See Python Initialization Configuration for details on how to configure the interpreter prior to initialization.
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:
the runtime pre-initialization functions covered in Python Initialization Configuration
Configuration functions:
PyInitFrozenExtensions()
the configuration functions covered in Python Initialization Configuration
Informative functions:
Utilities:
the status reporting and utility functions covered in Python Initialization Configuration
Memory allocators:
Synchronization:
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.
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.
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.
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 wonât 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.
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.
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.
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.
Deprecated since version 3.12, will be removed in version 3.14.
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.
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.
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 scriptâs directory nor the userâs 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.
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.
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.
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.
This API is kept for backward compatibility: setting PyConfig.user_site_directory should be used instead, see Python Initialization Configuration.
Donât 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.
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.
This API is kept for backward compatibility: setting PyConfig.quiet should be used instead, see Python Initialization Configuration.
Donât 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.
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.
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.
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.
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.
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.
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.
Return true (non-zero) if the main Python interpreter is shutting down. Return false (zero) otherwise.
Added in version 3.13.
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.
This is a backwards-compatible version of Py_FinalizeEx() that disregards the return value.
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.
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 programâs 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).
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__ moduleâs 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__ moduleâs 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().
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.
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 programâs 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.
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.
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.
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.
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.
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.
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.
Return the platform identifier for the current platform. On Unix, this is formed from the âofficialâ 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.
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.
Return an indication of the compiler used to build the current Python version, in square brackets, for example:
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.
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.
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 programâs 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 isnât 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] doesnât 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.
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.
This API is kept for backward compatibility: setting PyConfig.home should be used instead, see Python Initialization Configuration.
Set the default âhomeâ 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 programâs 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.
Return the default âhomeâ, 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.
The Python interpreter is not fully thread-safe. In order to support multi-threaded Python programs, thereâs 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. Thereâs also one global variable pointing to the current PyThreadState: it can be retrieved using PyThreadState_Get().
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 donât 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.
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 donât 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.
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 CPythonâs runtime.
The fact that only the âcurrentâ 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 Pythonâs 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 CPythonâs 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 âmainâ interpreter, fork() should only be called in that interpreterâs âmainâ thread, where the CPython global runtime was originally initialized. The only exception is if exec() will be called immediately after.
These are the most commonly used types and functions when writing C extension code, or when embedding the Python interpreter:
This data structure represents the state shared by a number of cooperating threads. Threads belonging to the same interpreter share their module administration and a few other internal items. There are no public members in this structure.
Threads belonging to different interpreters initially share nothing, except process state like available memory, open file descriptors and such. The global interpreter lock is also shared by all threads, regardless of to which interpreter they belong.
This data structure represents the state of a single thread. The only public data member is:
This threadâs interpreter state.
Deprecated function which does nothing.
In Python 3.6 and older, this function created the GIL if it didnât 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 donât 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.
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.
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.
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 neednât check for NULL).
See also PyThreadState_GetUnchecked().
Similar to PyThreadState_Get(), but donât kill the process with a fatal error if it is NULL. The caller is responsible to check if the result is NULL.
Added in version 3.13: In Python 3.5 to 3.12, the function was private and known as _PyThreadState_UncheckedGet().
Swap the current thread state with the thread state given by the argument tstate, which may be NULL.
The GIL does not need to be held, but will be held upon returning if tstate is non-NULL.
The following functions use thread-local storage, and are not compatible with sub-interpreters:
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 âhandleâ 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.
Release any resources previously acquired. After this call, Pythonâs 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.
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.
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.
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.
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.
This macro expands to PyEval_RestoreThread(_save);: it is equivalent to Py_END_ALLOW_THREADS without the closing brace.
This macro expands to _save = PyEval_SaveThread();: it is equivalent to Py_BEGIN_ALLOW_THREADS without the opening brace and variable declaration.
All of the following functions must be called after Py_Initialize().
Changed in version 3.7: Py_Initialize() now initializes the GIL.
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.
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.
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().
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.
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().
Changed in version 3.13: The PyThreadState.on_delete callback was removed.
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().
Destroy the current thread state and release the global interpreter lock. Like PyThreadState_Delete(), the global interpreter lock must be held. The thread state must have been reset with a previous call to PyThreadState_Clear().
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.
Get the unique thread state identifier of the Python thread state tstate.
tstate must not be NULL.
Added in version 3.9.
Get the interpreter of the Python thread state tstate.
tstate must not be NULL.
Added in version 3.9.
Suspend tracing and profiling in the Python thread state tstate.
Resume them using the PyThreadState_LeaveTracing() function.
Added in version 3.11.
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.
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.
Return the interpreterâs 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.
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.
Return a strong reference to the __main__ module object for the given interpreter.
The caller must hold the GIL.
Added in version 3.13.
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*.
Get the frame evaluation function.
See the PEP 523 âAdding a frame evaluation API to CPythonâ.
Added in version 3.9.
Set the frame evaluation function.
See the PEP 523 âAdding a frame evaluation API to CPythonâ.
Added in version 3.9.
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.
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 isnât 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.
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).
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 â if it isnât, a fatal error is reported.
PyEval_SaveThread() is a higher-level function which is always available (even when threads have not been initialized).
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 âmainâ 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:
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:
If this is 0 then the sub-interpreter will use its own âobjectâ allocator state. Otherwise it will use (share) the main interpreterâs.
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.
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.
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.
If this is 0 then the sub-interpreterâs threading module wonât create threads. Otherwise threads are allowed.
If this is 0 then the sub-interpreterâs threading module wonât create daemon threads. Otherwise daemon threads are allowed (as long as allow_threads is non-zero).
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.
This determines the operation of the GIL for the sub-interpreter. It may be one of the following:
Use the default selection (PyInterpreterConfig_SHARED_GIL).
Use (share) the main interpreterâs GIL.
Use the sub-interpreterâs own GIL.
If this is PyInterpreterConfig_OWN_GIL then PyInterpreterConfig.use_main_obmalloc must be 0.
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 interpreterâs GIL will be held by the current thread and the previously interpreterâs 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 configâs 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 moduleâs 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 extensionâs init function is not called. Objects in the moduleâs 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 extensionâs 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.
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 interpreterâs GIL, allows fork/exec, allows daemon threads, and allows single-phase init modules.
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 havenât been explicitly destroyed at that point.
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)) canât 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 isnât perfect â for example, using low-level file operations like os.close() they can (accidentally or maliciously) affect each otherâs 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-)interpreterâs 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 donât 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.
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.
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 wonât 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 doesnât need a current thread state to run, and it doesnât 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 wonât 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.
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.
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:
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.
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.
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.
The value for the what parameter to Py_tracefunc functions when a call is about to return.
The value for the what parameter to Py_tracefunc functions when a C function is about to be called.
The value for the what parameter to Py_tracefunc functions when a C function has raised an exception.
The value for the what parameter to Py_tracefunc functions when a C function has returned.
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.
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.
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.
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.
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.
Reference tracing¶Added in version 3.13.
The type of the trace function registered using PyRefTracer_SetTracer(). The first parameter is a Python object that has been just created (when event is set to PyRefTracer_CREATE) or about to be destroyed (when event is set to PyRefTracer_DESTROY). The data argument is the opaque pointer that was provided when PyRefTracer_SetTracer() was called.
Added in version 3.13.
The value for the event parameter to PyRefTracer functions when a Python object has been created.
The value for the event parameter to PyRefTracer functions when a Python object has been destroyed.
Register a reference tracer function. The function will be called when a new Python has been created or when an object is going to be destroyed. If data is provided it must be an opaque pointer that will be provided when the tracer function is called. Return 0 on success. Set an exception and return -1 on error.
Not that tracer functions must not create Python objects inside or otherwise the call will be re-entrant. The tracer also must not clear any existing exception or set an exception. The GIL will be held every time the tracer function is called.
The GIL must be held when calling this function.
Added in version 3.13.
Get the registered reference tracer function and the value of the opaque data pointer that was registered when PyRefTracer_SetTracer() was called. If no tracer was registered this function will return NULL and will set the data pointer to NULL.
The GIL must be held when calling this function.
Added in version 3.13.
Advanced Debugger Support¶These functions are only intended to be used by advanced debugging tools.
Return the interpreter state object at the head of the list of all such objects.
Return the main interpreter state object.
Return the next interpreter state object after interp from the list of all such objects.
Return the pointer to the first PyThreadState object in the list of threads associated with the interpreter interp.
Return the next thread state object after tstate from the list of all such objects belonging to the same PyInterpreterState object.
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 donât 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
âA New C-API for Thread-Local Storage in CPythonâ (PEP 539)
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 keyâs 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.
This macro expands to the initializer for Py_tss_t variables. Note that this macro wonât be defined with Py_LIMITED_API.
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.
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.
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.
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().
Return a non-zero value if the given Py_tss_t has been initialized by PyThread_tss_create().
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 â calling it on an already initialized key is a no-op and immediately returns success.
Destroy a TSS key to forget the values associated with the key across all threads, and change the keyâs 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 â calling it on an already destroyed key is a no-op.
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.
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.
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.
The C-API provides a basic mutual exclusion lock.
A mutual exclusion lock. The PyMutex should be initialized to zero to represent the unlocked state. For example:
Instances of PyMutex should not be copied or moved. Both the contents and address of a PyMutex are meaningful, and it must remain at a fixed, writable location in memory.
Note
A PyMutex currently occupies one byte, but the size should be considered unstable. The size may change in future Python releases without a deprecation period.
Added in version 3.13.
Lock mutex m. If another thread has already locked it, the calling thread will block until the mutex is unlocked. While blocked, the thread will temporarily release the GIL if it is held.
Added in version 3.13.
Unlock mutex m. The mutex must be locked â otherwise, the function will issue a fatal error.
Added in version 3.13.
The critical section API provides a deadlock avoidance layer on top of per-object locks for free-threaded CPython. They are intended to replace reliance on the global interpreter lock, and are no-ops in versions of Python with the global interpreter lock.
Critical sections avoid deadlocks by implicitly suspending active critical sections and releasing the locks during calls to PyEval_SaveThread(). When PyEval_RestoreThread() is called, the most recent critical section is resumed, and its locks reacquired. This means the critical section API provides weaker guarantees than traditional locks â they are useful because their behavior is similar to the GIL.
The functions and structs used by the macros are exposed for cases where C macros are not available. They should only be used as in the given macro expansions. Note that the sizes and contents of the structures may change in future Python versions.
Note
Operations that need to lock two objects at once must use Py_BEGIN_CRITICAL_SECTION2. You cannot use nested critical sections to lock more than one object at once, because the inner critical section may suspend the outer critical sections. This API does not provide a way to lock more than two objects at once.
Example usage:
static PyObject *
set_field(MyObject *self, PyObject *value)
{
Py_BEGIN_CRITICAL_SECTION(self);
Py_SETREF(self->field, Py_XNewRef(value));
Py_END_CRITICAL_SECTION();
Py_RETURN_NONE;
}
In the above example, Py_SETREF calls Py_DECREF, which can call arbitrary code through an objectâs deallocation function. The critical section API avoids potential deadlocks due to reentrancy and lock ordering by allowing the runtime to temporarily suspend the critical section if the code triggered by the finalizer blocks and calls PyEval_SaveThread().
Acquires the per-object lock for the object op and begins a critical section.
In the free-threaded build, this macro expands to:
{
PyCriticalSection _py_cs;
PyCriticalSection_Begin(&_py_cs, (PyObject*)(op))
In the default build, this macro expands to {.
Added in version 3.13.
Ends the critical section and releases the per-object lock.
In the free-threaded build, this macro expands to:
PyCriticalSection_End(&_py_cs); }
In the default build, this macro expands to }.
Added in version 3.13.
Acquires the per-objects locks for the objects a and b and begins a critical section. The locks are acquired in a consistent order (lowest address first) to avoid lock ordering deadlocks.
In the free-threaded build, this macro expands to:
{
PyCriticalSection2 _py_cs2;
PyCriticalSection2_Begin(&_py_cs2, (PyObject*)(a), (PyObject*)(b))
In the default build, this macro expands to {.
Added in version 3.13.
Ends the critical section and releases the per-object locks.
In the free-threaded build, this macro expands to:
PyCriticalSection2_End(&_py_cs2); }
In the default build, this macro expands to }.
Added in version 3.13.
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