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instance.
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.. data:: prefix
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A string giving the site-specific directory prefix where the platform
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independent Python files are installed; by default, this is the string
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``'/usr/local'``. This can be set at build time with the :option:`--prefix`
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argument to the :program:`configure` script. The main collection of Python
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library modules is installed in the directory ``prefix + '/lib/pythonversion'``
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while the platform independent header files (all except :file:`pyconfig.h`) are
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stored in ``prefix + '/include/pythonversion'``, where *version* is equal to
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``version[:3]``.
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.. data:: ps1
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ps2
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.. index::
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single: interpreter prompts
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single: prompts, interpreter
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Strings specifying the primary and secondary prompt of the interpreter. These
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are only defined if the interpreter is in interactive mode. Their initial
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values in this case are ``'>>> '`` and ``'... '``. If a non-string object is
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assigned to either variable, its :func:`str` is re-evaluated each time the
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interpreter prepares to read a new interactive command; this can be used to
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implement a dynamic prompt.
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.. function:: setcheckinterval(interval)
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Set the interpreter's "check interval". This integer value determines how often
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the interpreter checks for periodic things such as thread switches and signal
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handlers. The default is ``100``, meaning the check is performed every 100
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Python virtual instructions. Setting it to a larger value may increase
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performance for programs using threads. Setting it to a value ``<=`` 0 checks
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every virtual instruction, maximizing responsiveness as well as overhead.
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.. function:: setdefaultencoding(name)
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Set the current default string encoding used by the Unicode implementation. If
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*name* does not match any available encoding, :exc:`LookupError` is raised.
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This function is only intended to be used by the :mod:`site` module
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implementation and, where needed, by :mod:`sitecustomize`. Once used by the
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:mod:`site` module, it is removed from the :mod:`sys` module's namespace.
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.. % Note that \refmodule{site} is not imported if
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.. % the \programopt{-S} option is passed to the interpreter, in which
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.. % case this function will remain available.
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.. function:: setdlopenflags(n)
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Set the flags used by the interpreter for :cfunc:`dlopen` calls, such as when
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the interpreter loads extension modules. Among other things, this will enable a
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lazy resolving of symbols when importing a module, if called as
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``sys.setdlopenflags(0)``. To share symbols across extension modules, call as
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``sys.setdlopenflags(dl.RTLD_NOW | dl.RTLD_GLOBAL)``. Symbolic names for the
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flag modules can be either found in the :mod:`dl` module, or in the :mod:`DLFCN`
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module. If :mod:`DLFCN` is not available, it can be generated from
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:file:`/usr/include/dlfcn.h` using the :program:`h2py` script. Availability:
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Unix.
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.. function:: setprofile(profilefunc)
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.. index::
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single: profile function
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single: profiler
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Set the system's profile function, which allows you to implement a Python source
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code profiler in Python. See chapter :ref:`profile` for more information on the
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Python profiler. The system's profile function is called similarly to the
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system's trace function (see :func:`settrace`), but it isn't called for each
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executed line of code (only on call and return, but the return event is reported
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even when an exception has been set). The function is thread-specific, but
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there is no way for the profiler to know about context switches between threads,
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so it does not make sense to use this in the presence of multiple threads. Also,
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its return value is not used, so it can simply return ``None``.
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.. function:: setrecursionlimit(limit)
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|
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.
|
|
|
|
|
|
.. function:: settrace(tracefunc)
|
|
|
|
.. index::
|
|
single: trace function
|
|
single: debugger
|
|
|
|
Set the system's trace function, which allows you to implement a Python
|
|
source code debugger in Python. See section :ref:`debugger-hooks` in the
|
|
chapter on the Python debugger. The function is thread-specific; for a
|
|
debugger to support multiple threads, it must be registered using
|
|
:func:`settrace` for each thread being debugged.
|
|
|
|
.. note::
|
|
|
|
The :func:`settrace` function is intended only for implementing debuggers,
|
|
profilers, coverage tools and the like. Its behavior is part of the
|
|
implementation platform, rather than part of the language definition, and thus
|
|
may not be available in all Python implementations.
|
|
|
|
|
|
.. function:: settscdump(on_flag)
|
|
|
|
Activate dumping of VM measurements using the Pentium timestamp counter, if
|
|
*on_flag* is true. Deactivate these dumps if *on_flag* is off. The function is
|
|
available only if Python was compiled with :option:`--with-tsc`. To understand
|
|
the output of this dump, read :file:`Python/ceval.c` in the Python sources.
|
|
|
|
|
|
.. data:: 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.
|
|
``stdout`` is used for the output of :func:`print` and expression statements.
|
|
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 :meth:`write` method that takes a
|
|
string argument. (Changing these objects doesn't affect the standard I/O
|
|
streams of processes executed by :func:`os.popen`, :func:`os.system` or the
|
|
:func:`exec\*` family of functions in the :mod:`os` module.)
|
|
|
|
|
|
.. data:: __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.
|
|
|
|
.. note::
|
|
|
|
Under some conditions ``stdin``, ``stdout`` and ``stderr`` as well as the
|
|
original values ``__stdin__``, ``__stdout__`` and ``__stderr__`` can be
|
|
None. It is usually the case for Windows GUI apps that aren't connected to
|
|
a console and Python apps started with :program:`pythonw`.
|
|
|
|
|
|
.. data:: 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.
|
|
|
|
|
|
.. data:: 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)]'
|
|
|
|
|
|
.. data:: api_version
|
|
|
|
The C API version for this interpreter. Programmers may find this useful when
|
|
debugging version conflicts between Python and extension modules.
|
|
|
|
|
|
.. data:: 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)``.
|
|
|
|
|
|
.. data:: warnoptions
|
|
|
|
This is an implementation detail of the warnings framework; do not modify this
|
|
value. Refer to the :mod:`warnings` module for more information on the warnings
|
|
framework.
|
|
|
|
|
|
.. data:: 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 :const:`version`. It is provided in the :mod:`sys`
|
|
module for informational purposes; modifying this value has no effect on the
|
|
registry keys used by Python. Availability: Windows.
|
|
|
|
|
|
.. seealso::
|
|
|
|
Module :mod:`site`
|
|
This describes how to use .pth files to extend ``sys.path``.
|
|
|
|
|