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cpython/Doc/library/sys.rst
Martin v. Löwis 04dc25c537 Issue #3187: Add sys.setfilesystemencoding.
2008-10-03 16:09:28 +00:00

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:mod:`sys` --- System-specific parameters and functions
=======================================================
.. module:: sys
:synopsis: 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.
.. data:: 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 :option:`-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[0]`` is the empty string.
To loop over the standard input, or the list of files given on the
command line, see the :mod:`fileinput` module.
.. data:: byteorder
An indicator of the native byte order. This will have the value ``'big'`` on
big-endian (most-significant byte first) platforms, and ``'little'`` on
little-endian (least-significant byte first) platforms.
.. data:: subversion
A triple (repo, branch, version) representing the Subversion information of the
Python interpreter. *repo* is the name of the repository, ``'CPython'``.
*branch* is a string of one of the forms ``'trunk'``, ``'branches/name'`` or
``'tags/name'``. *version* is the output of ``svnversion``, if the interpreter
was built from a Subversion checkout; it contains the revision number (range)
and possibly a trailing 'M' if there were local modifications. If the tree was
exported (or svnversion was not available), it is the revision of
``Include/patchlevel.h`` if the branch is a tag. Otherwise, it is ``None``.
.. data:: 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.)
.. data:: copyright
A string containing the copyright pertaining to the Python interpreter.
.. function:: _clear_type_cache()
Clear the internal type cache. The type cache is used to speed up attribute
and method lookups. Use the function *only* to drop unnecessary references
during reference leak debugging.
This function should be used for internal and specialized purposes only.
.. function:: _current_frames()
Return a dictionary mapping each thread's identifier to the topmost stack frame
currently active in that thread at the time the function is called. Note that
functions in the :mod:`traceback` module can build the call stack given such a
frame.
This is most useful for debugging deadlock: this function does not require the
deadlocked threads' cooperation, and such threads' call stacks are frozen for as
long as they remain deadlocked. The frame returned for a non-deadlocked thread
may bear no relationship to that thread's current activity by the time calling
code examines the frame.
This function should be used for internal and specialized purposes only.
.. data:: dllhandle
Integer specifying the handle of the Python DLL. Availability: Windows.
.. function:: displayhook(value)
If *value* is not ``None``, this function prints it to ``sys.stdout``, and saves
it in ``builtins._``.
``sys.displayhook`` is called on the result of evaluating an :term:`expression`
entered in an interactive Python session. The display of these values can be
customized by assigning another one-argument function to ``sys.displayhook``.
.. function:: 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``.
.. data:: __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.
.. function:: 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
an except clause." For any stack frame, only information about the exception
being currently handled is accessible.
.. index:: object: traceback
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 class object); *value* gets the exception parameter (its
:dfn:`associated value` or the second argument to :keyword:`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. 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 :keyword:`try`
... :keyword:`finally` statement) or to call :func:`exc_info` in a
function that does not itself handle an exception.
Such cycles are normally automatically reclaimed when garbage collection
is enabled and they become unreachable, but it remains more efficient to
avoid creating cycles.
.. data:: 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 :option:`--exec-prefix` argument to the
:program:`configure` script. Specifically, all configuration files (e.g. the
:file:`pyconfig.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]``.
.. data:: executable
A string giving the name of the executable binary for the Python interpreter, on
systems where this makes sense.
.. function:: exit([arg])
Exit from Python. This is implemented by raising the :exc:`SystemExit`
exception, so cleanup actions specified by finally clauses of :keyword:`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.
.. data:: flags
The struct sequence *flags* exposes the status of command line flags. The
attributes are read only.
+------------------------------+------------------------------------------+
| attribute | flag |
+==============================+==========================================+
| :const:`debug` | -d |
+------------------------------+------------------------------------------+
| :const:`py3k_warning` | -3 |
+------------------------------+------------------------------------------+
| :const:`division_warning` | -Q |
+------------------------------+------------------------------------------+
| :const:`division_new` | -Qnew |
+------------------------------+------------------------------------------+
| :const:`inspect` | -i |
+------------------------------+------------------------------------------+
| :const:`interactive` | -i |
+------------------------------+------------------------------------------+
| :const:`optimize` | -O or -OO |
+------------------------------+------------------------------------------+
| :const:`dont_write_bytecode` | -B |
+------------------------------+------------------------------------------+
| :const:`no_site` | -S |
+------------------------------+------------------------------------------+
| :const:`ignore_environment` | -E |
+------------------------------+------------------------------------------+
| :const:`verbose` | -v |
+------------------------------+------------------------------------------+
| :const:`unicode` | -U |
+------------------------------+------------------------------------------+
.. data:: float_info
A structseq holding information about the float type. It contains low level
information about the precision and internal representation. Please study
your system's :file:`float.h` for more information.
+---------------------+--------------------------------------------------+
| attribute | explanation |
+=====================+==================================================+
| :const:`epsilon` | Difference between 1 and the next representable |
| | floating point number |
+---------------------+--------------------------------------------------+
| :const:`dig` | digits (see :file:`float.h`) |
+---------------------+--------------------------------------------------+
| :const:`mant_dig` | mantissa digits (see :file:`float.h`) |
+---------------------+--------------------------------------------------+
| :const:`max` | maximum representable finite float |
+---------------------+--------------------------------------------------+
| :const:`max_exp` | maximum int e such that radix**(e-1) is in the |
| | range of finite representable floats |
+---------------------+--------------------------------------------------+
| :const:`max_10_exp` | maximum int e such that 10**e is in the |
| | range of finite representable floats |
+---------------------+--------------------------------------------------+
| :const:`min` | Minimum positive normalizer float |
+---------------------+--------------------------------------------------+
| :const:`min_exp` | minimum int e such that radix**(e-1) is a |
| | normalized float |
+---------------------+--------------------------------------------------+
| :const:`min_10_exp` | minimum int e such that 10**e is a normalized |
| | float |
+---------------------+--------------------------------------------------+
| :const:`radix` | radix of exponent |
+---------------------+--------------------------------------------------+
| :const:`rounds` | addition rounds (see :file:`float.h`) |
+---------------------+--------------------------------------------------+
.. note::
The information in the table is simplified.
.. function:: getcheckinterval()
Return the interpreter's "check interval"; see :func:`setcheckinterval`.
.. function:: getdefaultencoding()
Return the name of the current default string encoding used by the Unicode
implementation.
.. function:: getdlopenflags()
Return the current value of the flags that are used for :cfunc:`dlopen` calls.
The flag constants are defined in the :mod:`ctypes` and :mod:`DLFCN` modules.
Availability: Unix.
.. function:: getfilesystemencoding()
Return the name of the encoding used to convert Unicode filenames into system
file names, or ``None`` if the system default encoding is used. The result value
depends on the operating system:
* On Windows 9x, the encoding is "mbcs".
* On Mac OS X, the encoding is "utf-8".
* On Unix, the encoding is the user's preference according to the result of
nl_langinfo(CODESET), or :const:`None` if the ``nl_langinfo(CODESET)`` failed.
* On Windows NT+, file names are Unicode natively, so no conversion is
performed. :func:`getfilesystemencoding` still returns ``'mbcs'``, as this is
the encoding that applications should use when they explicitly want to convert
Unicode strings to byte strings that are equivalent when used as file names.
.. function:: 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 :func:`getrefcount`.
.. function:: 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
:func:`setrecursionlimit`.
.. function:: getsizeof(object[, default])
Return the size of an object in bytes. The object can be any type of
object. All built-in objects will return correct results, but this
does not have to hold true for third-party extensions as it is implementation
specific.
The *default* argument allows to define a value which will be returned
if the object type does not provide means to retrieve the size and would
cause a `TypeError`.
func:`getsizeof` calls the object's __sizeof__ method and adds an additional
garbage collector overhead if the object is managed by the garbage collector.
.. function:: _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, :exc:`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.
.. function:: getprofile()
.. index::
single: profile function
single: profiler
Get the profiler function as set by :func:`setprofile`.
.. function:: gettrace()
.. index::
single: trace function
single: debugger
Get the trace function as set by :func:`settrace`.
.. note::
The :func:`gettrace` 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:: getwindowsversion()
Return a tuple containing five components, describing the Windows version
currently running. The elements are *major*, *minor*, *build*, *platform*, and
*text*. *text* contains a string while all other values are integers.
*platform* may be one of the following values:
+-----------------------------------------+-------------------------+
| Constant | Platform |
+=========================================+=========================+
| :const:`0 (VER_PLATFORM_WIN32s)` | Win32s on Windows 3.1 |
+-----------------------------------------+-------------------------+
| :const:`1 (VER_PLATFORM_WIN32_WINDOWS)` | Windows 95/98/ME |
+-----------------------------------------+-------------------------+
| :const:`2 (VER_PLATFORM_WIN32_NT)` | Windows NT/2000/XP/x64 |
+-----------------------------------------+-------------------------+
| :const:`3 (VER_PLATFORM_WIN32_CE)` | Windows CE |
+-----------------------------------------+-------------------------+
This function wraps the Win32 :cfunc:`GetVersionEx` function; see the Microsoft
documentation for more information about these fields.
Availability: Windows.
.. data:: 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 :func:`hex` function. The
``version_info`` value may be used for a more human-friendly encoding of the
same information.
.. function:: intern(string)
Enter *string* in the table of "interned" strings and return the interned string
-- which is *string* itself or a copy. Interning strings is useful to gain a
little performance on dictionary lookup -- if the keys in a dictionary are
interned, and the lookup key is interned, the key comparisons (after hashing)
can be done by a pointer compare instead of a string compare. Normally, the
names used in Python programs are automatically interned, and the dictionaries
used to hold module, class or instance attributes have interned keys.
Interned strings are not immortal; you must keep a reference to the return
value of :func:`intern` around to benefit from it.
.. data:: 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 chapter :ref:`debugger` for
more information.)
The meaning of the variables is the same as that of the return values from
:func:`exc_info` above. (Since there is only one interactive thread,
thread-safety is not a concern for these variables, unlike for ``exc_type``
etc.)
.. data:: maxsize
An integer giving the maximum value a variable of type :ctype:`Py_ssize_t` can
take. It's usually ``2**31 - 1`` on a 32-bit platform and ``2**63 - 1`` on a
64-bit platform.
.. data:: maxunicode
An integer giving the largest supported code point for a Unicode character. The
value of this depends on the configuration option that specifies whether Unicode
characters are stored as UCS-2 or UCS-4.
.. data:: 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.
.. data:: path
.. index:: triple: module; search; path
A list of strings that specifies the search path for modules. Initialized from
the environment variable :envvar:`PYTHONPATH`, plus an installation-dependent
default.
As initialized upon program startup, 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 :envvar:`PYTHONPATH`.
A program is free to modify this list for its own purposes.
.. data:: platform
This string contains a platform identifier that can be used to append
platform-specific components to :data:`sys.path`, for instance.
For Unix systems, this is the lowercased OS name as returned by ``uname -s``
with the first part of the version as returned by ``uname -r`` appended,
e.g. ``'sunos5'`` or ``'linux2'``, *at the time when Python was built*.
For other systems, the values are:
================ ===========================
System :data:`platform` value
================ ===========================
Windows ``'win32'``
Windows/Cygwin ``'cygwin'``
Mac OS X ``'darwin'``
Mac OS 9 ``'mac'``
OS/2 ``'os2'``
OS/2 EMX ``'os2emx'``
RiscOS ``'riscos'``
AtheOS ``'atheos'``
================ ===========================
.. data:: 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 :option:`--prefix`
argument to the :program:`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 :file:`pyconfig.h`) are
stored in ``prefix + '/include/pythonversion'``, where *version* is equal to
``version[:3]``.
.. data:: ps1
ps2
.. index::
single: interpreter prompts
single: prompts, interpreter
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 :func:`str` is re-evaluated each time the
interpreter prepares to read a new interactive command; this can be used to
implement a dynamic prompt.
.. data:: dont_write_bytecode
If this is true, Python won't try to write ``.pyc`` or ``.pyo`` files on the
import of source modules. This value is initially set to ``True`` or ``False``
depending on the ``-B`` command line option and the ``PYTHONDONTWRITEBYTECODE``
environment variable, but you can set it yourself to control bytecode file
generation.
.. function:: 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 ``100``, meaning the check is performed every 100
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.
.. function:: setdefaultencoding(name)
Set the current default string encoding used by the Unicode implementation. If
*name* does not match any available encoding, :exc:`LookupError` is raised.
This function is only intended to be used by the :mod:`site` module
implementation and, where needed, by :mod:`sitecustomize`. Once used by the
:mod:`site` module, it is removed from the :mod:`sys` module's namespace.
.. Note that :mod:`site` is not imported if the :option:`-S` option is passed
to the interpreter, in which case this function will remain available.
.. function:: setdlopenflags(n)
Set the flags used by the interpreter for :cfunc:`dlopen` calls, such as when
the interpreter loads extension modules. Among other things, this will enable a
lazy resolving of symbols when importing a module, if called as
``sys.setdlopenflags(0)``. To share symbols across extension modules, call as
``sys.setdlopenflags(ctypes.RTLD_GLOBAL)``. Symbolic names for the
flag modules can be either found in the :mod:`ctypes` module, or in the :mod:`DLFCN`
module. If :mod:`DLFCN` is not available, it can be generated from
:file:`/usr/include/dlfcn.h` using the :program:`h2py` script. Availability:
Unix.
.. function:: setfilesystemencoding(enc)
Set the encoding used when converting Python strings to file names to *enc*.
By default, Python tries to determine the encoding it should use automatically
on Unix; on Windows, it avoids such conversion completely. This function can
be used when Python's determination of the encoding needs to be overwritten,
e.g. when not all file names on disk can be decoded using the encoding that
Python had chosen.
.. function:: setprofile(profilefunc)
.. index::
single: profile function
single: profiler
Set the system's profile function, which allows you to implement a Python source
code profiler in Python. See chapter :ref:`profile` for more information on the
Python profiler. The system's profile function is called similarly to the
system's trace function (see :func:`settrace`), but it isn't called for each
executed line of code (only on call and return, but the return event is reported
even when an exception has been set). The function is thread-specific, but
there is no way for the profiler to know about context switches between threads,
so it does not make sense to use this in the presence of multiple threads. Also,
its return value is not used, so it can simply return ``None``.
.. function:: 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.
.. 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 but
including calls to :func:`input`. ``stdout`` is used for
the output of :func:`print` and :term:`expression` statements and for the
prompts of :func:`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 :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``.
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