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cpython/Doc/library/sys.rst
Vinay Sajip cd9b746b37 Added cross-references to venv definition.
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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:: abiflags
On POSIX systems where Python is build with the standard ``configure``
script, this contains the ABI flags as specified by :pep:`3149`.
.. versionadded:: 3.2
.. 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:: base_exec_prefix
Set during Python startup, before ``site.py`` is run, to the same value as
:data:`exec_prefix`. If not running in a
:ref:`virtual environment <venv-def>`, the values will stay the same; if
``site.py`` finds that a virtual environment is in use, the values of
:data:`prefix` and :data:`exec_prefix` will be changed to point to the
virtual environment, whereas :data:`base_prefix` and
:data:`base_exec_prefix` will remain pointing to the base Python
installation (the one which the virtual environment was created from).
.. versionadded:: 3.3
.. data:: base_prefix
Set during Python startup, before ``site.py`` is run, to the same value as
:data:`prefix`. If not running in a :ref:`virtual environment <venv-def>`, the values
will stay the same; if ``site.py`` finds that a virtual environment is in
use, the values of :data:`prefix` and :data:`exec_prefix` will be changed to
point to the virtual environment, whereas :data:`base_prefix` and
:data:`base_exec_prefix` will remain pointing to the base Python
installation (the one which the virtual environment was created from).
.. versionadded:: 3.3
.. 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:: 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.)
.. function:: call_tracing(func, args)
Call ``func(*args)``, while tracing is enabled. The tracing state is saved,
and restored afterwards. This is intended to be called from a debugger from
a checkpoint, to recursively debug some other code.
.. 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.
.. function:: _debugmallocstats()
Print low-level information to stderr about the state of CPython's memory
allocator.
If Python is configured --with-pydebug, it also performs some expensive
internal consistency checks.
.. versionadded:: 3.3
.. impl-detail::
This function is specific to CPython. The exact output format is not
defined here, and may change.
.. data:: dllhandle
Integer specifying the handle of the Python DLL. Availability: Windows.
.. function:: displayhook(value)
If *value* is not ``None``, this function prints ``repr(value)`` to
``sys.stdout``, and saves *value* in ``builtins._``. If ``repr(value)`` is
not encodable to ``sys.stdout.encoding`` with ``sys.stdout.errors`` error
handler (which is probably ``'strict'``), encode it to
``sys.stdout.encoding`` with ``'backslashreplace'`` error handler.
``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``.
Pseudo-code::
def displayhook(value):
if value is None:
return
# Set '_' to None to avoid recursion
builtins._ = None
text = repr(value)
try:
sys.stdout.write(text)
except UnicodeEncodeError:
bytes = text.encode(sys.stdout.encoding, 'backslashreplace')
if hasattr(sys.stdout, 'buffer'):
sys.stdout.buffer.write(bytes)
else:
text = bytes.decode(sys.stdout.encoding, 'strict')
sys.stdout.write(text)
sys.stdout.write("\n")
builtins._ = value
.. versionchanged:: 3.2
Use ``'backslashreplace'`` error handler on :exc:`UnicodeEncodeError`.
.. 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 :option:`-B` command line option and the
:envvar:`PYTHONDONTWRITEBYTECODE` environment variable, but you can set it
yourself to control bytecode file generation.
.. 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 type of the
exception being handled (a subclass of :exc:`BaseException`); *value* gets
the exception instance (an instance of the exception type); *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 ``--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
:file:`{exec_prefix}/lib/python{X.Y}/config`, and shared library modules are
installed in :file:`{exec_prefix}/lib/python{X.Y}/lib-dynload`, where *X.Y*
is the version number of Python, for example ``3.2``.
.. note:: If a :ref:`virtual environment <venv-def>` is in effect, this
value will be changed in ``site.py`` to point to the virtual environment.
The value for the Python installation will still be available, via
:data:`base_exec_prefix`.
.. data:: executable
A string giving the absolute path of the executable binary for the Python
interpreter, on systems where this makes sense. If Python is unable to retrieve
the real path to its executable, :data:`sys.executable` will be an empty string
or ``None``.
.. 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 :data:`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.
Since :func:`exit` ultimately "only" raises an exception, it will only exit
the process when called from the main thread, and the exception is not
intercepted.
.. data:: flags
The :term:`struct sequence` *flags* exposes the status of command line
flags. The attributes are read only.
============================= =============================
attribute flag
============================= =============================
:const:`debug` :option:`-d`
:const:`inspect` :option:`-i`
:const:`interactive` :option:`-i`
:const:`optimize` :option:`-O` or :option:`-OO`
:const:`dont_write_bytecode` :option:`-B`
:const:`no_user_site` :option:`-s`
:const:`no_site` :option:`-S`
:const:`ignore_environment` :option:`-E`
:const:`verbose` :option:`-v`
:const:`bytes_warning` :option:`-b`
:const:`quiet` :option:`-q`
:const:`hash_randomization` :option:`-R`
============================= =============================
.. versionchanged:: 3.2
Added ``quiet`` attribute for the new :option:`-q` flag.
.. versionadded:: 3.2.3
The ``hash_randomization`` attribute.
.. versionchanged:: 3.3
Removed obsolete ``division_warning`` attribute.
.. data:: float_info
A :term:`struct sequence` holding information about the float type. It
contains low level information about the precision and internal
representation. The values correspond to the various floating-point
constants defined in the standard header file :file:`float.h` for the 'C'
programming language; see section 5.2.4.2.2 of the 1999 ISO/IEC C standard
[C99]_, 'Characteristics of floating types', for details.
+---------------------+----------------+--------------------------------------------------+
| attribute | float.h macro | explanation |
+=====================+================+==================================================+
| :const:`epsilon` | DBL_EPSILON | difference between 1 and the least value greater |
| | | than 1 that is representable as a float |
+---------------------+----------------+--------------------------------------------------+
| :const:`dig` | DBL_DIG | maximum number of decimal digits that can be |
| | | faithfully represented in a float; see below |
+---------------------+----------------+--------------------------------------------------+
| :const:`mant_dig` | DBL_MANT_DIG | float precision: the number of base-``radix`` |
| | | digits in the significand of a float |
+---------------------+----------------+--------------------------------------------------+
| :const:`max` | DBL_MAX | maximum representable finite float |
+---------------------+----------------+--------------------------------------------------+
| :const:`max_exp` | DBL_MAX_EXP | maximum integer e such that ``radix**(e-1)`` is |
| | | a representable finite float |
+---------------------+----------------+--------------------------------------------------+
| :const:`max_10_exp` | DBL_MAX_10_EXP | maximum integer e such that ``10**e`` is in the |
| | | range of representable finite floats |
+---------------------+----------------+--------------------------------------------------+
| :const:`min` | DBL_MIN | minimum positive normalized float |
+---------------------+----------------+--------------------------------------------------+
| :const:`min_exp` | DBL_MIN_EXP | minimum integer e such that ``radix**(e-1)`` is |
| | | a normalized float |
+---------------------+----------------+--------------------------------------------------+
| :const:`min_10_exp` | DBL_MIN_10_EXP | minimum integer e such that ``10**e`` is a |
| | | normalized float |
+---------------------+----------------+--------------------------------------------------+
| :const:`radix` | FLT_RADIX | radix of exponent representation |
+---------------------+----------------+--------------------------------------------------+
| :const:`rounds` | FLT_ROUNDS | integer constant representing the rounding mode |
| | | used for arithmetic operations. This reflects |
| | | the value of the system FLT_ROUNDS macro at |
| | | interpreter startup time. See section 5.2.4.2.2 |
| | | of the C99 standard for an explanation of the |
| | | possible values and their meanings. |
+---------------------+----------------+--------------------------------------------------+
The attribute :attr:`sys.float_info.dig` needs further explanation. If
``s`` is any string representing a decimal number with at most
:attr:`sys.float_info.dig` significant digits, then converting ``s`` to a
float and back again will recover a string representing the same decimal
value::
>>> import sys
>>> sys.float_info.dig
15
>>> s = '3.14159265358979' # decimal string with 15 significant digits
>>> format(float(s), '.15g') # convert to float and back -> same value
'3.14159265358979'
But for strings with more than :attr:`sys.float_info.dig` significant digits,
this isn't always true::
>>> s = '9876543211234567' # 16 significant digits is too many!
>>> format(float(s), '.16g') # conversion changes value
'9876543211234568'
.. data:: float_repr_style
A string indicating how the :func:`repr` function behaves for
floats. If the string has value ``'short'`` then for a finite
float ``x``, ``repr(x)`` aims to produce a short string with the
property that ``float(repr(x)) == x``. This is the usual behaviour
in Python 3.1 and later. Otherwise, ``float_repr_style`` has value
``'legacy'`` and ``repr(x)`` behaves in the same way as it did in
versions of Python prior to 3.1.
.. versionadded:: 3.1
.. function:: getcheckinterval()
Return the interpreter's "check interval"; see :func:`setcheckinterval`.
.. deprecated:: 3.2
Use :func:`getswitchinterval` instead.
.. 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 :c:func:`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. The result value depends on the operating system:
* 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 ``'utf-8'`` if ``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.
* On Windows 9x, the encoding is ``'mbcs'``.
.. versionchanged:: 3.2
On Unix, use ``'utf-8'`` instead of ``None`` if ``nl_langinfo(CODESET)``
failed. :func:`getfilesystemencoding` result cannot be ``None``.
.. 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.
Only the memory consumption directly attributed to the object is
accounted for, not the memory consumption of objects it refers to.
If given, *default* will be returned if the object does not provide means to
retrieve the size. Otherwise a :exc:`TypeError` will be raised.
:func:`getsizeof` calls the object's ``__sizeof__`` method and adds an
additional garbage collector overhead if the object is managed by the garbage
collector.
See `recursive sizeof recipe <http://code.activestate.com/recipes/577504>`_
for an example of using :func:`getsizeof` recursively to find the size of
containers and all their contents.
.. function:: getswitchinterval()
Return the interpreter's "thread switch interval"; see
:func:`setswitchinterval`.
.. versionadded:: 3.2
.. 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.
.. impl-detail::
This function should be used for internal and specialized purposes only.
It is not guaranteed to exist in all implementations of Python.
.. 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`.
.. impl-detail::
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 named tuple describing the Windows version
currently running. The named elements are *major*, *minor*,
*build*, *platform*, *service_pack*, *service_pack_minor*,
*service_pack_major*, *suite_mask*, and *product_type*.
*service_pack* contains a string while all other values are
integers. The components can also be accessed by name, so
``sys.getwindowsversion()[0]`` is equivalent to
``sys.getwindowsversion().major``. For compatibility with prior
versions, only the first 5 elements are retrievable by indexing.
*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 |
+-----------------------------------------+-------------------------+
*product_type* may be one of the following values:
+---------------------------------------+---------------------------------+
| Constant | Meaning |
+=======================================+=================================+
| :const:`1 (VER_NT_WORKSTATION)` | The system is a workstation. |
+---------------------------------------+---------------------------------+
| :const:`2 (VER_NT_DOMAIN_CONTROLLER)` | The system is a domain |
| | controller. |
+---------------------------------------+---------------------------------+
| :const:`3 (VER_NT_SERVER)` | The system is a server, but not |
| | a domain controller. |
+---------------------------------------+---------------------------------+
This function wraps the Win32 :c:func:`GetVersionEx` function; see the
Microsoft documentation on :c:func:`OSVERSIONINFOEX` for more information
about these fields.
Availability: Windows.
.. versionchanged:: 3.2
Changed to a named tuple and added *service_pack_minor*,
*service_pack_major*, *suite_mask*, and *product_type*.
.. data:: hash_info
A :term:`struct sequence` giving parameters of the numeric hash
implementation. For more details about hashing of numeric types, see
:ref:`numeric-hash`.
+---------------------+--------------------------------------------------+
| attribute | explanation |
+=====================+==================================================+
| :const:`width` | width in bits used for hash values |
+---------------------+--------------------------------------------------+
| :const:`modulus` | prime modulus P used for numeric hash scheme |
+---------------------+--------------------------------------------------+
| :const:`inf` | hash value returned for a positive infinity |
+---------------------+--------------------------------------------------+
| :const:`nan` | hash value returned for a nan |
+---------------------+--------------------------------------------------+
| :const:`imag` | multiplier used for the imaginary part of a |
| | complex number |
+---------------------+--------------------------------------------------+
.. versionadded:: 3.2
.. 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
:term:`struct sequence` :data:`sys.version_info` may be used for a more
human-friendly encoding of the same information.
The ``hexversion`` is a 32-bit number with the following layout:
+-------------------------+------------------------------------------------+
| Bits (big endian order) | Meaning |
+=========================+================================================+
| :const:`1-8` | ``PY_MAJOR_VERSION`` (the ``2`` in |
| | ``2.1.0a3``) |
+-------------------------+------------------------------------------------+
| :const:`9-16` | ``PY_MINOR_VERSION`` (the ``1`` in |
| | ``2.1.0a3``) |
+-------------------------+------------------------------------------------+
| :const:`17-24` | ``PY_MICRO_VERSION`` (the ``0`` in |
| | ``2.1.0a3``) |
+-------------------------+------------------------------------------------+
| :const:`25-28` | ``PY_RELEASE_LEVEL`` (``0xA`` for alpha, |
| | ``0xB`` for beta, ``0xC`` for release |
| | candidate and ``0xF`` for final) |
+-------------------------+------------------------------------------------+
| :const:`29-32` | ``PY_RELEASE_SERIAL`` (the ``3`` in |
| | ``2.1.0a3``, zero for final releases) |
+-------------------------+------------------------------------------------+
Thus ``2.1.0a3`` is hexversion ``0x020100a3``.
.. data:: implementation
An object containing information about the implementation of the
currently running Python interpreter. The following attributes are
required to exist in all Python implementations.
*name* is the implementation's identifier, e.g. ``'cpython'``. The actual
string is defined by the Python implementation, but it is guaranteed to be
lower case.
*version* is a named tuple, in the same format as
:data:`sys.version_info`. It represents the version of the Python
*implementation*. This has a distinct meaning from the specific
version of the Python *language* to which the currently running
interpreter conforms, which ``sys.version_info`` represents. For
example, for PyPy 1.8 ``sys.implementation.version`` might be
``sys.version_info(1, 8, 0, 'final', 0)``, whereas ``sys.version_info``
would be ``sys.version_info(2, 7, 2, 'final', 0)``. For CPython they
are the same value, since it is the reference implementation.
*hexversion* is the implementation version in hexadecimal format, like
:data:`sys.hexversion`.
*cache_tag* is the tag used by the import machinery in the filenames of
cached modules. By convention, it would be a composite of the
implementation's name and version, like ``'cpython-33'``. However, a
Python implementation may use some other value if appropriate. If
``cache_tag`` is set to ``None``, it indicates that module caching should
be disabled.
:data:`sys.implementation` may contain additional attributes specific to
the Python implementation. These non-standard attributes must start with
an underscore, and are not described here. Regardless of its contents,
:data:`sys.implementation` will not change during a run of the interpreter,
nor between implementation versions. (It may change between Python
language versions, however.) See `PEP 421` for more information.
.. versionadded:: 3.3
.. data:: int_info
A :term:`struct sequence` that holds information about Python's internal
representation of integers. The attributes are read only.
+-------------------------+----------------------------------------------+
| Attribute | Explanation |
+=========================+==============================================+
| :const:`bits_per_digit` | number of bits held in each digit. Python |
| | integers are stored internally in base |
| | ``2**int_info.bits_per_digit`` |
+-------------------------+----------------------------------------------+
| :const:`sizeof_digit` | size in bytes of the C type used to |
| | represent a digit |
+-------------------------+----------------------------------------------+
.. versionadded:: 3.1
.. 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 :mod:`pdb` module for
more information.)
The meaning of the variables is the same as that of the return values from
:func:`exc_info` above.
.. data:: maxsize
An integer giving the maximum value a variable of type :c:type:`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 value of the largest Unicode code point,
i.e. ``1114111`` (``0x10FFFF`` in hexadecimal).
.. versionchanged:: 3.3
Before :pep:`393`, ``sys.maxunicode`` used to be either ``0xFFFF``
or ``0x10FFFF``, depending on the configuration option that specified
whether Unicode characters were stored as UCS-2 or UCS-4.
.. data:: meta_path
A list of :term:`finder` objects that have their :meth:`find_module`
methods called to see if one of the objects can find the module to be
imported. The :meth:`find_module` method is called at least with the
absolute name of the module being imported. If the module to be imported is
contained in package then the parent package's :attr:`__path__` attribute
is passed in as a second argument. The method returns ``None`` if
the module cannot be found, else returns a :term:`loader`.
:data:`sys.meta_path` is searched before any implicit default finders or
:data:`sys.path`.
See :pep:`302` for the original specification.
.. 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.
.. seealso::
Module :mod:`site` This describes how to use .pth files to extend
:data:`sys.path`.
.. data:: path_hooks
A list of callables that take a path argument to try to create a
:term:`finder` for the path. If a finder can be created, it is to be
returned by the callable, else raise :exc:`ImportError`.
Originally specified in :pep:`302`.
.. data:: path_importer_cache
A dictionary acting as a cache for :term:`finder` objects. The keys are
paths that have been passed to :data:`sys.path_hooks` and the values are
the finders that are found. If a path is a valid file system path but no
explicit finder is found on :data:`sys.path_hooks` then ``None`` is
stored to represent the implicit default finder should be used. If the path
is not an existing path then :class:`imp.NullImporter` is set.
Originally specified in :pep:`302`.
.. 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, except on Linux, 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 ``'freebsd8'``, *at the time
when Python was built*. Unless you want to test for a specific system
version, it is therefore recommended to use the following idiom::
if sys.platform.startswith('freebsd'):
# FreeBSD-specific code here...
elif sys.platform.startswith('linux'):
# Linux-specific code here...
For other systems, the values are:
================ ===========================
System :data:`platform` value
================ ===========================
Linux ``'linux'``
Windows ``'win32'``
Windows/Cygwin ``'cygwin'``
Mac OS X ``'darwin'``
OS/2 ``'os2'``
OS/2 EMX ``'os2emx'``
================ ===========================
.. versionchanged:: 3.3
On Linux, :attr:`sys.platform` doesn't contain the major version anymore.
It is always ``'linux'``, instead of ``'linux2'`` or ``'linux3'``. Since
older Python versions include the version number, it is recommended to
always use the ``startswith`` idiom presented above.
.. seealso::
:attr:`os.name` has a coarser granularity. :func:`os.uname` gives
system-dependent version information.
The :mod:`platform` module provides detailed checks for the
system's identity.
.. 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 ``--prefix``
argument to the :program:`configure` script. The main collection of Python
library modules is installed in the directory :file:`{prefix}/lib/python{X.Y}`
while the platform independent header files (all except :file:`pyconfig.h`) are
stored in :file:`{prefix}/include/python{X.Y}`, where *X.Y* is the version
number of Python, for example ``3.2``.
.. note:: If a :ref:`virtual environment <venv-def>` is in effect, this
value will be changed in ``site.py`` to point to the virtual
environment. The value for the Python installation will still be
available, via :data:`base_prefix`.
.. 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.
.. 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.
.. deprecated:: 3.2
This function doesn't have an effect anymore, as the internal logic for
thread switching and asynchronous tasks has been rewritten. Use
:func:`setswitchinterval` instead.
.. function:: setdlopenflags(n)
Set the flags used by the interpreter for :c:func:`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(os.RTLD_GLOBAL)``. Symbolic names for the flag modules
can be found in the :mod:`os` module (``RTLD_xxx`` constants, e.g.
:data:`os.RTLD_LAZY`).
Availability: Unix.
.. 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 they have 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:: setswitchinterval(interval)
Set the interpreter's thread switch interval (in seconds). This floating-point
value determines the ideal duration of the "timeslices" allocated to
concurrently running Python threads. Please note that the actual value
can be higher, especially if long-running internal functions or methods
are used. Also, which thread becomes scheduled at the end of the interval
is the operating system's decision. The interpreter doesn't have its
own scheduler.
.. versionadded:: 3.2
.. 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. The function is thread-specific; for a
debugger to support multiple threads, it must be registered using
:func:`settrace` for each thread being debugged.
Trace functions should have three arguments: *frame*, *event*, and
*arg*. *frame* is the current stack frame. *event* is a string: ``'call'``,
``'line'``, ``'return'``, ``'exception'``, ``'c_call'``, ``'c_return'``, or
``'c_exception'``. *arg* depends on the event type.
The trace function is invoked (with *event* set to ``'call'``) whenever a new
local scope is entered; it should return a reference to a local trace
function to be used that scope, or ``None`` if the scope shouldn't be traced.
The local trace function should return a reference to itself (or to another
function for further tracing in that scope), or ``None`` to turn off tracing
in that scope.
The events have the following meaning:
``'call'``
A function is called (or some other code block entered). The
global trace function is called; *arg* is ``None``; the return value
specifies the local trace function.
``'line'``
The interpreter is about to execute a new line of code or re-execute the
condition of a loop. The local trace function is called; *arg* is
``None``; the return value specifies the new local trace function. See
:file:`Objects/lnotab_notes.txt` for a detailed explanation of how this
works.
``'return'``
A function (or other code block) is about to return. The local trace
function is called; *arg* is the value that will be returned, or ``None``
if the event is caused by an exception being raised. The trace function's
return value is ignored.
``'exception'``
An exception has occurred. The local trace function is called; *arg* is a
tuple ``(exception, value, traceback)``; the return value specifies the
new local trace function.
``'c_call'``
A C function is about to be called. This may be an extension function or
a built-in. *arg* is the C function object.
``'c_return'``
A C function has returned. *arg* is the C function object.
``'c_exception'``
A C function has raised an exception. *arg* is the C function object.
Note that as an exception is propagated down the chain of callers, an
``'exception'`` event is generated at each level.
For more information on code and frame objects, refer to :ref:`types`.
.. impl-detail::
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 ``--with-tsc``. To understand
the output of this dump, read :file:`Python/ceval.c` in the Python sources.
.. impl-detail::
This function is intimately bound to CPython implementation details and
thus not likely to be implemented elsewhere.
.. data:: stdin
stdout
stderr
:term:`File objects <file object>` used by the interpreter for standard
input, output and errors:
* ``stdin`` is used for all interactive input (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 its error messages go to ``stderr``.
By default, these streams are regular text streams as returned by the
:func:`open` function. Their parameters are chosen as follows:
* The character encoding is platform-dependent. Under Windows, if the stream
is interactive (that is, if its :meth:`isatty` method returns True), the
console codepage is used, otherwise the ANSI code page. Under other
platforms, the locale encoding is used (see :meth:`locale.getpreferredencoding`).
Under all platforms though, you can override this value by setting the
:envvar:`PYTHONIOENCODING` environment variable.
* When interactive, standard streams are line-buffered. Otherwise, they
are block-buffered like regular text files. You can override this
value with the :option:`-u` command-line option.
To write or read binary data from/to the standard streams, use the
underlying binary :data:`~io.TextIOBase.buffer`. For example, to write
bytes to :data:`stdout`, use ``sys.stdout.buffer.write(b'abc')``. Using
:meth:`io.TextIOBase.detach`, streams can be made binary by default. This
function sets :data:`stdin` and :data:`stdout` to binary::
def make_streams_binary():
sys.stdin = sys.stdin.detach()
sys.stdout = sys.stdout.detach()
Note that the streams may be replaced with objects (like :class:`io.StringIO`)
that do not support the :attr:`~io.BufferedIOBase.buffer` attribute or the
:meth:`~io.BufferedIOBase.detach` method and can raise :exc:`AttributeError`
or :exc:`io.UnsupportedOperation`.
.. 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 print to the actual standard stream no matter if the
``sys.std*`` object has been redirected.
It can also be used to restore the actual files to known working file objects
in case they have been overwritten with a broken object. However, the
preferred way to do this is to explicitly save the previous stream before
replacing it, and restore the saved 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:: thread_info
A :term:`struct sequence` holding information about the thread
implementation.
+------------------+---------------------------------------------------------+
| Attribute | Explanation |
+==================+=========================================================+
| :const:`name` | Name of the thread implementation: |
| | |
| | * ``'nt'``: Windows threads |
| | * ``'os2'``: OS/2 threads |
| | * ``'pthread'``: POSIX threads |
| | * ``'solaris'``: Solaris threads |
+------------------+---------------------------------------------------------+
| :const:`lock` | Name of the lock implementation: |
| | |
| | * ``'semaphore'``: a lock uses a semaphore |
| | * ``'mutex+cond'``: a lock uses a mutex |
| | and a condition variable |
| | * ``None`` if this information is unknown |
+------------------+---------------------------------------------------------+
| :const:`version` | Name and version of the thread library. It is a string, |
| | or ``None`` if these informations are unknown. |
+------------------+---------------------------------------------------------+
.. versionadded:: 3.3
.. 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. This string is displayed
when the interactive interpreter is started. Do not extract version information
out of it, rather, use :data:`version_info` and the functions provided by the
:mod:`platform` module.
.. 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)``. The components can also be accessed by name,
so ``sys.version_info[0]`` is equivalent to ``sys.version_info.major``
and so on.
.. versionchanged:: 3.1
Added named component attributes.
.. 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.
.. data:: _xoptions
A dictionary of the various implementation-specific flags passed through
the :option:`-X` command-line option. Option names are either mapped to
their values, if given explicitly, or to :const:`True`. Example::
$ ./python -Xa=b -Xc
Python 3.2a3+ (py3k, Oct 16 2010, 20:14:50)
[GCC 4.4.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> sys._xoptions
{'a': 'b', 'c': True}
.. impl-detail::
This is a CPython-specific way of accessing options passed through
:option:`-X`. Other implementations may export them through other
means, or not at all.
.. versionadded:: 3.2
.. rubric:: Citations
.. [C99] ISO/IEC 9899:1999. "Programming languages -- C." A public draft of this standard is available at http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf .
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