# Initialization, Finalization, and Threads

See also Python Initialization Configuration.

## Before Python Initialization

In an application embedding Python, the `Py_Initialize()` function must be called before using any other Python/C API functions; with the exception of a few functions and the global configuration variables.

The following functions can be safely called before Python is initialized:

- Configuration functions:

  - `PyImport_AppendInittab()`

  - `PyImport_ExtendInittab()`

  - `PyInitFrozenExtensions()`

  - `PyMem_SetAllocator()`

  - `PyMem_SetupDebugHooks()`

  - `PyObject_SetArenaAllocator()`

  - `Py_SetPath()`

  - `Py_SetProgramName()`

  - `Py_SetPythonHome()`

  - `Py_SetStandardStreamEncoding()`

  - `PySys_AddWarnOption()`

  - `PySys_AddXOption()`

  - `PySys_ResetWarnOptions()`

- Informative functions:

  - `Py_IsInitialized()`

  - `PyMem_GetAllocator()`

  - `PyObject_GetArenaAllocator()`

  - `Py_GetBuildInfo()`

  - `Py_GetCompiler()`

  - `Py_GetCopyright()`

  - `Py_GetPlatform()`

  - `Py_GetVersion()`

- Utilities:

  - `Py_DecodeLocale()`

- Memory allocators:

  - `PyMem_RawMalloc()`

  - `PyMem_RawRealloc()`

  - `PyMem_RawCalloc()`

  - `PyMem_RawFree()`

Note

The following functions should not be called before `Py_Initialize()`: `Py_EncodeLocale()`, `Py_GetPath()`, `Py_GetPrefix()`, `Py_GetExecPrefix()`, `Py_GetProgramFullPath()`, `Py_GetPythonHome()`, `Py_GetProgramName()` and `PyEval_InitThreads()`.

## Global configuration variables

Python has variables for the global configuration to control different features and options. By default, these flags are controlled by command line 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 `Py_BytesWarningFlag` to 1 and `-bb` sets `Py_BytesWarningFlag` to 2.

int Py_BytesWarningFlag

  This API is kept for backward compatibility: setting `PyConfig.bytes_warning` should be used instead, see Python Initialization Configuration.

  Issue a warning when comparing `bytes` or `bytearray` with `str` or `bytes` with `int`. Issue an error if greater or equal to `2`.

  Set by the `-b` option.

  Deprecated since version 3.12.

int Py_DebugFlag

  This API is kept for backward compatibility: setting `PyConfig.parser_debug` should be used instead, see Python Initialization Configuration.

  Turn on parser debugging output (for expert only, depending on compilation options).

  Set by the `-d` option and the `PYTHONDEBUG` environment variable.

  Deprecated since version 3.12.

int Py_DontWriteBytecodeFlag

  This API is kept for backward compatibility: setting `PyConfig.write_bytecode` should be used instead, see Python Initialization Configuration.

  If set to non-zero, Python won’t try to write `.pyc` files on the import of source modules.

  Set by the `-B` option and the `PYTHONDONTWRITEBYTECODE` environment variable.

  Deprecated since version 3.12.

int Py_FrozenFlag

  This API is kept for backward compatibility: setting `PyConfig.pathconfig_warnings` should be used instead, see Python Initialization Configuration.

  Suppress error messages when calculating the module search path in `Py_GetPath()`.

  Private flag used by `_freeze_module` and `frozenmain` programs.

  Deprecated since version 3.12.

int Py_HashRandomizationFlag

  This API is kept for backward compatibility: setting `PyConfig.hash_seed` and `PyConfig.use_hash_seed` should be used instead, see Python Initialization Configuration.

  Set to `1` if the `PYTHONHASHSEED` environment variable is set to a non-empty string.

  If the flag is non-zero, read the `PYTHONHASHSEED` environment variable to initialize the secret hash seed.

  Deprecated since version 3.12.

int Py_IgnoreEnvironmentFlag

  This API is kept for backward compatibility: setting `PyConfig.use_environment` should be used instead, see Python Initialization Configuration.

  Ignore all `PYTHON*` environment variables, e.g. `PYTHONPATH` and `PYTHONHOME`, that might be set.

  Set by the `-E` and `-I` options.

  Deprecated since version 3.12.

int Py_InspectFlag

  This API is kept for backward compatibility: setting `PyConfig.inspect` should be used instead, see Python Initialization Configuration.

  When a script is passed as first argument or the `-c` option is used, enter interactive mode after executing the script or the command, even when `sys.stdin` does not appear to be a terminal.

  Set by the `-i` option and the `PYTHONINSPECT` environment variable.

  Deprecated since version 3.12.

int Py_InteractiveFlag

  This API is kept for backward compatibility: setting `PyConfig.interactive` should be used instead, see Python Initialization Configuration.

  Set by the `-i` option.

  Deprecated since version 3.12.

int Py_IsolatedFlag

  This API is kept for backward compatibility: setting `PyConfig.isolated` should be used instead, see Python Initialization Configuration.

  Run Python in isolated mode. In isolated mode `sys.path` contains neither the script’s directory nor the user’s site-packages directory.

  Set by the `-I` option.

  Added in version 3.4.

  Deprecated since version 3.12.

int Py_LegacyWindowsFSEncodingFlag

  This API is kept for backward compatibility: setting `PyPreConfig.legacy_windows_fs_encoding` should be used instead, see Python Initialization Configuration.

  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 filesystem encoding and error handler.

  Set to `1` if the `PYTHONLEGACYWINDOWSFSENCODING` environment variable is set to a non-empty string.

  See PEP 529 for more details.

  Availability: Windows.

  Deprecated since version 3.12.

int Py_LegacyWindowsStdioFlag

  This API is kept for backward compatibility: setting `PyConfig.legacy_windows_stdio` should be used instead, see Python Initialization Configuration.

  If the flag is non-zero, use `io.FileIO` instead of `io._WindowsConsoleIO` for `sys` standard streams.

  Set to `1` if the `PYTHONLEGACYWINDOWSSTDIO` environment variable is set to a non-empty string.

  See PEP 528 for more details.

  Availability: Windows.

  Deprecated since version 3.12.

int Py_NoSiteFlag

  This API is kept for backward compatibility: setting `PyConfig.site_import` should be used instead, see Python Initialization Configuration.

  Disable the import of the module `site` and the site-dependent manipulations of `sys.path` that it entails. Also disable these manipulations if `site` is explicitly imported later (call `site.main()` if you want them to be triggered).

  Set by the `-S` option.

  Deprecated since version 3.12.

int Py_NoUserSiteDirectory

  This API is kept for backward compatibility: setting `PyConfig.user_site_directory` should be used instead, see Python Initialization Configuration.

  Don’t add the `user site-packages directory` to `sys.path`.

  Set by the `-s` and `-I` options, and the `PYTHONNOUSERSITE` environment variable.

  Deprecated since version 3.12.

int Py_OptimizeFlag

  This API is kept for backward compatibility: setting `PyConfig.optimization_level` should be used instead, see Python Initialization Configuration.

  Set by the `-O` option and the `PYTHONOPTIMIZE` environment variable.

  Deprecated since version 3.12.

int Py_QuietFlag

  This API is kept for backward compatibility: setting `PyConfig.quiet` should be used instead, see Python Initialization Configuration.

  Don’t display the copyright and version messages even in interactive mode.

  Set by the `-q` option.

  Added in version 3.2.

  Deprecated since version 3.12.

int Py_UnbufferedStdioFlag

  This API is kept for backward compatibility: setting `PyConfig.buffered_stdio` should be used instead, see Python Initialization Configuration.

  Force the stdout and stderr streams to be unbuffered.

  Set by the `-u` option and the `PYTHONUNBUFFERED` environment variable.

  Deprecated since version 3.12.

int Py_VerboseFlag

  This API is kept for backward compatibility: setting `PyConfig.verbose` should be used instead, see Python Initialization Configuration.

  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 `-v` option and the `PYTHONVERBOSE` environment variable.

  Deprecated since version 3.12.

## Initializing and finalizing the interpreter

void Py_Initialize()
  Part of the Stable ABI.

  Initialize the Python interpreter. In an application embedding Python, this should be called before using any other Python/C API functions; see Before Python Initialization for the few exceptions.

  This initializes the table of loaded modules (`sys.modules`), and creates the fundamental modules `builtins`, `__main__` and `sys`. It also initializes the module search path (`sys.path`). It does not set `sys.argv`; use `PySys_SetArgvEx()` for that. This is a no-op when called for a second time (without calling `Py_FinalizeEx()` first). There is no return value; it is a fatal error if the initialization fails.

  Use the `Py_InitializeFromConfig()` function to customize the Python Initialization Configuration.

  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.

void Py_InitializeEx(int initsigs)
  Part of the Stable ABI.

  This function works like `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.

  Use the `Py_InitializeFromConfig()` function to customize the Python Initialization Configuration.

int Py_IsInitialized()
  Part of the Stable ABI.

  Return true (nonzero) when the Python interpreter has been initialized, false (zero) if not. After `Py_FinalizeEx()` is called, this returns false until `Py_Initialize()` is called again.

int Py_FinalizeEx()
  Part of the Stable ABI since version 3.6.

  Undo all initializations made by `Py_Initialize()` and subsequent use of Python/C API functions, and destroy all sub-interpreters (see `Py_NewInterpreter()` below) that were created and not yet destroyed since the last call to `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 `Py_Initialize()` again first).

  Since this is the reverse of `Py_Initialize()`, it should be called in the same thread with the same interpreter active. That means the main thread and the main interpreter. This should never be called while `Py_RunMain()` is running.

  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 (`__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 `Py_Initialize()` and `Py_FinalizeEx()` more than once.

  Raises an auditing event `cpython._PySys_ClearAuditHooks` with no arguments.

  Added in version 3.6.

void Py_Finalize()
  Part of the Stable ABI.

  This is a backwards-compatible version of `Py_FinalizeEx()` that disregards the return value.

## Process-wide parameters

int Py_SetStandardStreamEncoding(const char *encoding, const char *errors)

  This API is kept for backward compatibility: setting `PyConfig.stdio_encoding` and `PyConfig.stdio_errors` should be used instead, see Python Initialization Configuration.

  This function should be called before `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 `str.encode()`.

  It overrides `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 `PYTHONIOENCODING` and/or default values (depending on other settings).

  Note that `sys.stderr` always uses the “backslashreplace” error handler, regardless of this (or any other) setting.

  If `Py_FinalizeEx()` is called, this function will need to be called again in order to affect subsequent calls to `Py_Initialize()`.

  Returns `0` if successful, a nonzero value on error (e.g. calling after the interpreter has already been initialized).

  Added in version 3.4.

  Deprecated since version 3.11.

void Py_SetProgramName(const wchar_t *name)
  Part of the Stable ABI.

  This API is kept for backward compatibility: setting `PyConfig.program_name` should be used instead, see Python Initialization Configuration.

  This function should be called before `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 `main()` function of the program (converted to wide characters). This is used by `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 `Py_DecodeLocale()` to decode a bytes string to get a wchar_t* string.

  Deprecated since version 3.11.

wchar_t *Py_GetProgramName()
  Part of the Stable ABI.

  Return the program name set with `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 `Py_Initialize()`, otherwise it returns `NULL`.

  Changed in version 3.10: It now returns `NULL` if called before `Py_Initialize()`.

wchar_t *Py_GetPrefix()
  Part of the Stable ABI.

  Return the prefix for installed platform-independent files. This is derived through a number of complicated rules from the program name set with `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 prefix variable in the top-level `Makefile` and the `--prefix` argument to the 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 `Py_Initialize()`, otherwise it returns `NULL`.

  Changed in version 3.10: It now returns `NULL` if called before `Py_Initialize()`.

wchar_t *Py_GetExecPrefix()
  Part of the Stable ABI.

  Return the exec-prefix for installed platform-dependent files. This is derived through a number of complicated rules from the program name set with `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 exec_prefix variable in the top-level `Makefile` and the `--exec-prefix` argument to the 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 `/usr/local/plat` subtree while platform independent may be installed in `/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 mount or automount programs to share `/usr/local` between platforms while having `/usr/local/plat` be a different filesystem for each platform.

  This function should not be called before `Py_Initialize()`, otherwise it returns `NULL`.

  Changed in version 3.10: It now returns `NULL` if called before `Py_Initialize()`.

wchar_t *Py_GetProgramFullPath()
  Part of the Stable ABI.

  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 `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 `Py_Initialize()`, otherwise it returns `NULL`.

  Changed in version 3.10: It now returns `NULL` if called before `Py_Initialize()`.

wchar_t *Py_GetPath()
  Part of the Stable ABI.

  Return the default module search path; this is computed from the program name (set by `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 `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 `Py_Initialize()`, otherwise it returns `NULL`.

  Changed in version 3.10: It now returns `NULL` if called before `Py_Initialize()`.

void Py_SetPath(const wchar_t*)
  Part of the Stable ABI since version 3.7.

  This API is kept for backward compatibility: setting `PyConfig.module_search_paths` and `PyConfig.module_search_paths_set` should be used instead, see Python Initialization Configuration.

  Set the default module search path. If this function is called before `Py_Initialize()`, then `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 `sys.executable` to be set to the program full path (see `Py_GetProgramFullPath()`) and for `sys.prefix` and `sys.exec_prefix` to be empty. It is up to the caller to modify these if required after calling `Py_Initialize()`.

  Use `Py_DecodeLocale()` to decode a bytes string to get a wchar_* string.

  The path argument is copied internally, so the caller may free it after the call completes.

  Changed in version 3.8: The program full path is now used for `sys.executable`, instead of the program name.

  Deprecated since version 3.11.

const char *Py_GetVersion()
  Part of the Stable ABI.

  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]"

  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 `sys.version`.

  See also the `Py_Version` constant.

const char *Py_GetPlatform()
  Part of the Stable ABI.

  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`.

const char *Py_GetCopyright()
  Part of the Stable ABI.

  Return the official copyright string for the current Python version, for example

  `'Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam'`

  The returned string points into static storage; the caller should not modify its value. The value is available to Python code as `sys.copyright`.

const char *Py_GetCompiler()
  Part of the Stable ABI.

  Return an indication of the compiler used to build the current Python version, in square brackets, for example:

      "[GCC 2.7.2.2]"

  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`.

const char *Py_GetBuildInfo()
  Part of the Stable ABI.

  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"

  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`.

void PySys_SetArgvEx(int argc, wchar_t **argv, int updatepath)
  Part of the Stable ABI.

  This API is kept for backward compatibility: setting `PyConfig.argv`, `PyConfig.parse_argv` and `PyConfig.safe_path` should be used instead, see Python Initialization Configuration.

  Set `sys.argv` based on argc and argv. These parameters are similar to those passed to the program’s `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 `sys.argv`, a fatal condition is signalled using `Py_FatalError()`.

  If updatepath is zero, this is all the function does. If updatepath is non-zero, the function also modifies `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 `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 `sys.path`, which is the same as prepending the current working directory (`"."`).

  Use `Py_DecodeLocale()` to decode a bytes string to get a wchar_* string.

  See also `PyConfig.orig_argv` and `PyConfig.argv` members of the Python Initialization Configuration.

  Note

  It is recommended that applications embedding the Python interpreter for purposes other than executing a single script pass `0` as updatepath, and update `sys.path` themselves if desired. See CVE-2008-5983.

  On versions before 3.1.3, you can achieve the same effect by manually popping the first `sys.path` element after having called `PySys_SetArgv()`, for example using:

      PyRun_SimpleString("import sys; sys.path.pop(0)\n");

  Added in version 3.1.3.

  Deprecated since version 3.11.

void PySys_SetArgv(int argc, wchar_t **argv)
  Part of the Stable ABI.

  This API is kept for backward compatibility: setting `PyConfig.argv` and `PyConfig.parse_argv` should be used instead, see Python Initialization Configuration.

  This function works like `PySys_SetArgvEx()` with updatepath set to `1` unless the python interpreter was started with the `-I`.

  Use `Py_DecodeLocale()` to decode a bytes string to get a wchar_* string.

  See also `PyConfig.orig_argv` and `PyConfig.argv` members of the Python Initialization Configuration.

  Changed in version 3.4: The updatepath value depends on `-I`.

  Deprecated since version 3.11.

void Py_SetPythonHome(const wchar_t *home)
  Part of the Stable ABI.

  This API is kept for backward compatibility: setting `PyConfig.home` should be used instead, see Python Initialization Configuration.

  Set the default “home” directory, that is, the location of the standard Python libraries. See `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 `Py_DecodeLocale()` to decode a bytes string to get a wchar_* string.

  Deprecated since version 3.11.

wchar_t *Py_GetPythonHome()
  Part of the Stable ABI.

  Return the default “home”, that is, the value set by a previous call to `Py_SetPythonHome()`, or the value of the `PYTHONHOME` environment variable if it is set.

  This function should not be called before `Py_Initialize()`, otherwise it returns `NULL`.

  Changed in version 3.10: It now returns `NULL` if called before `Py_Initialize()`.

## Thread State and the Global Interpreter Lock

The Python interpreter is not fully thread-safe. In order to support multi-threaded Python programs, there’s a global lock, called the global interpreter lock or 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.

Therefore, the rule exists that only the thread that has acquired the 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 `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.

The Python interpreter keeps some thread-specific bookkeeping information inside a data structure called `PyThreadState`. There’s also one global variable pointing to the current `PyThreadState`: it can be retrieved using `PyThreadState_Get()`.

### Releasing the GIL from extension code

Most extension code manipulating the 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

The `Py_BEGIN_ALLOW_THREADS` macro opens a new block and declares a hidden local variable; the `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);

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 `zlib` and `hashlib` modules release the GIL when compressing or hashing data.

### Non-Python created threads

When threads are created using the dedicated Python APIs (such as the `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 `PyGILState_Ensure()` and `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 `Py_Initialize()`). Python supports the creation of additional interpreters (using `Py_NewInterpreter()`), but mixing multiple interpreters and the `PyGILState_*` API is unsupported.

### Cautions about fork()

Another important thing to note about threads is their behaviour in the face of the C `fork()` call. On most systems with `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 `os.fork()` by acquiring the locks it uses internally before the fork, and releasing them afterwards. In addition, it resets any 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 `pthread_atfork()` would need to be used to accomplish the same thing. Additionally, when extending or embedding Python, calling `fork()` directly rather than through `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. `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 `os.fork()` does. This means finalizing all other `PyThreadState` objects belonging to the current interpreter and all other `PyInterpreterState` objects. Due to this and the special nature of the “main” interpreter, `fork()` should only be called in that interpreter’s “main” thread, where the CPython global runtime was originally initialized. The only exception is if `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:

type PyInterpreterState
  Part of the Limited API (as an opaque struct).

  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.

type PyThreadState
  Part of the Limited API (as an opaque struct).

  This data structure represents the state of a single thread. The only public data member is:

  PyInterpreterState *interp

    This thread’s interpreter state.

void PyEval_InitThreads()
  Part of the Stable ABI.

  Deprecated function which does nothing.

  In Python 3.6 and older, this function created the GIL if it didn’t exist.

  Changed in version 3.9: The function now does nothing.

  Changed in version 3.7: This function is now called by `Py_Initialize()`, so you don’t have to call it yourself anymore.

  Changed in version 3.2: This function cannot be called before `Py_Initialize()` anymore.

  Deprecated since version 3.9.

int PyEval_ThreadsInitialized()
  Part of the Stable ABI.

  Returns a non-zero value if `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.

  Changed in version 3.7: The GIL is now initialized by `Py_Initialize()`.

  Deprecated since version 3.9.

PyThreadState *PyEval_SaveThread()
  Part of the Stable ABI.

  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.

void PyEval_RestoreThread(PyThreadState *tstate)
  Part of the Stable ABI.

  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 `_Py_IsFinalizing()` or `sys.is_finalizing()` to check if the interpreter is in process of being finalized before calling this function to avoid unwanted termination.

PyThreadState *PyThreadState_Get()
  Part of the Stable ABI.

  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`).

PyThreadState *PyThreadState_Swap(PyThreadState *tstate)
  Part of the Stable ABI.

  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:

PyGILState_STATE PyGILState_Ensure()
  Part of the Stable ABI.

  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 `PyGILState_Release()`. In general, other thread-related APIs may be used between `PyGILState_Ensure()` and `PyGILState_Release()` calls as long as the thread state is restored to its previous state before the Release(). For example, normal usage of the `Py_BEGIN_ALLOW_THREADS` and `Py_END_ALLOW_THREADS` macros is acceptable.

  The return value is an opaque “handle” to the thread state when `PyGILState_Ensure()` was called, and must be passed to `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 `PyGILState_Ensure()` must save the handle for its call to `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 `_Py_IsFinalizing()` or `sys.is_finalizing()` to check if the interpreter is in process of being finalized before calling this function to avoid unwanted termination.

void PyGILState_Release(PyGILState_STATE)
  Part of the Stable ABI.

  Release any resources previously acquired. After this call, Python’s state will be the same as it was prior to the corresponding `PyGILState_Ensure()` call (but generally this state will be unknown to the caller, hence the use of the GILState API).

  Every call to `PyGILState_Ensure()` must be matched by a call to `PyGILState_Release()` on the same thread.

PyThreadState *PyGILState_GetThisThreadState()
  Part of the Stable ABI.

  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.

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.

  Added in version 3.4.

The following macros are normally used without a trailing semicolon; look for example usage in the Python source distribution.

Py_BEGIN_ALLOW_THREADS
  Part of the Stable ABI.

  This macro expands to `{ PyThreadState *_save; _save = PyEval_SaveThread();`. Note that it contains an opening brace; it must be matched with a following `Py_END_ALLOW_THREADS` macro. See above for further discussion of this macro.

Py_END_ALLOW_THREADS
  Part of the Stable ABI.

  This macro expands to `PyEval_RestoreThread(_save); }`. Note that it contains a closing brace; it must be matched with an earlier `Py_BEGIN_ALLOW_THREADS` macro. See above for further discussion of this macro.

Py_BLOCK_THREADS
  Part of the Stable ABI.

  This macro expands to `PyEval_RestoreThread(_save);`: it is equivalent to `Py_END_ALLOW_THREADS` without the closing brace.

Py_UNBLOCK_THREADS
  Part of the Stable ABI.

  This macro expands to `_save = PyEval_SaveThread();`: it is equivalent to `Py_BEGIN_ALLOW_THREADS` without the opening brace and variable declaration.

### Low-level API

All of the following functions must be called after `Py_Initialize()`.

Changed in version 3.7: `Py_Initialize()` now initializes the GIL.

PyInterpreterState *PyInterpreterState_New()
  Part of the Stable ABI.

  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.

  Raises an auditing event `cpython.PyInterpreterState_New` with no arguments.

void PyInterpreterState_Clear(PyInterpreterState *interp)
  Part of the Stable ABI.

  Reset all information in an interpreter state object. The global interpreter lock must be held.

  Raises an auditing event `cpython.PyInterpreterState_Clear` with no arguments.

void PyInterpreterState_Delete(PyInterpreterState *interp)
  Part of the Stable ABI.

  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 `PyInterpreterState_Clear()`.

PyThreadState *PyThreadState_New(PyInterpreterState *interp)
  Part of the Stable ABI.

  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.

void PyThreadState_Clear(PyThreadState *tstate)
  Part of the Stable ABI.

  Reset all information in a thread state object. The global interpreter lock must be held.

  Changed in version 3.9: This function now calls the `PyThreadState.on_delete` callback. Previously, that happened in `PyThreadState_Delete()`.

void PyThreadState_Delete(PyThreadState *tstate)
  Part of the Stable ABI.

  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 `PyThreadState_Clear()`.

void PyThreadState_DeleteCurrent(void)

  Destroy the current thread state and release the global interpreter lock. Like `PyThreadState_Delete()`, the global interpreter lock need not be held. The thread state must have been reset with a previous call to `PyThreadState_Clear()`.

PyFrameObject *PyThreadState_GetFrame(PyThreadState *tstate)
  Part of the Stable ABI since version 3.10.

  Get the current frame of the Python thread state tstate.

  Return a strong reference. Return `NULL` if no frame is currently executing.

  See also `PyEval_GetFrame()`.

  tstate must not be `NULL`.

  Added in version 3.9.

uint64_t PyThreadState_GetID(PyThreadState *tstate)
  Part of the Stable ABI since version 3.10.

  Get the unique thread state identifier of the Python thread state tstate.

  tstate must not be `NULL`.

  Added in version 3.9.

PyInterpreterState *PyThreadState_GetInterpreter(PyThreadState *tstate)
  Part of the Stable ABI since version 3.10.

  Get the interpreter of the Python thread state tstate.

  tstate must not be `NULL`.

  Added in version 3.9.

void PyThreadState_EnterTracing(PyThreadState *tstate)

  Suspend tracing and profiling in the Python thread state tstate.

  Resume them using the `PyThreadState_LeaveTracing()` function.

  Added in version 3.11.

void PyThreadState_LeaveTracing(PyThreadState *tstate)

  Resume tracing and profiling in the Python thread state tstate suspended by the `PyThreadState_EnterTracing()` function.

  See also `PyEval_SetTrace()` and `PyEval_SetProfile()` functions.

  Added in version 3.11.

PyInterpreterState *PyInterpreterState_Get(void)
  Part of the Stable ABI since version 3.9.

  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.

  Added in version 3.9.

int64_t PyInterpreterState_GetID(PyInterpreterState *interp)
  Part of the Stable ABI since version 3.7.

  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.

  Added in version 3.7.

PyObject *PyInterpreterState_GetDict(PyInterpreterState *interp)
  Part of the Stable ABI since version 3.8.

  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 `PyModule_GetState()`, which extensions should use to store interpreter-specific state information.

  Added in version 3.8.

typedef 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.

  Changed in version 3.9: The function now takes a tstate parameter.

  Changed in version 3.11: The frame parameter changed from `PyFrameObject*` to `_PyInterpreterFrame*`.

_PyFrameEvalFunction _PyInterpreterState_GetEvalFrameFunc(PyInterpreterState *interp)

  Get the frame evaluation function.

  See the PEP 523 “Adding a frame evaluation API to CPython”.

  Added in version 3.9.

void _PyInterpreterState_SetEvalFrameFunc(PyInterpreterState *interp, _PyFrameEvalFunction eval_frame)

  Set the frame evaluation function.

  See the PEP 523 “Adding a frame evaluation API to CPython”.

  Added in version 3.9.

PyObject *PyThreadState_GetDict()
  Return value: Borrowed reference. Part of the Stable ABI.

  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.

int PyThreadState_SetAsyncExc(unsigned long id, PyObject *exc)
  Part of the Stable ABI.

  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.

  Changed in version 3.7: The type of the id parameter changed from long to unsigned long.

void PyEval_AcquireThread(PyThreadState *tstate)
  Part of the Stable ABI.

  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 `_Py_IsFinalizing()` or `sys.is_finalizing()` to check if the interpreter is in process of being finalized before calling this function to avoid unwanted termination.

  Changed in version 3.8: Updated to be consistent with `PyEval_RestoreThread()`, `Py_END_ALLOW_THREADS()`, and `PyGILState_Ensure()`, and terminate the current thread if called while the interpreter is finalizing.

  `PyEval_RestoreThread()` is a higher-level function which is always available (even when threads have not been initialized).

void PyEval_ReleaseThread(PyThreadState *tstate)
  Part of the Stable ABI.

  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.

  `PyEval_SaveThread()` is a higher-level function which is always available (even when threads have not been initialized).

void PyEval_AcquireLock()
  Part of the Stable ABI.

  Acquire the global interpreter lock. The lock must have been created earlier. If this thread already has the lock, a deadlock ensues.

  Deprecated since version 3.2: This function does not update the current thread state. Please use `PyEval_RestoreThread()` or `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 `_Py_IsFinalizing()` or `sys.is_finalizing()` to check if the interpreter is in process of being finalized before calling this function to avoid unwanted termination.

  Changed in version 3.8: Updated to be consistent with `PyEval_RestoreThread()`, `Py_END_ALLOW_THREADS()`, and `PyGILState_Ensure()`, and terminate the current thread if called while the interpreter is finalizing.

void PyEval_ReleaseLock()
  Part of the Stable ABI.

  Release the global interpreter lock. The lock must have been created earlier.

  Deprecated since version 3.2: This function does not update the current thread state. Please use `PyEval_SaveThread()` or `PyEval_ReleaseThread()` instead.

## 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 `PyInterpreterState_Main()` function returns a pointer to its state.

You can switch between sub-interpreters using the `PyThreadState_Swap()` function. You can create and destroy them using the following functions:

type PyInterpreterConfig

  Structure containing most parameters to configure a sub-interpreter. Its values are used only in `Py_NewInterpreterFromConfig()` and never modified by the runtime.

  Added in version 3.12.

  Structure fields:

  int use_main_obmalloc

    If this is `0` then the sub-interpreter will use its own “object” allocator state. Otherwise it will use (share) the main interpreter’s.

    If this is `0` then `check_multi_interp_extensions` must be `1` (non-zero). If this is `1` then `gil` must not be `PyInterpreterConfig_OWN_GIL`.

  int allow_fork

    If this is `0` then the runtime will not support forking the process in any thread where the sub-interpreter is currently active. Otherwise fork is unrestricted.

    Note that the `subprocess` module still works when fork is disallowed.

  int allow_exec

    If this is `0` then the runtime will not support replacing the current process via exec (e.g. `os.execv()`) in any thread where the sub-interpreter is currently active. Otherwise exec is unrestricted.

    Note that the `subprocess` module still works when exec is disallowed.

  int allow_threads

    If this is `0` then the sub-interpreter’s `threading` module won’t create threads. Otherwise threads are allowed.

  int allow_daemon_threads

    If this is `0` then the sub-interpreter’s `threading` module won’t create daemon threads. Otherwise daemon threads are allowed (as long as `allow_threads` is non-zero).

  int check_multi_interp_extensions

    If this is `0` then all extension modules may be imported, including legacy (single-phase init) modules, in any thread where the sub-interpreter is currently active. Otherwise only multi-phase init extension modules (see PEP 489) may be imported. (Also see `Py_mod_multiple_interpreters`.)

    This must be `1` (non-zero) if `use_main_obmalloc` is `0`.

  int gil

    This determines the operation of the GIL for the sub-interpreter. It may be one of the following:

    PyInterpreterConfig_DEFAULT_GIL

      Use the default selection (`PyInterpreterConfig_SHARED_GIL`).

    PyInterpreterConfig_SHARED_GIL

      Use (share) the main interpreter’s GIL.

    PyInterpreterConfig_OWN_GIL

      Use the sub-interpreter’s own GIL.

    If this is `PyInterpreterConfig_OWN_GIL` then `PyInterpreterConfig.use_main_obmalloc` must be `0`.

PyStatus Py_NewInterpreterFromConfig(PyThreadState **tstate_p, const PyInterpreterConfig *config)

  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 `builtins`, `__main__` and `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 given config controls the options with which the interpreter is initialized.

  Upon success, tstate_p will be set 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, tstate_p is set to `NULL`; 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. Likewise a current thread state must be set on entry. On success, the returned thread state will be set as current. If the sub-interpreter is created with its own GIL then the GIL of the calling interpreter will be released. When the function returns, the new interpreter’s GIL will be held by the current thread and the previously interpreter’s GIL will remain released here.

  Added in version 3.12.

  Sub-interpreters are most effective when isolated from each other, with certain functionality restricted:

      PyInterpreterConfig config = {
          .use_main_obmalloc = 0,
          .allow_fork = 0,
          .allow_exec = 0,
          .allow_threads = 1,
          .allow_daemon_threads = 0,
          .check_multi_interp_extensions = 1,
          .gil = PyInterpreterConfig_OWN_GIL,
      };
      PyThreadState *tstate = Py_NewInterpreterFromConfig(&config);

  Note that the config is used only briefly and does not get modified. During initialization the config’s values are converted into various `PyInterpreterState` values. A read-only copy of the config may be stored internally on the `PyInterpreterState`.

  Extension modules are shared between (sub-)interpreters as follows:

  - For modules using multi-phase initialization, e.g. `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. `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 `Py_FinalizeEx()` and `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.

PyThreadState *Py_NewInterpreter(void)
  Part of the Stable ABI.

  Create a new sub-interpreter. This is essentially just a wrapper around `Py_NewInterpreterFromConfig()` with a config that preserves the existing behavior. The result is an unisolated sub-interpreter that shares the main interpreter’s GIL, allows fork/exec, allows daemon threads, and allows single-phase init modules.

void Py_EndInterpreter(PyThreadState *tstate)
  Part of the Stable ABI.

  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 used by the target interpreter must be held before calling this function. No GIL is held when it returns.

  `Py_FinalizeEx()` will destroy all sub-interpreters that haven’t been explicitly destroyed at that point.

### A Per-Interpreter GIL

Using `Py_NewInterpreterFromConfig()` you can create a sub-interpreter that is completely isolated from other interpreters, including having its own GIL. The most important benefit of this isolation is that such an interpreter can execute Python code without being blocked by other interpreters or blocking any others. Thus a single Python process can truly take advantage of multiple CPU cores when running Python code. The isolation also encourages a different approach to concurrency than that of just using threads. (See PEP 554.)

Using an isolated interpreter requires vigilance in preserving that isolation. That especially means not sharing any objects or mutable state without guarantees about thread-safety. Even objects that are otherwise immutable (e.g. `None`, `(1, 5)`) can’t normally be shared because of the refcount. One simple but less-efficient approach around this is to use a global lock around all use of some state (or object). Alternately, effectively immutable objects (like integers or strings) can be made safe in spite of their refcounts by making them “immortal”. In fact, this has been done for the builtin singletons, small integers, and a number of other builtin objects.

If you preserve isolation then you will have access to proper multi-core computing without the complications that come with free-threading. Failure to preserve isolation will expose you to the full consequences of free-threading, including races and hard-to-debug crashes.

Aside from that, one of the main challenges of using multiple isolated interpreters is how to communicate between them safely (not break isolation) and efficiently. The runtime and stdlib do not provide any standard approach to this yet. A future stdlib module would help mitigate the effort of preserving isolation and expose effective tools for communicating (and sharing) data between interpreters.

Added in version 3.12.

### 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 `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 `PyGILState_Ensure()` and `PyGILState_Release()` calls. Furthermore, extensions (such as `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.

int Py_AddPendingCall(int (*func)(void*), void *arg)
  Part of the Stable ABI.

  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 bytecode boundary;

  - with the main thread holding the 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 PyGILState API.

  Added in version 3.1.

  Changed in version 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 and Tracing

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.

typedef int (*Py_tracefunc)(PyObject *obj, PyFrameObject *frame, int what, PyObject *arg)

  The type of the trace function registered using `PyEval_SetProfile()` and `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 `PyTrace_CALL`, `PyTrace_EXCEPTION`, `PyTrace_LINE`, `PyTrace_RETURN`, `PyTrace_C_CALL`, `PyTrace_C_EXCEPTION`, `PyTrace_C_RETURN`, or `PyTrace_OPCODE`, and arg depends on the value of what:

  Value of what | Meaning of arg
  --- | ---
  `PyTrace_CALL` | Always `Py_None`.
  `PyTrace_EXCEPTION` | Exception information as returned by `sys.exc_info()`.
  `PyTrace_LINE` | Always `Py_None`.
  `PyTrace_RETURN` | Value being returned to the caller, or `NULL` if caused by an exception.
  `PyTrace_C_CALL` | Function object being called.
  `PyTrace_C_EXCEPTION` | Function object being called.
  `PyTrace_C_RETURN` | Function object being called.
  `PyTrace_OPCODE` | Always `Py_None`.

int PyTrace_CALL

  The value of the what parameter to a `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.

int PyTrace_EXCEPTION

  The value of the what parameter to a `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.

int PyTrace_LINE

  The value passed as the what parameter to a `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 `f_trace_lines` to 0 on that frame.

int PyTrace_RETURN

  The value for the what parameter to `Py_tracefunc` functions when a call is about to return.

int PyTrace_C_CALL

  The value for the what parameter to `Py_tracefunc` functions when a C function is about to be called.

int PyTrace_C_EXCEPTION

  The value for the what parameter to `Py_tracefunc` functions when a C function has raised an exception.

int PyTrace_C_RETURN

  The value for the what parameter to `Py_tracefunc` functions when a C function has returned.

int PyTrace_OPCODE

  The value for the what parameter to `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 `f_trace_opcodes` to 1 on the frame.

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 `PyTrace_LINE` `PyTrace_OPCODE` and `PyTrace_EXCEPTION`.

  See also the `sys.setprofile()` function.

  The caller must hold the GIL.

void PyEval_SetProfileAllThreads(Py_tracefunc func, PyObject *obj)

  Like `PyEval_SetProfile()` but sets the profile function in all running threads belonging to the current interpreter instead of the setting it only on the current thread.

  The caller must hold the GIL.

  As `PyEval_SetProfile()`, this function ignores any exceptions raised while setting the profile functions in all threads.

Added in version 3.12.

void PyEval_SetTrace(Py_tracefunc func, PyObject *obj)

  Set the tracing function to func. This is similar to `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 `PyEval_SetTrace()` will not receive `PyTrace_C_CALL`, `PyTrace_C_EXCEPTION` or `PyTrace_C_RETURN` as a value for the what parameter.

  See also the `sys.settrace()` function.

  The caller must hold the GIL.

void PyEval_SetTraceAllThreads(Py_tracefunc func, PyObject *obj)

  Like `PyEval_SetTrace()` but sets the tracing function in all running threads belonging to the current interpreter instead of the setting it only on the current thread.

  The caller must hold the GIL.

  As `PyEval_SetTrace()`, this function ignores any exceptions raised while setting the trace functions in all threads.

Added in version 3.12.

## Advanced Debugger Support

These functions are only intended to be used by advanced debugging tools.

PyInterpreterState *PyInterpreterState_Head()

  Return the interpreter state object at the head of the list of all such objects.

PyInterpreterState *PyInterpreterState_Main()

  Return the main interpreter state object.

PyInterpreterState *PyInterpreterState_Next(PyInterpreterState *interp)

  Return the next interpreter state object after interp from the list of all such objects.

PyThreadState *PyInterpreterState_ThreadHead(PyInterpreterState *interp)

  Return the pointer to the first `PyThreadState` object in the list of threads associated with the interpreter interp.

PyThreadState *PyThreadState_Next(PyThreadState *tstate)

  Return the next thread state object after tstate from the list of all such objects belonging to the same `PyInterpreterState` object.

## Thread Local Storage Support

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 (`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 void* value per thread.

The GIL does not need to be held when calling these functions; they supply their own locking.

Note that `Python.h` does not include the declaration of the TLS APIs, you need to include `pythread.h` to use thread-local storage.

Note

None of these API functions handle memory management on behalf of the void* values. You need to allocate and deallocate them yourself. If the void* values happen to be PyObject*, these functions don’t do refcount operations on them either.

### 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 `Py_tss_t` instead of int to represent thread keys.

Added in version 3.7.

See also

“A New C-API for Thread-Local Storage in CPython” (PEP 539)

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 Py_LIMITED_API is not defined, static allocation of this type by `Py_tss_NEEDS_INIT` is allowed.

Py_tss_NEEDS_INIT

  This macro expands to the initializer for `Py_tss_t` variables. Note that this macro won’t be defined with Py_LIMITED_API.

#### Dynamic Allocation

Dynamic allocation of the `Py_tss_t`, required in extension modules built with Py_LIMITED_API, where static allocation of this type is not possible due to its implementation being opaque at build time.

Py_tss_t *PyThread_tss_alloc()
  Part of the Stable ABI since version 3.7.

  Return a value which is the same state as a value initialized with `Py_tss_NEEDS_INIT`, or `NULL` in the case of dynamic allocation failure.

void PyThread_tss_free(Py_tss_t *key)
  Part of the Stable ABI since version 3.7.

  Free the given key allocated by `PyThread_tss_alloc()`, after first calling `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 `PyThread_tss_set()` and `PyThread_tss_get()` are undefined if the given `Py_tss_t` has not been initialized by `PyThread_tss_create()`.

int PyThread_tss_is_created(Py_tss_t *key)
  Part of the Stable ABI since version 3.7.

  Return a non-zero value if the given `Py_tss_t` has been initialized by `PyThread_tss_create()`.

int PyThread_tss_create(Py_tss_t *key)
  Part of the Stable ABI since version 3.7.

  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 `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.

void PyThread_tss_delete(Py_tss_t *key)
  Part of the Stable ABI since version 3.7.

  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 `PyThread_tss_create()`. This function can be called repeatedly on the same key – calling it on an already destroyed key is a no-op.

int PyThread_tss_set(Py_tss_t *key, void *value)
  Part of the Stable ABI since version 3.7.

  Return a zero value to indicate successfully associating a void* value with a TSS key in the current thread. Each thread has a distinct mapping of the key to a void* value.

void *PyThread_tss_get(Py_tss_t *key)
  Part of the Stable ABI since version 3.7.

  Return the 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 (TLS) API

Deprecated since version 3.7: This API is superseded by Thread Specific Storage (TSS) 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, `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.

int PyThread_create_key()
  Part of the Stable ABI.

void PyThread_delete_key(int key)
  Part of the Stable ABI.

int PyThread_set_key_value(int key, void *value)
  Part of the Stable ABI.

void *PyThread_get_key_value(int key)
  Part of the Stable ABI.

void PyThread_delete_key_value(int key)
  Part of the Stable ABI.

void PyThread_ReInitTLS()
  Part of the Stable ABI.