Initialization, Finalization, and Threads¶

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:

• Functions that initialize the interpreter:

  • Py_Initialize()

  • Py_InitializeEx()

  • Py_InitializeFromConfig()

  • Py_BytesMain()

  • Py_Main()

  • the runtime pre-initialization functions covered in Python Initialization Configuration

• Configuration functions:

  • PyImport_AppendInittab()

  • PyImport_ExtendInittab()

  • PyInitFrozenExtensions()

  • PyMem_SetAllocator()

  • PyMem_SetupDebugHooks()

  • PyObject_SetArenaAllocator()

  • Py_SetProgramName()

  • Py_SetPythonHome()

  • PySys_ResetWarnOptions()

  • the configuration functions covered in Python Initialization Configuration

• Informative functions:

  • Py_IsInitialized()

  • PyMem_GetAllocator()

  • PyObject_GetArenaAllocator()

  • Py_GetBuildInfo()

  • Py_GetCompiler()

  • Py_GetCopyright()

  • Py_GetPlatform()

  • Py_GetVersion()

  • Py_IsInitialized()

• Utilities:

  • Py_DecodeLocale()

  • the status reporting and utility functions covered in Python Initialization Configuration

• Memory allocators:

  • PyMem_RawMalloc()

  • PyMem_RawRealloc()

  • PyMem_RawCalloc()

  • PyMem_RawFree()

• Synchronization:

  • PyMutex_Lock()

  • PyMutex_Unlock()

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, will be removed in version 3.14.

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, will be removed in version 3.14.

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, will be removed in version 3.14.

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, will be removed in version 3.14.

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, will be removed in version 3.14.

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, will be removed in version 3.14.

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, will be removed in version 3.14.

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, will be removed in version 3.15.

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, will be removed in version 3.14.

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, will be removed in version 3.14.

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, will be removed in version 3.14.

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, will be removed in version 3.14.

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, will be removed in version 3.14.

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, will be removed in version 3.14.

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, will be removed in version 3.14.

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, will be removed in version 3.14.

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, will be removed in version 3.14.

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 the Python Initialization Configuration API 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 Py_InitializeFromConfig() 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 may be useful when CPython is embedded as part of a larger application.

Use Py_InitializeFromConfig() to customize the Python Initialization Configuration.

PyStatus Py_InitializeFromConfig(const PyConfig *config)¶

Initialize Python from config configuration, as described in Initialization with PyConfig.

See the Python Initialization Configuration section for details on pre-initializing the interpreter, populating the runtime configuration structure, and querying the returned status structure.

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_IsFinalizing()¶

Part of the Stable ABI since version 3.13.

Return true (non-zero) if the main Python interpreter is shutting down. Return false (zero) otherwise.

Added in version 3.13.

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. Py_FinalizeEx() must not be called recursively from within itself. Therefore, it must not be called by any code that may be run as part of the interpreter shutdown process, such as atexit handlers, object finalizers, or any code that may be run while flushing the stdout and stderr files.

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.

int Py_BytesMain(int argc, char **argv)¶

Part of the Stable ABI since version 3.8.

Similar to Py_Main() but argv is an array of bytes strings, allowing the calling application to delegate the text decoding step to the CPython runtime.

Added in version 3.8.

int Py_Main(int argc, wchar_t **argv)¶

Part of the Stable ABI.

The main program for the standard interpreter, encapsulating a full initialization/finalization cycle, as well as additional behaviour to implement reading configurations settings from the environment and command line, and then executing __main__ in accordance with Command line.

This is made available for programs which wish to support the full CPython command line interface, rather than just embedding a Python runtime in a larger application.

The argc and argv parameters are similar to those which are passed to a C program’s main() function, except that the argv entries are first converted to wchar_t using Py_DecodeLocale(). It is also important to note that the argument list entries may be modified to point to strings other than those passed in (however, the contents of the strings pointed to by the argument list are not modified).

The return value is 2 if the argument list does not represent a valid Python command line, and otherwise the same as Py_RunMain().

In terms of the CPython runtime configuration APIs documented in the runtime configuration section (and without accounting for error handling), Py_Main is approximately equivalent to:

PyConfig config;
PyConfig_InitPythonConfig(&config);
PyConfig_SetArgv(&config, argc, argv);
Py_InitializeFromConfig(&config);
PyConfig_Clear(&config);

Py_RunMain();

In normal usage, an embedding application will call this function instead of calling Py_Initialize(), Py_InitializeEx() or Py_InitializeFromConfig() directly, and all settings will be applied as described elsewhere in this documentation. If this function is instead called after a preceding runtime initialization API call, then exactly which environmental and command line configuration settings will be updated is version dependent (as it depends on which settings correctly support being modified after they have already been set once when the runtime was first initialized).

int Py_RunMain(void)¶

Executes the main module in a fully configured CPython runtime.

Executes the command (PyConfig.run_command), the script (PyConfig.run_filename) or the module (PyConfig.run_module) specified on the command line or in the configuration. If none of these values are set, runs the interactive Python prompt (REPL) using the __main__ module’s global namespace.

If PyConfig.inspect is not set (the default), the return value will be 0 if the interpreter exits normally (that is, without raising an exception), the exit status of an unhandled SystemExit, or 1 for any other unhandled exception.

If PyConfig.inspect is set (such as when the -i option is used), rather than returning when the interpreter exits, execution will instead resume in an interactive Python prompt (REPL) using the __main__ module’s global namespace. If the interpreter exited with an exception, it is immediately raised in the REPL session. The function return value is then determined by the way the REPL session terminates: 0, 1, or the status of a SystemExit, as specified above.

This function always finalizes the Python interpreter before it returns.

See Python Configuration for an example of a customized Python that always runs in isolated mode using Py_RunMain().

int PyUnstable_AtExit(PyInterpreterState *interp, void (*func)(void*), void *data)¶

This is Unstable API. It may change without warning in minor releases.

Register an atexit callback for the target interpreter interp. This is similar to Py_AtExit(), but takes an explicit interpreter and data pointer for the callback.

The GIL must be held for interp.

Added in version 3.13.

Process-wide parameters¶

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 PyConfig.program_name, 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().

Deprecated since version 3.13, will be removed in version 3.15: Get sys.executable instead.

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 PyConfig.program_name 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.base_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().

Deprecated since version 3.13, will be removed in version 3.15: Get sys.base_prefix instead, or sys.prefix if virtual environments need to be handled.

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 PyConfig.program_name 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.base_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().

Deprecated since version 3.13, will be removed in version 3.15: Get sys.base_exec_prefix instead, or sys.exec_prefix if virtual environments need to be handled.

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 PyConfig.program_name). 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().

Deprecated since version 3.13, will be removed in version 3.15: Get sys.executable instead.

wchar_t *Py_GetPath()¶

Part of the Stable ABI.

Return the default module search path; this is computed from the program name (set by PyConfig.program_name) 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().

Deprecated since version 3.13, will be removed in version 3.15: Get sys.path instead.

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_t* 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_t* 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_t* 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 PyConfig.home, 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().

Deprecated since version 3.13, will be removed in version 3.15: Get PyConfig.home or PYTHONHOME environment variable instead.

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().

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.

Py_BEGIN_ALLOW_THREADS
... Do some blocking I/O operation ...
Py_END_ALLOW_THREADS

PyThreadState *_save;

_save = PyEval_SaveThread();
... Do some blocking I/O operation ...
PyEval_RestoreThread(_save);

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);

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 = NULL;
PyStatus status = Py_NewInterpreterFromConfig(&tstate, &config);
if (PyStatus_Exception(status)) {
    Py_ExitStatusException(status);
}

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