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v3.11.15
Doc/c-api/init.rst
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Serhiy Storchaka
[3.11] gh-115664: Fix ordering of more versionadded and versionchanged directives (GH-116298) (GH-116450) (GH-116452)
07 мар 2024, 11:35
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07 мар 2024, 11:35
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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`, :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`. 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:`!io._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) pair: module; builtins pair: module; __main__ pair: module; sys triple: module; search; path single: PySys_SetArgv (C function) single: PySys_SetArgvEx (C function) single: Py_FinalizeEx (C function) 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 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:`~object.__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. Process-wide parameters ======================= .. 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 .. 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:expr:`wchar_*` string. .. deprecated:: 3.11 .. c:function:: wchar_t* 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 :option:`--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) single: Py_SetPath() 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, ``';'`` on Windows. 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:expr:`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) This API is kept for backward compatibility: setting :c:member:`PyConfig.argv`, :c:member:`PyConfig.parse_argv` and :c:member:`PyConfig.safe_path` should be used instead, see :ref:`Python Initialization Configuration <init-config>`. 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:expr:`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. 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; check w/ Guido. .. deprecated:: 3.11 .. c:function:: void PySys_SetArgv(int argc, wchar_t **argv) This API is kept for backward compatibility: setting :c:member:`PyConfig.argv` and :c:member:`PyConfig.parse_argv` should be used instead, see :ref:`Python Initialization Configuration <init-config>`. 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:expr:`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`. .. deprecated:: 3.11 .. 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:expr:`wchar_*` string. .. deprecated:: 3.11 .. c:function:: wchar_t* 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` environment variable if it is set. 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 (C type) 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. Release the global interpreter lock. ... Do some blocking I/O operation ... Reacquire the global interpreter lock. 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 (C macro) single: Py_END_ALLOW_THREADS (C macro) 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 block. 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 (C function) single: PyEval_SaveThread (C function) 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. */ result = CallSomeFunction(); /* evaluate result or handle exception */ /* Release the thread. No Python API allowed beyond this point. */ PyGILState_Release(gstate); Note that the ``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 ``PyGILState_*`` API is unsupported. .. _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 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 always able to. 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. .. c:type:: PyThreadState This data structure represents the state of a single thread. The only public data member is: .. c:member:: PyInterpreterState *interp This thread's interpreter state. .. c:function:: void PyEval_InitThreads() .. index:: single: PyEval_AcquireThread() 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. .. versionchanged:: 3.2 This function cannot be called before :c:func:`Py_Initialize()` anymore. .. deprecated:: 3.9 .. index:: pair: 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:: 3.9 .. c:function:: PyThreadState* PyEval_SaveThread() Release the global interpreter lock (if it has been created) and reset the thread state to ``NULL``, returning the previous thread state (which is not ``NULL``). If the lock has been created, the current thread must have acquired it. .. c:function:: void PyEval_RestoreThread(PyThreadState *tstate) Acquire the global interpreter lock (if it has been created) and set the 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. 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 *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 though recursive calls are allowed, these handles *cannot* be shared - each unique call to :c:func:`PyGILState_Ensure` must save the handle for its call to :c:func:`PyGILState_Release`. When the function returns, the current thread will hold the GIL and be able to call arbitrary Python code. Failure is a fatal error. .. 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:: void PyGILState_Release(PyGILState_STATE) Release any resources previously acquired. After this call, Python's state will be the same as it was prior to the corresponding :c:func:`PyGILState_Ensure` call (but generally this state will be unknown to the caller, hence the use of the GILState API). Every call to :c:func:`PyGILState_Ensure` must be matched by a call to :c:func:`PyGILState_Release` on the same thread. .. c:function:: PyThreadState* PyGILState_GetThisThreadState() Get the current thread state for this thread. May return ``NULL`` if no GILState API has been used on the current thread. Note that the main thread always has such a thread-state, even if no auto-thread-state call has been made on the main thread. This is mainly a helper/diagnostic function. .. c:function:: int PyGILState_Check() Return ``1`` if the current thread is holding the GIL and ``0`` otherwise. This function can be called from any thread at any time. Only if it has had its Python thread state initialized and currently is holding the GIL will it return ``1``. This is mainly a helper/diagnostic function. It can be useful for example in callback contexts or memory allocation functions when knowing that the GIL is locked can allow the caller to perform sensitive actions or otherwise behave differently. .. versionadded:: 3.4 The following macros are normally used without a trailing semicolon; look for example usage in the Python source distribution. .. c:macro:: Py_BEGIN_ALLOW_THREADS This macro expands to ``{ PyThreadState *_save; _save = PyEval_SaveThread();``. Note that it contains an opening brace; it must be matched with a following :c:macro:`Py_END_ALLOW_THREADS` macro. See above for further discussion of this macro. .. c:macro:: Py_END_ALLOW_THREADS This macro expands to ``PyEval_RestoreThread(_save); }``. Note that it contains a closing brace; it must be matched with an earlier :c:macro:`Py_BEGIN_ALLOW_THREADS` macro. See above for further discussion of this macro. .. c:macro:: Py_BLOCK_THREADS This macro expands to ``PyEval_RestoreThread(_save);``: it is equivalent to :c:macro:`Py_END_ALLOW_THREADS` without the closing brace. .. c:macro:: Py_UNBLOCK_THREADS This macro expands to ``_save = PyEval_SaveThread();``: it is equivalent to :c:macro:`Py_BEGIN_ALLOW_THREADS` without the opening brace and variable declaration. Low-level API ------------- All of the following functions must be called after :c:func:`Py_Initialize`. .. versionchanged:: 3.7 :c:func:`Py_Initialize()` now initializes the :term:`GIL`. .. c:function:: PyInterpreterState* PyInterpreterState_New() Create a new interpreter state object. The global interpreter lock need not be held, but may be held if it is necessary to serialize calls to this function. .. audit-event:: cpython.PyInterpreterState_New "" c.PyInterpreterState_New .. c:function:: void PyInterpreterState_Clear(PyInterpreterState *interp) Reset all information in an interpreter state object. The global interpreter lock must be held. .. audit-event:: cpython.PyInterpreterState_Clear "" c.PyInterpreterState_Clear .. c:function:: void PyInterpreterState_Delete(PyInterpreterState *interp) Destroy an interpreter state object. The global interpreter lock need not be held. The interpreter state must have been reset with a previous call to :c:func:`PyInterpreterState_Clear`. .. c:function:: PyThreadState* PyThreadState_New(PyInterpreterState *interp) Create a new thread state object belonging to the given interpreter object. The global interpreter lock need not be held, but may be held if it is necessary to serialize calls to this function. .. c:function:: void PyThreadState_Clear(PyThreadState *tstate) Reset all information in a thread state object. The global interpreter lock must be held. .. versionchanged:: 3.9 This function now calls the :c:member:`PyThreadState.on_delete` callback. Previously, that happened in :c:func:`PyThreadState_Delete`. .. c:function:: void PyThreadState_Delete(PyThreadState *tstate) Destroy a thread state object. The global interpreter lock need not be held. The thread state must have been reset with a previous call to :c:func:`PyThreadState_Clear`. .. c:function:: void PyThreadState_DeleteCurrent(void) Destroy the current thread state and release the global interpreter lock. Like :c:func:`PyThreadState_Delete`, the global interpreter lock need not be held. The thread state must have been reset with a previous call to :c:func:`PyThreadState_Clear`. .. c:function:: PyFrameObject* PyThreadState_GetFrame(PyThreadState *tstate) Get the current frame of the Python thread state *tstate*. Return a :term:`strong reference`. Return ``NULL`` if no frame is currently executing. See also :c:func:`PyEval_GetFrame`. *tstate* must not be ``NULL``. .. versionadded:: 3.9 .. c:function:: uint64_t PyThreadState_GetID(PyThreadState *tstate) Get the unique thread state identifier of the Python thread state *tstate*. *tstate* must not be ``NULL``. .. versionadded:: 3.9 .. c:function:: PyInterpreterState* PyThreadState_GetInterpreter(PyThreadState *tstate) Get the interpreter of the Python thread state *tstate*. *tstate* must not be ``NULL``. .. versionadded:: 3.9 .. c:function:: void PyThreadState_EnterTracing(PyThreadState *tstate) Suspend tracing and profiling in the Python thread state *tstate*. Resume them using the :c:func:`PyThreadState_LeaveTracing` function. .. versionadded:: 3.11 .. c:function:: void PyThreadState_LeaveTracing(PyThreadState *tstate) Resume tracing and profiling in the Python thread state *tstate* suspended by the :c:func:`PyThreadState_EnterTracing` function. See also :c:func:`PyEval_SetTrace` and :c:func:`PyEval_SetProfile` functions. .. versionadded:: 3.11 .. c:function:: PyInterpreterState* PyInterpreterState_Get(void) Get the current interpreter. Issue a fatal error if there no current Python thread state or no current interpreter. It cannot return NULL. The caller must hold the GIL. .. versionadded:: 3.9 .. c:function:: int64_t PyInterpreterState_GetID(PyInterpreterState *interp) Return the interpreter's unique ID. If there was any error in doing so then ``-1`` is returned and an error is set. The caller must hold the GIL. .. versionadded:: 3.7 .. c:function:: PyObject* PyInterpreterState_GetDict(PyInterpreterState *interp) Return a dictionary in which interpreter-specific data may be stored. If this function returns ``NULL`` then no exception has been raised and the caller should assume no interpreter-specific dict is available. This is not a replacement for :c:func:`PyModule_GetState()`, which extensions should use to store interpreter-specific state information. .. versionadded:: 3.8 .. c:type:: PyObject* (*_PyFrameEvalFunction)(PyThreadState *tstate, _PyInterpreterFrame *frame, int throwflag) Type of a frame evaluation function. The *throwflag* parameter is used by the ``throw()`` method of generators: if non-zero, handle the current exception. .. versionchanged:: 3.9 The function now takes a *tstate* parameter. .. versionchanged:: 3.11 The *frame* parameter changed from ``PyFrameObject*`` to ``_PyInterpreterFrame*``. .. c:function:: _PyFrameEvalFunction _PyInterpreterState_GetEvalFrameFunc(PyInterpreterState *interp) Get the frame evaluation function. See the :pep:`523` "Adding a frame evaluation API to CPython". .. versionadded:: 3.9 .. c:function:: void _PyInterpreterState_SetEvalFrameFunc(PyInterpreterState *interp, _PyFrameEvalFunction eval_frame) Set the frame evaluation function. See the :pep:`523` "Adding a frame evaluation API to CPython". .. versionadded:: 3.9 .. c:function:: PyObject* PyThreadState_GetDict() Return a dictionary in which extensions can store thread-specific state information. Each extension should use a unique key to use to store state in the dictionary. It is okay to call this function when no current thread state is available. If this function returns ``NULL``, no exception has been raised and the caller should assume no current thread state is available. .. c:function:: int PyThreadState_SetAsyncExc(unsigned long id, PyObject *exc) Asynchronously raise an exception in a thread. The *id* argument is the thread id of the target thread; *exc* is the exception object to be raised. This function does not steal any references to *exc*. To prevent naive misuse, you must write your own C extension to call this. Must be called with the GIL held. Returns the number of thread states modified; this is normally one, but will be zero if the thread id isn't found. If *exc* is ``NULL``, the pending exception (if any) for the thread is cleared. This raises no exceptions. .. versionchanged:: 3.7 The type of the *id* parameter changed from :c:expr:`long` to :c:expr:`unsigned long`. .. c:function:: void PyEval_AcquireThread(PyThreadState *tstate) Acquire the global interpreter lock and set the current thread state to *tstate*, which must not be ``NULL``. The lock must have been created earlier. If this thread already has the lock, 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. .. versionchanged:: 3.8 Updated to be consistent with :c:func:`PyEval_RestoreThread`, :c:func:`Py_END_ALLOW_THREADS`, and :c:func:`PyGILState_Ensure`, and terminate the current thread if called while the interpreter is finalizing. :c:func:`PyEval_RestoreThread` is a higher-level function which is always available (even when threads have not been initialized). .. c:function:: void PyEval_ReleaseThread(PyThreadState *tstate) Reset the current thread state to ``NULL`` and release the global interpreter lock. The lock must have been created earlier and must be held by the current thread. The *tstate* argument, which must not be ``NULL``, is only used to check that it represents the current thread state --- if it isn't, a fatal error is reported. :c:func:`PyEval_SaveThread` is a higher-level function which is always available (even when threads have not been initialized). .. c:function:: void PyEval_AcquireLock() Acquire the global interpreter lock. The lock must have been created earlier. If this thread already has the lock, a deadlock ensues. .. deprecated:: 3.2 This function does not update the current thread state. Please use :c:func:`PyEval_RestoreThread` or :c:func:`PyEval_AcquireThread` instead. .. 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. .. versionchanged:: 3.8 Updated to be consistent with :c:func:`PyEval_RestoreThread`, :c:func:`Py_END_ALLOW_THREADS`, and :c:func:`PyGILState_Ensure`, and terminate the current thread if called while the interpreter is finalizing. .. c:function:: void PyEval_ReleaseLock() Release the global interpreter lock. The lock must have been created earlier. .. deprecated:: 3.2 This function does not update the current thread state. Please use :c:func:`PyEval_SaveThread` or :c:func:`PyEval_ReleaseThread` instead. .. _sub-interpreter-support: Sub-interpreter support ======================= While in most uses, you will only embed a single Python interpreter, there are cases where you need to create several independent interpreters in the same process and perhaps even in the same thread. Sub-interpreters allow you to do that. The "main" interpreter is the first one created when the runtime initializes. It is usually the only Python interpreter in a process. Unlike sub-interpreters, the main interpreter has unique process-global responsibilities like signal handling. It is also responsible for execution during runtime initialization and is usually the active interpreter during runtime finalization. The :c:func:`PyInterpreterState_Main` function returns a pointer to its state. You can switch between sub-interpreters using the :c:func:`PyThreadState_Swap` function. You can create and destroy them using the following functions: .. c:function:: PyThreadState* Py_NewInterpreter() .. index:: pair: module; builtins pair: module; __main__ pair: module; sys single: stdout (in module sys) single: stderr (in module sys) single: stdin (in module sys) Create a new sub-interpreter. This is an (almost) totally separate environment for the execution of Python code. In particular, the new interpreter has separate, independent versions of all imported modules, including the fundamental modules :mod:`builtins`, :mod:`__main__` and :mod:`sys`. The table of loaded modules (``sys.modules``) and the module search path (``sys.path``) are also separate. The new environment has no ``sys.argv`` variable. It has new standard I/O stream file objects ``sys.stdin``, ``sys.stdout`` and ``sys.stderr`` (however these refer to the same underlying file descriptors). The return value points to the first thread state created in the new sub-interpreter. This thread state is made in the current thread state. Note that no actual thread is created; see the discussion of thread states below. If creation of the new interpreter is unsuccessful, ``NULL`` is returned; no exception is set since the exception state is stored in the current thread state and there may not be a current thread state. (Like all other Python/C API functions, the global interpreter lock must be held before calling this function and is still held when it returns; however, unlike most other Python/C API functions, there needn't be a current thread state on entry.) .. index:: single: Py_FinalizeEx (C function) single: Py_Initialize (C function) Extension modules are shared between (sub-)interpreters as follows: * For modules using multi-phase initialization, e.g. :c:func:`PyModule_FromDefAndSpec`, a separate module object is created and initialized for each interpreter. Only C-level static and global variables are shared between these module objects. * For modules using single-phase initialization, e.g. :c:func:`PyModule_Create`, the first time a particular extension is imported, it is initialized normally, and a (shallow) copy of its module's dictionary is squirreled away. When the same extension is imported by another (sub-)interpreter, a new module is initialized and filled with the contents of this copy; the extension's ``init`` function is not called. Objects in the module's dictionary thus end up shared across (sub-)interpreters, which might cause unwanted behavior (see `Bugs and caveats`_ below). Note that this is different from what happens when an extension is imported after the interpreter has been completely re-initialized by calling :c:func:`Py_FinalizeEx` and :c:func:`Py_Initialize`; in that case, the extension's ``initmodule`` function *is* called again. As with multi-phase initialization, this means that only C-level static and global variables are shared between these modules. .. index:: single: close (in module os) .. c:function:: void Py_EndInterpreter(PyThreadState *tstate) .. index:: single: Py_FinalizeEx (C function) Destroy the (sub-)interpreter represented by the given thread state. The given thread state must be the current thread state. See the discussion of thread states below. When the call returns, the current thread state is ``NULL``. All thread states associated with this interpreter are destroyed. (The global interpreter lock must be held before calling this function and is still held when it returns.) :c:func:`Py_FinalizeEx` will destroy all sub-interpreters that haven't been explicitly destroyed at that point. Bugs and caveats ---------------- Because sub-interpreters (and the main interpreter) are part of the same process, the insulation between them isn't perfect --- for example, using low-level file operations like :func:`os.close` they can (accidentally or maliciously) affect each other's open files. Because of the way extensions are shared between (sub-)interpreters, some extensions may not work properly; this is especially likely when using single-phase initialization or (static) global variables. It is possible to insert objects created in one sub-interpreter into a namespace of another (sub-)interpreter; this should be avoided if possible. Special care should be taken to avoid sharing user-defined functions, methods, instances or classes between sub-interpreters, since import operations executed by such objects may affect the wrong (sub-)interpreter's dictionary of loaded modules. It is equally important to avoid sharing objects from which the above are reachable. Also note that combining this functionality with ``PyGILState_*`` APIs is delicate, because these APIs assume a bijection between Python thread states and OS-level threads, an assumption broken by the presence of sub-interpreters. It is highly recommended that you don't switch sub-interpreters between a pair of matching :c:func:`PyGILState_Ensure` and :c:func:`PyGILState_Release` calls. Furthermore, extensions (such as :mod:`ctypes`) using these APIs to allow calling of Python code from non-Python created threads will probably be broken when using sub-interpreters. Asynchronous Notifications ========================== A mechanism is provided to make asynchronous notifications to the main interpreter thread. These notifications take the form of a function pointer and a void pointer argument. .. c:function:: int Py_AddPendingCall(int (*func)(void *), void *arg) Schedule a function to be called from the main interpreter thread. On success, ``0`` is returned and *func* is queued for being called in the main thread. On failure, ``-1`` is returned without setting any exception. When successfully queued, *func* will be *eventually* called from the main interpreter thread with the argument *arg*. It will be called asynchronously with respect to normally running Python code, but with both these conditions met: * on a :term:`bytecode` boundary; * with the main thread holding the :term:`global interpreter lock` (*func* can therefore use the full C API). *func* must return ``0`` on success, or ``-1`` on failure with an exception set. *func* won't be interrupted to perform another asynchronous notification recursively, but it can still be interrupted to switch threads if the global interpreter lock is released. This function doesn't need a current thread state to run, and it doesn't need the global interpreter lock. To call this function in a subinterpreter, the caller must hold the GIL. Otherwise, the function *func* can be scheduled to be called from the wrong interpreter. .. warning:: This is a low-level function, only useful for very special cases. There is no guarantee that *func* will be called as quick as possible. If the main thread is busy executing a system call, *func* won't be called before the system call returns. This function is generally **not** suitable for calling Python code from arbitrary C threads. Instead, use the :ref:`PyGILState API<gilstate>`. .. versionadded:: 3.1 .. versionchanged:: 3.9 If this function is called in a subinterpreter, the function *func* is now scheduled to be called from the subinterpreter, rather than being called from the main interpreter. Each subinterpreter now has its own list of scheduled calls. .. _profiling: Profiling and Tracing ===================== .. sectionauthor:: Fred L. Drake, Jr. <fdrake@acm.org> The Python interpreter provides some low-level support for attaching profiling and execution tracing facilities. These are used for profiling, debugging, and coverage analysis tools. This C interface allows the profiling or tracing code to avoid the overhead of calling through Python-level callable objects, making a direct C function call instead. The essential attributes of the facility have not changed; the interface allows trace functions to be installed per-thread, and the basic events reported to the trace function are the same as had been reported to the Python-level trace functions in previous versions. .. c:type:: int (*Py_tracefunc)(PyObject *obj, PyFrameObject *frame, int what, PyObject *arg) The type of the trace function registered using :c:func:`PyEval_SetProfile` and :c:func:`PyEval_SetTrace`. The first parameter is the object passed to the registration function as *obj*, *frame* is the frame object to which the event pertains, *what* is one of the constants :c:data:`PyTrace_CALL`, :c:data:`PyTrace_EXCEPTION`, :c:data:`PyTrace_LINE`, :c:data:`PyTrace_RETURN`, :c:data:`PyTrace_C_CALL`, :c:data:`PyTrace_C_EXCEPTION`, :c:data:`PyTrace_C_RETURN`, or :c:data:`PyTrace_OPCODE`, and *arg* depends on the value of *what*: +-------------------------------+----------------------------------------+ | Value of *what* | Meaning of *arg* | +===============================+========================================+ | :c:data:`PyTrace_CALL` | Always :c:data:`Py_None`. | +-------------------------------+----------------------------------------+ | :c:data:`PyTrace_EXCEPTION` | Exception information as returned by | | | :func:`sys.exc_info`. | +-------------------------------+----------------------------------------+ | :c:data:`PyTrace_LINE` | Always :c:data:`Py_None`. | +-------------------------------+----------------------------------------+ | :c:data:`PyTrace_RETURN` | Value being returned to the caller, | | | or ``NULL`` if caused by an exception. | +-------------------------------+----------------------------------------+ | :c:data:`PyTrace_C_CALL` | Function object being called. | +-------------------------------+----------------------------------------+ | :c:data:`PyTrace_C_EXCEPTION` | Function object being called. | +-------------------------------+----------------------------------------+ | :c:data:`PyTrace_C_RETURN` | Function object being called. | +-------------------------------+----------------------------------------+ | :c:data:`PyTrace_OPCODE` | Always :c:data:`Py_None`. | +-------------------------------+----------------------------------------+ .. c:var:: int PyTrace_CALL The value of the *what* parameter to a :c:type:`Py_tracefunc` function when a new call to a function or method is being reported, or a new entry into a generator. Note that the creation of the iterator for a generator function is not reported as there is no control transfer to the Python bytecode in the corresponding frame. .. c:var:: int PyTrace_EXCEPTION The value of the *what* parameter to a :c:type:`Py_tracefunc` function when an exception has been raised. The callback function is called with this value for *what* when after any bytecode is processed after which the exception becomes set within the frame being executed. The effect of this is that as exception propagation causes the Python stack to unwind, the callback is called upon return to each frame as the exception propagates. Only trace functions receives these events; they are not needed by the profiler. .. c:var:: int PyTrace_LINE The value passed as the *what* parameter to a :c:type:`Py_tracefunc` function (but not a profiling function) when a line-number event is being reported. It may be disabled for a frame by setting :attr:`~frame.f_trace_lines` to *0* on that frame. .. c:var:: int PyTrace_RETURN The value for the *what* parameter to :c:type:`Py_tracefunc` functions when a call is about to return. .. c:var:: int PyTrace_C_CALL The value for the *what* parameter to :c:type:`Py_tracefunc` functions when a C function is about to be called. .. c:var:: int PyTrace_C_EXCEPTION The value for the *what* parameter to :c:type:`Py_tracefunc` functions when a C function has raised an exception. .. c:var:: int PyTrace_C_RETURN The value for the *what* parameter to :c:type:`Py_tracefunc` functions when a C function has returned. .. c:var:: int PyTrace_OPCODE The value for the *what* parameter to :c:type:`Py_tracefunc` functions (but not profiling functions) when a new opcode is about to be executed. This event is not emitted by default: it must be explicitly requested by setting :attr:`~frame.f_trace_opcodes` to *1* on the frame. .. c:function:: void PyEval_SetProfile(Py_tracefunc func, PyObject *obj) Set the profiler function to *func*. The *obj* parameter is passed to the function as its first parameter, and may be any Python object, or ``NULL``. If the profile function needs to maintain state, using a different value for *obj* for each thread provides a convenient and thread-safe place to store it. The profile function is called for all monitored events except :c:data:`PyTrace_LINE` :c:data:`PyTrace_OPCODE` and :c:data:`PyTrace_EXCEPTION`. See also the :func:`sys.setprofile` function. The caller must hold the :term:`GIL`. .. c:function:: void PyEval_SetTrace(Py_tracefunc func, PyObject *obj) Set the tracing function to *func*. This is similar to :c:func:`PyEval_SetProfile`, except the tracing function does receive line-number events and per-opcode events, but does not receive any event related to C function objects being called. Any trace function registered using :c:func:`PyEval_SetTrace` will not receive :c:data:`PyTrace_C_CALL`, :c:data:`PyTrace_C_EXCEPTION` or :c:data:`PyTrace_C_RETURN` as a value for the *what* parameter. See also the :func:`sys.settrace` function. The caller must hold the :term:`GIL`. .. _advanced-debugging: Advanced Debugger Support ========================= .. sectionauthor:: Fred L. Drake, Jr. <fdrake@acm.org> These functions are only intended to be used by advanced debugging tools. .. c:function:: PyInterpreterState* PyInterpreterState_Head() Return the interpreter state object at the head of the list of all such objects. .. c:function:: PyInterpreterState* PyInterpreterState_Main() Return the main interpreter state object. .. c:function:: PyInterpreterState* PyInterpreterState_Next(PyInterpreterState *interp) Return the next interpreter state object after *interp* from the list of all such objects. .. c:function:: PyThreadState * PyInterpreterState_ThreadHead(PyInterpreterState *interp) Return the pointer to the first :c:type:`PyThreadState` object in the list of threads associated with the interpreter *interp*. .. c:function:: PyThreadState* PyThreadState_Next(PyThreadState *tstate) Return the next thread state object after *tstate* from the list of all such objects belonging to the same :c:type:`PyInterpreterState` object. .. _thread-local-storage: Thread Local Storage Support ============================ .. sectionauthor:: Masayuki Yamamoto <ma3yuki.8mamo10@gmail.com> The Python interpreter provides low-level support for thread-local storage (TLS) which wraps the underlying native TLS implementation to support the Python-level thread local storage API (:class:`threading.local`). The CPython C level APIs are similar to those offered by pthreads and Windows: use a thread key and functions to associate a :c:expr:`void*` value per thread. The GIL does *not* need to be held when calling these functions; they supply their own locking. Note that :file:`Python.h` does not include the declaration of the TLS APIs, you need to include :file:`pythread.h` to use thread-local storage. .. note:: None of these API functions handle memory management on behalf of the :c:expr:`void*` values. You need to allocate and deallocate them yourself. If the :c:expr:`void*` values happen to be :c:expr:`PyObject*`, these functions don't do refcount operations on them either. .. _thread-specific-storage-api: Thread Specific Storage (TSS) API --------------------------------- TSS API is introduced to supersede the use of the existing TLS API within the CPython interpreter. This API uses a new type :c:type:`Py_tss_t` instead of :c:expr:`int` to represent thread keys. .. versionadded:: 3.7 .. seealso:: "A New C-API for Thread-Local Storage in CPython" (:pep:`539`) .. c:type:: Py_tss_t This data structure represents the state of a thread key, the definition of which may depend on the underlying TLS implementation, and it has an internal field representing the key's initialization state. There are no public members in this structure. When :ref:`Py_LIMITED_API <stable>` is not defined, static allocation of this type by :c:macro:`Py_tss_NEEDS_INIT` is allowed. .. c:macro:: Py_tss_NEEDS_INIT This macro expands to the initializer for :c:type:`Py_tss_t` variables. Note that this macro won't be defined with :ref:`Py_LIMITED_API <stable>`. Dynamic Allocation ~~~~~~~~~~~~~~~~~~ Dynamic allocation of the :c:type:`Py_tss_t`, required in extension modules built with :ref:`Py_LIMITED_API <stable>`, where static allocation of this type is not possible due to its implementation being opaque at build time. .. c:function:: Py_tss_t* PyThread_tss_alloc() Return a value which is the same state as a value initialized with :c:macro:`Py_tss_NEEDS_INIT`, or ``NULL`` in the case of dynamic allocation failure. .. c:function:: void PyThread_tss_free(Py_tss_t *key) Free the given *key* allocated by :c:func:`PyThread_tss_alloc`, after first calling :c:func:`PyThread_tss_delete` to ensure any associated thread locals have been unassigned. This is a no-op if the *key* argument is ``NULL``. .. note:: A freed key becomes a dangling pointer. You should reset the key to ``NULL``. Methods ~~~~~~~ The parameter *key* of these functions must not be ``NULL``. Moreover, the behaviors of :c:func:`PyThread_tss_set` and :c:func:`PyThread_tss_get` are undefined if the given :c:type:`Py_tss_t` has not been initialized by :c:func:`PyThread_tss_create`. .. c:function:: int PyThread_tss_is_created(Py_tss_t *key) Return a non-zero value if the given :c:type:`Py_tss_t` has been initialized by :c:func:`PyThread_tss_create`. .. c:function:: int PyThread_tss_create(Py_tss_t *key) Return a zero value on successful initialization of a TSS key. The behavior is undefined if the value pointed to by the *key* argument is not initialized by :c:macro:`Py_tss_NEEDS_INIT`. This function can be called repeatedly on the same key -- calling it on an already initialized key is a no-op and immediately returns success. .. c:function:: void PyThread_tss_delete(Py_tss_t *key) Destroy a TSS key to forget the values associated with the key across all threads, and change the key's initialization state to uninitialized. A destroyed key is able to be initialized again by :c:func:`PyThread_tss_create`. This function can be called repeatedly on the same key -- calling it on an already destroyed key is a no-op. .. c:function:: int PyThread_tss_set(Py_tss_t *key, void *value) Return a zero value to indicate successfully associating a :c:expr:`void*` value with a TSS key in the current thread. Each thread has a distinct mapping of the key to a :c:expr:`void*` value. .. c:function:: void* PyThread_tss_get(Py_tss_t *key) Return the :c:expr:`void*` value associated with a TSS key in the current thread. This returns ``NULL`` if no value is associated with the key in the current thread. .. _thread-local-storage-api: Thread Local Storage (TLS) API ------------------------------ .. deprecated:: 3.7 This API is superseded by :ref:`Thread Specific Storage (TSS) API <thread-specific-storage-api>`. .. note:: This version of the API does not support platforms where the native TLS key is defined in a way that cannot be safely cast to ``int``. On such platforms, :c:func:`PyThread_create_key` will return immediately with a failure status, and the other TLS functions will all be no-ops on such platforms. Due to the compatibility problem noted above, this version of the API should not be used in new code. .. c:function:: int PyThread_create_key() .. c:function:: void PyThread_delete_key(int key) .. c:function:: int PyThread_set_key_value(int key, void *value) .. c:function:: void* PyThread_get_key_value(int key) .. c:function:: void PyThread_delete_key_value(int key) .. c:function:: void PyThread_ReInitTLS()