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.. highlight:: c
.. _initialization:
*****************************************
Initialization, Finalization, and Threads
*****************************************
See also :ref:`Python Initialization Configuration <init-config>`.
.. _pre-init-safe:
Before Python Initialization
============================
In an application embedding Python, the :c:func:`Py_Initialize` function must
be called before using any other Python/C API functions; with the exception of
a few functions and the :ref:`global configuration variables
<global-conf-vars>`.
The following functions can be safely called before Python is initialized:
* Configuration functions:
* :c:func:`PyImport_AppendInittab`
* :c:func:`PyImport_ExtendInittab`
* :c:func:`PyInitFrozenExtensions`
* :c:func:`PyMem_SetAllocator`
* :c:func:`PyMem_SetupDebugHooks`
* :c:func:`PyObject_SetArenaAllocator`
* :c:func:`Py_SetPath`
* :c:func:`Py_SetProgramName`
* :c:func:`Py_SetPythonHome`
* :c:func:`Py_SetStandardStreamEncoding`
* :c:func:`PySys_AddWarnOption`
* :c:func:`PySys_AddXOption`
* :c:func:`PySys_ResetWarnOptions`
* Informative functions:
* :c:func:`Py_IsInitialized`
* :c:func:`PyMem_GetAllocator`
* :c:func:`PyObject_GetArenaAllocator`
* :c:func:`Py_GetBuildInfo`
* :c:func:`Py_GetCompiler`
* :c:func:`Py_GetCopyright`
* :c:func:`Py_GetPlatform`
* :c:func:`Py_GetVersion`
* Utilities:
* :c:func:`Py_DecodeLocale`
* Memory allocators:
* :c:func:`PyMem_RawMalloc`
* :c:func:`PyMem_RawRealloc`
* :c:func:`PyMem_RawCalloc`
* :c:func:`PyMem_RawFree`
.. note::
The following functions **should not be called** before
:c:func:`Py_Initialize`: :c:func:`Py_EncodeLocale`, :c:func:`Py_GetPath`,
:c:func:`Py_GetPrefix`, :c:func:`Py_GetExecPrefix`,
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:c:func:`Py_GetProgramFullPath`, :c:func:`Py_GetPythonHome`,
:c:func:`Py_GetProgramName` and :c:func:`PyEval_InitThreads`.
.. _global-conf-vars:
Global configuration variables
==============================
Python has variables for the global configuration to control different features
and options. By default, these flags are controlled by :ref:`command line
options <using-on-interface-options>`.
When a flag is set by an option, the value of the flag is the number of times
that the option was set. For example, ``-b`` sets :c:data:`Py_BytesWarningFlag`
to 1 and ``-bb`` sets :c:data:`Py_BytesWarningFlag` to 2.
.. c:var:: int Py_BytesWarningFlag
Issue a warning when comparing :class:`bytes` or :class:`bytearray` with
:class:`str` or :class:`bytes` with :class:`int`. Issue an error if greater
or equal to ``2``.
Set by the :option:`-b` option.
.. c:var:: int Py_DebugFlag
Turn on parser debugging output (for expert only, depending on compilation
options).
Set by the :option:`-d` option and the :envvar:`PYTHONDEBUG` environment
variable.
.. c:var:: int Py_DontWriteBytecodeFlag
If set to non-zero, Python won't try to write ``.pyc`` files on the
import of source modules.
Set by the :option:`-B` option and the :envvar:`PYTHONDONTWRITEBYTECODE`
environment variable.
.. c:var:: int Py_FrozenFlag
Suppress error messages when calculating the module search path in
:c:func:`Py_GetPath`.
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Private flag used by ``_freeze_module`` and ``frozenmain`` programs.
.. c:var:: int Py_HashRandomizationFlag
Set to ``1`` if the :envvar:`PYTHONHASHSEED` environment variable is set to
a non-empty string.
If the flag is non-zero, read the :envvar:`PYTHONHASHSEED` environment
variable to initialize the secret hash seed.
.. c:var:: int Py_IgnoreEnvironmentFlag
Ignore all :envvar:`PYTHON*` environment variables, e.g.
:envvar:`PYTHONPATH` and :envvar:`PYTHONHOME`, that might be set.
Set by the :option:`-E` and :option:`-I` options.
.. c:var:: int Py_InspectFlag
When a script is passed as first argument or the :option:`-c` option is used,
enter interactive mode after executing the script or the command, even when
:data:`sys.stdin` does not appear to be a terminal.
Set by the :option:`-i` option and the :envvar:`PYTHONINSPECT` environment
variable.
.. c:var:: int Py_InteractiveFlag
Set by the :option:`-i` option.
.. c:var:: int Py_IsolatedFlag
Run Python in isolated mode. In isolated mode :data:`sys.path` contains
neither the script's directory nor the user's site-packages directory.
Set by the :option:`-I` option.
.. versionadded:: 3.4
.. c:var:: int Py_LegacyWindowsFSEncodingFlag
If the flag is non-zero, use the ``mbcs`` encoding with ``replace`` error
handler, instead of the UTF-8 encoding with ``surrogatepass`` error handler,
for the :term:`filesystem encoding and error handler`.
Set to ``1`` if the :envvar:`PYTHONLEGACYWINDOWSFSENCODING` environment
variable is set to a non-empty string.
See :pep:`529` for more details.
.. availability:: Windows.
.. c:var:: int Py_LegacyWindowsStdioFlag
If the flag is non-zero, use :class:`io.FileIO` instead of
:class:`WindowsConsoleIO` for :mod:`sys` standard streams.
Set to ``1`` if the :envvar:`PYTHONLEGACYWINDOWSSTDIO` environment
variable is set to a non-empty string.
See :pep:`528` for more details.
.. availability:: Windows.
.. c:var:: int Py_NoSiteFlag
Disable the import of the module :mod:`site` and the site-dependent
manipulations of :data:`sys.path` that it entails. Also disable these
manipulations if :mod:`site` is explicitly imported later (call
:func:`site.main` if you want them to be triggered).
Set by the :option:`-S` option.
.. c:var:: int Py_NoUserSiteDirectory
Don't add the :data:`user site-packages directory <site.USER_SITE>` to
:data:`sys.path`.
Set by the :option:`-s` and :option:`-I` options, and the
:envvar:`PYTHONNOUSERSITE` environment variable.
.. c:var:: int Py_OptimizeFlag
Set by the :option:`-O` option and the :envvar:`PYTHONOPTIMIZE` environment
variable.
.. c:var:: int Py_QuietFlag
Don't display the copyright and version messages even in interactive mode.
Set by the :option:`-q` option.
.. versionadded:: 3.2
.. c:var:: int Py_UnbufferedStdioFlag
Force the stdout and stderr streams to be unbuffered.
Set by the :option:`-u` option and the :envvar:`PYTHONUNBUFFERED`
environment variable.
.. c:var:: int Py_VerboseFlag
Print a message each time a module is initialized, showing the place
(filename or built-in module) from which it is loaded. If greater or equal
to ``2``, print a message for each file that is checked for when
searching for a module. Also provides information on module cleanup at exit.
Set by the :option:`-v` option and the :envvar:`PYTHONVERBOSE` environment
variable.
Initializing and finalizing the interpreter
===========================================
.. c:function:: void Py_Initialize()
.. index::
single: Py_SetProgramName()
single: PyEval_InitThreads()
single: modules (in module sys)
single: path (in module sys)
module: __main__
module: sys
triple: module; search; path
single: PySys_SetArgv()
single: Py_FinalizeEx()
Initialize the Python interpreter. In an application embedding Python,
this should be called before using any other Python/C API functions; see
:ref:`Before Python Initialization <pre-init-safe>` for the few exceptions.
This initializes
the table of loaded modules (``sys.modules``), and creates the fundamental
modules :mod:`builtins`, :mod:`__main__` and :mod:`sys`. It also initializes
the module search path (``sys.path``). It does not set ``sys.argv``; use
:c:func:`PySys_SetArgvEx` for that. This is a no-op when called for a second time
(without calling :c:func:`Py_FinalizeEx` first). There is no return value; it is a
fatal error if the initialization fails.
.. note::
On Windows, changes the console mode from ``O_TEXT`` to ``O_BINARY``, which will
also affect non-Python uses of the console using the C Runtime.
.. c:function:: void Py_InitializeEx(int initsigs)
This function works like :c:func:`Py_Initialize` if *initsigs* is ``1``. If
*initsigs* is ``0``, it skips initialization registration of signal handlers, which
might be useful when Python is embedded.
.. c:function:: int Py_IsInitialized()
Return true (nonzero) when the Python interpreter has been initialized, false
(zero) if not. After :c:func:`Py_FinalizeEx` is called, this returns false until
:c:func:`Py_Initialize` is called again.
.. c:function:: int Py_FinalizeEx()
Undo all initializations made by :c:func:`Py_Initialize` and subsequent use of
Python/C API functions, and destroy all sub-interpreters (see
:c:func:`Py_NewInterpreter` below) that were created and not yet destroyed since
the last call to :c:func:`Py_Initialize`. Ideally, this frees all memory
allocated by the Python interpreter. This is a no-op when called for a second
time (without calling :c:func:`Py_Initialize` again first). Normally the
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return value is ``0``. If there were errors during finalization
(flushing buffered data), ``-1`` is returned.
This function is provided for a number of reasons. An embedding application
might want to restart Python without having to restart the application itself.
An application that has loaded the Python interpreter from a dynamically
loadable library (or DLL) might want to free all memory allocated by Python
before unloading the DLL. During a hunt for memory leaks in an application a
developer might want to free all memory allocated by Python before exiting from
the application.
**Bugs and caveats:** The destruction of modules and objects in modules is done
in random order; this may cause destructors (:meth:`__del__` methods) to fail
when they depend on other objects (even functions) or modules. Dynamically
loaded extension modules loaded by Python are not unloaded. Small amounts of
memory allocated by the Python interpreter may not be freed (if you find a leak,
please report it). Memory tied up in circular references between objects is not
freed. Some memory allocated by extension modules may not be freed. Some
extensions may not work properly if their initialization routine is called more
than once; this can happen if an application calls :c:func:`Py_Initialize` and
:c:func:`Py_FinalizeEx` more than once.
.. audit-event:: cpython._PySys_ClearAuditHooks "" c.Py_FinalizeEx
.. versionadded:: 3.6
.. c:function:: void Py_Finalize()
This is a backwards-compatible version of :c:func:`Py_FinalizeEx` that
disregards the return value.
.. c:function:: int Py_SetStandardStreamEncoding(const char *encoding, const char *errors)
.. index::
single: Py_Initialize()
single: main()
triple: stdin; stdout; sdterr
This API is kept for backward compatibility: setting
:c:member:`PyConfig.stdio_encoding` and :c:member:`PyConfig.stdio_errors`
should be used instead, see :ref:`Python Initialization Configuration
<init-config>`.
This function should be called before :c:func:`Py_Initialize`, if it is
called at all. It specifies which encoding and error handling to use
with standard IO, with the same meanings as in :func:`str.encode`.
It overrides :envvar:`PYTHONIOENCODING` values, and allows embedding code
to control IO encoding when the environment variable does not work.
*encoding* and/or *errors* may be ``NULL`` to use
:envvar:`PYTHONIOENCODING` and/or default values (depending on other
settings).
Note that :data:`sys.stderr` always uses the "backslashreplace" error
handler, regardless of this (or any other) setting.
If :c:func:`Py_FinalizeEx` is called, this function will need to be called
again in order to affect subsequent calls to :c:func:`Py_Initialize`.
Returns ``0`` if successful, a nonzero value on error (e.g. calling after the
interpreter has already been initialized).
.. versionadded:: 3.4
.. deprecated:: 3.11
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.. c:function:: void Py_SetProgramName(const wchar_t *name)
.. index::
single: Py_Initialize()
single: main()
single: Py_GetPath()
This API is kept for backward compatibility: setting
:c:member:`PyConfig.program_name` should be used instead, see :ref:`Python
Initialization Configuration <init-config>`.
This function should be called before :c:func:`Py_Initialize` is called for
the first time, if it is called at all. It tells the interpreter the value
of the ``argv[0]`` argument to the :c:func:`main` function of the program
(converted to wide characters).
This is used by :c:func:`Py_GetPath` and some other functions below to find
the Python run-time libraries relative to the interpreter executable. The
default value is ``'python'``. The argument should point to a
zero-terminated wide character string in static storage whose contents will not
change for the duration of the program's execution. No code in the Python
interpreter will change the contents of this storage.
Use :c:func:`Py_DecodeLocale` to decode a bytes string to get a
:c:type:`wchar_*` string.
.. deprecated:: 3.11
.. c:function:: wchar* Py_GetProgramName()
.. index:: single: Py_SetProgramName()
Return the program name set with :c:func:`Py_SetProgramName`, or the default.
The returned string points into static storage; the caller should not modify its
value.
This function should not be called before :c:func:`Py_Initialize`, otherwise
it returns ``NULL``.
.. versionchanged:: 3.10
It now returns ``NULL`` if called before :c:func:`Py_Initialize`.
.. c:function:: wchar_t* Py_GetPrefix()
Return the *prefix* for installed platform-independent files. This is derived
through a number of complicated rules from the program name set with
:c:func:`Py_SetProgramName` and some environment variables; for example, if the
program name is ``'/usr/local/bin/python'``, the prefix is ``'/usr/local'``. The
returned string points into static storage; the caller should not modify its
value. This corresponds to the :makevar:`prefix` variable in the top-level
:file:`Makefile` and the ``--prefix`` argument to the :program:`configure`
script at build time. The value is available to Python code as ``sys.prefix``.
It is only useful on Unix. See also the next function.
This function should not be called before :c:func:`Py_Initialize`, otherwise
it returns ``NULL``.
.. versionchanged:: 3.10
It now returns ``NULL`` if called before :c:func:`Py_Initialize`.
.. c:function:: wchar_t* Py_GetExecPrefix()
Return the *exec-prefix* for installed platform-*dependent* files. This is
derived through a number of complicated rules from the program name set with
:c:func:`Py_SetProgramName` and some environment variables; for example, if the
program name is ``'/usr/local/bin/python'``, the exec-prefix is
``'/usr/local'``. The returned string points into static storage; the caller
should not modify its value. This corresponds to the :makevar:`exec_prefix`
variable in the top-level :file:`Makefile` and the ``--exec-prefix``
argument to the :program:`configure` script at build time. The value is
available to Python code as ``sys.exec_prefix``. It is only useful on Unix.
Background: The exec-prefix differs from the prefix when platform dependent
files (such as executables and shared libraries) are installed in a different
directory tree. In a typical installation, platform dependent files may be
installed in the :file:`/usr/local/plat` subtree while platform independent may
be installed in :file:`/usr/local`.
Generally speaking, a platform is a combination of hardware and software
families, e.g. Sparc machines running the Solaris 2.x operating system are
considered the same platform, but Intel machines running Solaris 2.x are another
platform, and Intel machines running Linux are yet another platform. Different
major revisions of the same operating system generally also form different
platforms. Non-Unix operating systems are a different story; the installation
strategies on those systems are so different that the prefix and exec-prefix are
meaningless, and set to the empty string. Note that compiled Python bytecode
files are platform independent (but not independent from the Python version by
which they were compiled!).
System administrators will know how to configure the :program:`mount` or
:program:`automount` programs to share :file:`/usr/local` between platforms
while having :file:`/usr/local/plat` be a different filesystem for each
platform.
This function should not be called before :c:func:`Py_Initialize`, otherwise
it returns ``NULL``.
.. versionchanged:: 3.10
It now returns ``NULL`` if called before :c:func:`Py_Initialize`.
.. c:function:: wchar_t* Py_GetProgramFullPath()
.. index::
single: Py_SetProgramName()
single: executable (in module sys)
Return the full program name of the Python executable; this is computed as a
side-effect of deriving the default module search path from the program name
(set by :c:func:`Py_SetProgramName` above). The returned string points into
static storage; the caller should not modify its value. The value is available
to Python code as ``sys.executable``.
This function should not be called before :c:func:`Py_Initialize`, otherwise
it returns ``NULL``.
.. versionchanged:: 3.10
It now returns ``NULL`` if called before :c:func:`Py_Initialize`.
.. c:function:: wchar_t* Py_GetPath()
.. index::
triple: module; search; path
single: path (in module sys)
Return the default module search path; this is computed from the program name
(set by :c:func:`Py_SetProgramName` above) and some environment variables.
The returned string consists of a series of directory names separated by a
platform dependent delimiter character. The delimiter character is ``':'``
on Unix and macOS, ``';'`` on Windows. The returned string points into
static storage; the caller should not modify its value. The list
:data:`sys.path` is initialized with this value on interpreter startup; it
can be (and usually is) modified later to change the search path for loading
modules.
This function should not be called before :c:func:`Py_Initialize`, otherwise
it returns ``NULL``.
.. XXX should give the exact rules
.. versionchanged:: 3.10
It now returns ``NULL`` if called before :c:func:`Py_Initialize`.
.. c:function:: void Py_SetPath(const wchar_t *)
.. index::
triple: module; search; path
single: path (in module sys)
single: Py_GetPath()
This API is kept for backward compatibility: setting
:c:member:`PyConfig.module_search_paths` and
:c:member:`PyConfig.module_search_paths_set` should be used instead, see
:ref:`Python Initialization Configuration <init-config>`.
Set the default module search path. If this function is called before
:c:func:`Py_Initialize`, then :c:func:`Py_GetPath` won't attempt to compute a
default search path but uses the one provided instead. This is useful if
Python is embedded by an application that has full knowledge of the location
of all modules. The path components should be separated by the platform
dependent delimiter character, which is ``':'`` on Unix and macOS, ``';'``
This also causes :data:`sys.executable` to be set to the program
full path (see :c:func:`Py_GetProgramFullPath`) and for :data:`sys.prefix` and
:data:`sys.exec_prefix` to be empty. It is up to the caller to modify these
if required after calling :c:func:`Py_Initialize`.
Use :c:func:`Py_DecodeLocale` to decode a bytes string to get a
:c:type:`wchar_*` string.
The path argument is copied internally, so the caller may free it after the
call completes.
.. versionchanged:: 3.8
The program full path is now used for :data:`sys.executable`, instead
of the program name.
.. deprecated:: 3.11
.. c:function:: const char* Py_GetVersion()
Return the version of this Python interpreter. This is a string that looks
something like ::
"3.0a5+ (py3k:63103M, May 12 2008, 00:53:55) \n[GCC 4.2.3]"
.. index:: single: version (in module sys)
The first word (up to the first space character) is the current Python version;
the first characters are the major and minor version separated by a
period. The returned string points into static storage; the caller should not
modify its value. The value is available to Python code as :data:`sys.version`.
See also the :c:var:`Py_Version` constant.
.. c:function:: const char* Py_GetPlatform()
.. index:: single: platform (in module sys)
Return the platform identifier for the current platform. On Unix, this is
formed from the "official" name of the operating system, converted to lower
case, followed by the major revision number; e.g., for Solaris 2.x, which is
also known as SunOS 5.x, the value is ``'sunos5'``. On macOS, it is
``'darwin'``. On Windows, it is ``'win'``. The returned string points into
static storage; the caller should not modify its value. The value is available
to Python code as ``sys.platform``.
.. c:function:: const char* Py_GetCopyright()
Return the official copyright string for the current Python version, for example
``'Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam'``
.. index:: single: copyright (in module sys)
The returned string points into static storage; the caller should not modify its
value. The value is available to Python code as ``sys.copyright``.
.. c:function:: const char* Py_GetCompiler()
Return an indication of the compiler used to build the current Python version,
in square brackets, for example::
"[GCC 2.7.2.2]"
.. index:: single: version (in module sys)
The returned string points into static storage; the caller should not modify its
value. The value is available to Python code as part of the variable
``sys.version``.
.. c:function:: const char* Py_GetBuildInfo()
Return information about the sequence number and build date and time of the
current Python interpreter instance, for example ::
"#67, Aug 1 1997, 22:34:28"
.. index:: single: version (in module sys)
The returned string points into static storage; the caller should not modify its
value. The value is available to Python code as part of the variable
``sys.version``.
.. c:function:: void PySys_SetArgvEx(int argc, wchar_t **argv, int updatepath)
.. index::
single: main()
single: Py_FatalError()
single: argv (in module sys)
Set :data:`sys.argv` based on *argc* and *argv*. These parameters are
similar to those passed to the program's :c:func:`main` function with the
difference that the first entry should refer to the script file to be
executed rather than the executable hosting the Python interpreter. If there
isn't a script that will be run, the first entry in *argv* can be an empty
string. If this function fails to initialize :data:`sys.argv`, a fatal
condition is signalled using :c:func:`Py_FatalError`.
If *updatepath* is zero, this is all the function does. If *updatepath*
is non-zero, the function also modifies :data:`sys.path` according to the
following algorithm:
- If the name of an existing script is passed in ``argv[0]``, the absolute
path of the directory where the script is located is prepended to
:data:`sys.path`.
- Otherwise (that is, if *argc* is ``0`` or ``argv[0]`` doesn't point
to an existing file name), an empty string is prepended to
:data:`sys.path`, which is the same as prepending the current working
directory (``"."``).
Use :c:func:`Py_DecodeLocale` to decode a bytes string to get a
:c:type:`wchar_*` string.
See also :c:member:`PyConfig.orig_argv` and :c:member:`PyConfig.argv`
members of the :ref:`Python Initialization Configuration <init-config>`.
.. note::
It is recommended that applications embedding the Python interpreter
for purposes other than executing a single script pass ``0`` as *updatepath*,
and update :data:`sys.path` themselves if desired.
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See `CVE-2008-5983 <https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2008-5983>`_.
On versions before 3.1.3, you can achieve the same effect by manually
popping the first :data:`sys.path` element after having called
:c:func:`PySys_SetArgv`, for example using::
PyRun_SimpleString("import sys; sys.path.pop(0)\n");
.. versionadded:: 3.1.3
.. XXX impl. doesn't seem consistent in allowing ``0``/``NULL`` for the params;
.. c:function:: void PySys_SetArgv(int argc, wchar_t **argv)
This function works like :c:func:`PySys_SetArgvEx` with *updatepath* set
to ``1`` unless the :program:`python` interpreter was started with the
:option:`-I`.
Use :c:func:`Py_DecodeLocale` to decode a bytes string to get a
:c:type:`wchar_*` string.
See also :c:member:`PyConfig.orig_argv` and :c:member:`PyConfig.argv`
members of the :ref:`Python Initialization Configuration <init-config>`.
.. versionchanged:: 3.4 The *updatepath* value depends on :option:`-I`.
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.. c:function:: void Py_SetPythonHome(const wchar_t *home)
This API is kept for backward compatibility: setting
:c:member:`PyConfig.home` should be used instead, see :ref:`Python
Initialization Configuration <init-config>`.
Set the default "home" directory, that is, the location of the standard
Python libraries. See :envvar:`PYTHONHOME` for the meaning of the
argument string.
The argument should point to a zero-terminated character string in static
storage whose contents will not change for the duration of the program's
execution. No code in the Python interpreter will change the contents of
this storage.
Use :c:func:`Py_DecodeLocale` to decode a bytes string to get a
:c:type:`wchar_*` string.
.. deprecated:: 3.11
.. c:function:: w_char* Py_GetPythonHome()
Return the default "home", that is, the value set by a previous call to
:c:func:`Py_SetPythonHome`, or the value of the :envvar:`PYTHONHOME`
This function should not be called before :c:func:`Py_Initialize`, otherwise
it returns ``NULL``.
.. versionchanged:: 3.10
It now returns ``NULL`` if called before :c:func:`Py_Initialize`.
.. _threads:
Thread State and the Global Interpreter Lock
============================================
.. index::
single: global interpreter lock
single: interpreter lock
single: lock, interpreter
The Python interpreter is not fully thread-safe. In order to support
multi-threaded Python programs, there's a global lock, called the :term:`global
interpreter lock` or :term:`GIL`, that must be held by the current thread before
it can safely access Python objects. Without the lock, even the simplest
operations could cause problems in a multi-threaded program: for example, when
two threads simultaneously increment the reference count of the same object, the
reference count could end up being incremented only once instead of twice.
.. index:: single: setswitchinterval() (in module sys)
Therefore, the rule exists that only the thread that has acquired the
:term:`GIL` may operate on Python objects or call Python/C API functions.
In order to emulate concurrency of execution, the interpreter regularly
tries to switch threads (see :func:`sys.setswitchinterval`). The lock is also
released around potentially blocking I/O operations like reading or writing
a file, so that other Python threads can run in the meantime.
.. index::
single: PyThreadState
single: PyThreadState
The Python interpreter keeps some thread-specific bookkeeping information
inside a data structure called :c:type:`PyThreadState`. There's also one
global variable pointing to the current :c:type:`PyThreadState`: it can
be retrieved using :c:func:`PyThreadState_Get`.
Releasing the GIL from extension code
-------------------------------------
Most extension code manipulating the :term:`GIL` has the following simple
structure::
Save the thread state in a local variable.
... Do some blocking I/O operation ...
Restore the thread state from the local variable.
This is so common that a pair of macros exists to simplify it::
Py_BEGIN_ALLOW_THREADS
... Do some blocking I/O operation ...
Py_END_ALLOW_THREADS
.. index::
single: Py_BEGIN_ALLOW_THREADS
single: Py_END_ALLOW_THREADS
The :c:macro:`Py_BEGIN_ALLOW_THREADS` macro opens a new block and declares a
hidden local variable; the :c:macro:`Py_END_ALLOW_THREADS` macro closes the
The block above expands to the following code::
PyThreadState *_save;
_save = PyEval_SaveThread();
... Do some blocking I/O operation ...
PyEval_RestoreThread(_save);
.. index::
single: PyEval_RestoreThread()
single: PyEval_SaveThread()
Here is how these functions work: the global interpreter lock is used to protect the pointer to the
current thread state. When releasing the lock and saving the thread state,
the current thread state pointer must be retrieved before the lock is released
(since another thread could immediately acquire the lock and store its own thread
state in the global variable). Conversely, when acquiring the lock and restoring
the thread state, the lock must be acquired before storing the thread state
pointer.
.. note::
Calling system I/O functions is the most common use case for releasing
the GIL, but it can also be useful before calling long-running computations
which don't need access to Python objects, such as compression or
cryptographic functions operating over memory buffers. For example, the
standard :mod:`zlib` and :mod:`hashlib` modules release the GIL when
compressing or hashing data.
.. _gilstate:
Non-Python created threads
--------------------------
When threads are created using the dedicated Python APIs (such as the
:mod:`threading` module), a thread state is automatically associated to them
and the code showed above is therefore correct. However, when threads are
created from C (for example by a third-party library with its own thread
management), they don't hold the GIL, nor is there a thread state structure
for them.
If you need to call Python code from these threads (often this will be part
of a callback API provided by the aforementioned third-party library),
you must first register these threads with the interpreter by
creating a thread state data structure, then acquiring the GIL, and finally
storing their thread state pointer, before you can start using the Python/C
API. When you are done, you should reset the thread state pointer, release
the GIL, and finally free the thread state data structure.
The :c:func:`PyGILState_Ensure` and :c:func:`PyGILState_Release` functions do
all of the above automatically. The typical idiom for calling into Python
from a C thread is::
PyGILState_STATE gstate;
gstate = PyGILState_Ensure();
/* Perform Python actions here. */
/* evaluate result or handle exception */
/* Release the thread. No Python API allowed beyond this point. */
PyGILState_Release(gstate);
Note that the :c:func:`PyGILState_\*` functions assume there is only one global
interpreter (created automatically by :c:func:`Py_Initialize`). Python
supports the creation of additional interpreters (using
:c:func:`Py_NewInterpreter`), but mixing multiple interpreters and the
:c:func:`PyGILState_\*` API is unsupported.
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.. _fork-and-threads:
Cautions about fork()
---------------------
Another important thing to note about threads is their behaviour in the face
of the C :c:func:`fork` call. On most systems with :c:func:`fork`, after a
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process forks only the thread that issued the fork will exist. This has a
concrete impact both on how locks must be handled and on all stored state
in CPython's runtime.
The fact that only the "current" thread remains
means any locks held by other threads will never be released. Python solves
this for :func:`os.fork` by acquiring the locks it uses internally before
the fork, and releasing them afterwards. In addition, it resets any
:ref:`lock-objects` in the child. When extending or embedding Python, there
is no way to inform Python of additional (non-Python) locks that need to be
acquired before or reset after a fork. OS facilities such as
:c:func:`pthread_atfork` would need to be used to accomplish the same thing.
Additionally, when extending or embedding Python, calling :c:func:`fork`
directly rather than through :func:`os.fork` (and returning to or calling
into Python) may result in a deadlock by one of Python's internal locks
being held by a thread that is defunct after the fork.
:c:func:`PyOS_AfterFork_Child` tries to reset the necessary locks, but is not
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The fact that all other threads go away also means that CPython's
runtime state there must be cleaned up properly, which :func:`os.fork`
does. This means finalizing all other :c:type:`PyThreadState` objects
belonging to the current interpreter and all other
:c:type:`PyInterpreterState` objects. Due to this and the special
nature of the :ref:`"main" interpreter <sub-interpreter-support>`,
:c:func:`fork` should only be called in that interpreter's "main"
thread, where the CPython global runtime was originally initialized.
The only exception is if :c:func:`exec` will be called immediately
after.
High-level API
--------------
These are the most commonly used types and functions when writing C extension
code, or when embedding the Python interpreter:
.. c:type:: PyInterpreterState
This data structure represents the state shared by a number of cooperating
threads. Threads belonging to the same interpreter share their module
administration and a few other internal items. There are no public members in
this structure.
Threads belonging to different interpreters initially share nothing, except
process state like available memory, open file descriptors and such. The global
interpreter lock is also shared by all threads, regardless of to which
interpreter they belong.
This data structure represents the state of a single thread. The only public
data member is :attr:`interp` (:c:type:`PyInterpreterState *`), which points to
this thread's interpreter state.
.. c:function:: void PyEval_InitThreads()
single: PyEval_ReleaseThread()
single: PyEval_SaveThread()
single: PyEval_RestoreThread()
Deprecated function which does nothing.
In Python 3.6 and older, this function created the GIL if it didn't exist.
.. versionchanged:: 3.9
The function now does nothing.
.. versionchanged:: 3.7
This function is now called by :c:func:`Py_Initialize()`, so you don't
have to call it yourself anymore.
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.. versionchanged:: 3.2
This function cannot be called before :c:func:`Py_Initialize()` anymore.
.. deprecated-removed:: 3.9 3.11
.. index:: module: _thread
.. c:function:: int PyEval_ThreadsInitialized()
Returns a non-zero value if :c:func:`PyEval_InitThreads` has been called. This
function can be called without holding the GIL, and therefore can be used to
avoid calls to the locking API when running single-threaded.
.. versionchanged:: 3.7
The :term:`GIL` is now initialized by :c:func:`Py_Initialize()`.
.. deprecated-removed:: 3.9 3.11
.. c:function:: PyThreadState* PyEval_SaveThread()
Release the global interpreter lock (if it has been created) and reset the
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thread state to ``NULL``, returning the previous thread state (which is not
``NULL``). If the lock has been created, the current thread must have
.. c:function:: void PyEval_RestoreThread(PyThreadState *tstate)
Acquire the global interpreter lock (if it has been created) and set the
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thread state to *tstate*, which must not be ``NULL``. If the lock has been
created, the current thread must not have acquired it, otherwise deadlock
ensues.
.. note::
Calling this function from a thread when the runtime is finalizing
will terminate the thread, even if the thread was not created by Python.
You can use :c:func:`_Py_IsFinalizing` or :func:`sys.is_finalizing` to
check if the interpreter is in process of being finalized before calling
this function to avoid unwanted termination.
.. c:function:: PyThreadState* PyThreadState_Get()
Return the current thread state. The global interpreter lock must be held.
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When the current thread state is ``NULL``, this issues a fatal error (so that
the caller needn't check for ``NULL``).
.. c:function:: PyThreadState* PyThreadState_Swap(PyThreadState *tstate)
Swap the current thread state with the thread state given by the argument
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*tstate*, which may be ``NULL``. The global interpreter lock must be held
and is not released.
The following functions use thread-local storage, and are not compatible
with sub-interpreters:
.. c:function:: PyGILState_STATE PyGILState_Ensure()
Ensure that the current thread is ready to call the Python C API regardless
of the current state of Python, or of the global interpreter lock. This may
be called as many times as desired by a thread as long as each call is
matched with a call to :c:func:`PyGILState_Release`. In general, other
thread-related APIs may be used between :c:func:`PyGILState_Ensure` and
:c:func:`PyGILState_Release` calls as long as the thread state is restored to
its previous state before the Release(). For example, normal usage of the
:c:macro:`Py_BEGIN_ALLOW_THREADS` and :c:macro:`Py_END_ALLOW_THREADS` macros is
acceptable.
The return value is an opaque "handle" to the thread state when
:c:func:`PyGILState_Ensure` was called, and must be passed to
:c:func:`PyGILState_Release` to ensure Python is left in the same state. Even