Initialization, Finalization, and Threads

Before Python Initialization

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

for the few exceptions.

This initializes the table of loaded modules (sys.modules), and creates the fundamental modules

builtins

__main__

and

sys

. It also initializes the module search path (sys.path). It does not set sys.argv; use

PySys_SetArgvEx()

for that. This is a no-op when called for a second time (without calling

first). There is no return value; it is a fatal error if the initialization fails.

Note

On Windows, changes the console mode from O_TEXT to O_BINARY, which will also affect non-Python uses of the console using the C Runtime.

voidPy_InitializeEx(intinitsigs)

Part of the

.This function works like

if initsigs is 1. If initsigs is 0, it skips initialization registration of signal handlers, which might be useful when Python is embedded.

intPy_IsInitialized()

Part of the

.Return true (nonzero) when the Python interpreter has been initialized, false (zero) if not. After

is called, this returns false until

is called again.

intPy_FinalizeEx()

Part of the

since version 3.6.Undo all initializations made by

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

. Ideally, this frees all memory allocated by the Python interpreter. This is a no-op when called for a second time (without calling

again first). Normally the return value is 0. If there were errors during finalization (flushing buffered data), -1 is returned.

This function is provided for a number of reasons. An embedding application might want to restart Python without having to restart the application itself. An application that has loaded the Python interpreter from a dynamically loadable library (or DLL) might want to free all memory allocated by Python before unloading the DLL. During a hunt for memory leaks in an application a developer might want to free all memory allocated by Python before exiting from the application.

Bugs and caveats: The destruction of modules and objects in modules is done in random order; this may cause destructors (

__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

and

more than once.

Raises an

auditing event

cpython._PySys_ClearAuditHooks with no arguments.

New in version 3.6.

voidPy_Finalize()

Part of the

.This is a backwards-compatible version of

that disregards the return value.

Process-wide parameters

intPy_SetStandardStreamEncoding(constchar*encoding, constchar*errors)

This API is kept for backward compatibility: setting

PyConfig.stdio_encoding

and

PyConfig.stdio_errors

should be used instead, see

This function should be called before

, if it is called at all. It specifies which encoding and error handling to use with standard IO, with the same meanings as in

str.encode()

It overrides

PYTHONIOENCODING

values, and allows embedding code to control IO encoding when the environment variable does not work.

encoding and/or errors may be NULL to use

PYTHONIOENCODING

and/or default values (depending on other settings).

Note that

sys.stderr

always uses the “backslashreplace” error handler, regardless of this (or any other) setting.

is called, this function will need to be called again in order to affect subsequent calls to

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

New in version 3.4.

Deprecated since version 3.11.

voidPy_SetProgramName(constwchar_t*name)

Part of the

.This API is kept for backward compatibility: setting

PyConfig.program_name

should be used instead, see

This function should be called before

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

to decode a bytes string to get a wchar_* string.

Deprecated since version 3.11.

wchar_t*Py_GetProgramName()

Part of the

.Return the program name set with

, or the default. The returned string points into static storage; the caller should not modify its value.

This function should not be called before

, otherwise it returns NULL.

Changed in version 3.10: It now returns NULL if called before

wchar_t*Py_GetPrefix()

Part of the

.Return the prefix for installed platform-independent files. This is derived through a number of complicated rules from the program name set with

and some environment variables; for example, if the program name is '/usr/local/bin/python', the prefix is '/usr/local'. The returned string points into static storage; the caller should not modify its value. This corresponds to the prefix variable in the top-level Makefile and the

--prefix

argument to the configure script at build time. The value is available to Python code as sys.prefix. It is only useful on Unix. See also the next function.

This function should not be called before

, otherwise it returns NULL.

Changed in version 3.10: It now returns NULL if called before

wchar_t*Py_GetExecPrefix()

Part of the

.Return the exec-prefix for installed platform-dependent files. This is derived through a number of complicated rules from the program name set with

and some environment variables; for example, if the program name is '/usr/local/bin/python', the exec-prefix is '/usr/local'. The returned string points into static storage; the caller should not modify its value. This corresponds to the exec_prefix variable in the top-level Makefile and the --exec-prefix argument to the configure script at build time. The value is available to Python code as sys.exec_prefix. It is only useful on Unix.

Background: The exec-prefix differs from the prefix when platform dependent files (such as executables and shared libraries) are installed in a different directory tree. In a typical installation, platform dependent files may be installed in the /usr/local/plat subtree while platform independent may be installed in /usr/local.

Generally speaking, a platform is a combination of hardware and software families, e.g. Sparc machines running the Solaris 2.x operating system are considered the same platform, but Intel machines running Solaris 2.x are another platform, and Intel machines running Linux are yet another platform. Different major revisions of the same operating system generally also form different platforms. Non-Unix operating systems are a different story; the installation strategies on those systems are so different that the prefix and exec-prefix are meaningless, and set to the empty string. Note that compiled Python bytecode files are platform independent (but not independent from the Python version by which they were compiled!).

System administrators will know how to configure the mount or automount programs to share /usr/local between platforms while having /usr/local/plat be a different filesystem for each platform.

This function should not be called before

, otherwise it returns NULL.

Changed in version 3.10: It now returns NULL if called before

wchar_t*Py_GetProgramFullPath()

Part of the

.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

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

This function should not be called before

, otherwise it returns NULL.

Changed in version 3.10: It now returns NULL if called before

wchar_t*Py_GetPath()

Part of the

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

above) and some environment variables. The returned string consists of a series of directory names separated by a platform dependent delimiter character. The delimiter character is ':' on Unix and macOS, ';' on Windows. The returned string points into static storage; the caller should not modify its value. The list

sys.path

is initialized with this value on interpreter startup; it can be (and usually is) modified later to change the search path for loading modules.

This function should not be called before

, otherwise it returns NULL.

Changed in version 3.10: It now returns NULL if called before

voidPy_SetPath(constwchar_t*)

Part of the

PyConfig.module_search_paths

since version 3.7.This API is kept for backward compatibility: setting

and

PyConfig.module_search_paths_set

should be used instead, see

Set the default module search path. If this function is called before

, then

Py_GetPath()

won’t attempt to compute a default search path but uses the one provided instead. This is useful if Python is embedded by an application that has full knowledge of the location of all modules. The path components should be separated by the platform dependent delimiter character, which is ':' on Unix and macOS, ';' on Windows.

This also causes

sys.executable

to be set to the program full path (see

Py_GetProgramFullPath()

) and for

sys.prefix

and

sys.exec_prefix

to be empty. It is up to the caller to modify these if required after calling

Use

to decode a bytes string to get a wchar_* string.

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

Changed in version 3.8: The program full path is now used for

sys.executable

, instead of the program name.

Deprecated since version 3.11.

constchar*Py_GetVersion()

Part of the

.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

PyConfig.orig_argv

and

PyConfig.argv

members of the

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");New in version 3.1.3.

Deprecated since version 3.11.

voidPySys_SetArgv(intargc, wchar_t**argv)

Part of the

.This API is kept for backward compatibility: setting

PyConfig.argv

and

PyConfig.parse_argv

should be used instead, see

This function works like

PySys_SetArgvEx()

with updatepath set to 1 unless the python interpreter was started with the

-I

Use

to decode a bytes string to get a wchar_* string.

See also

PyConfig.orig_argv

and

PyConfig.argv

members of the

Changed in version 3.4: The updatepath value depends on

-I

Deprecated since version 3.11.

voidPy_SetPythonHome(constwchar_t*home)

Part of the

.This API is kept for backward compatibility: setting

PyConfig.home

should be used instead, see

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

to decode a bytes string to get a wchar_* string.

Deprecated since version 3.11.

wchar_t*Py_GetPythonHome()

Part of the

.Return the default “home”, that is, the value set by a previous call to

Py_SetPythonHome()

, or the value of the

PYTHONHOME

environment variable if it is set.

This function should not be called before

, otherwise it returns NULL.

Changed in version 3.10: It now returns NULL if called before

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

, 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

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

. There’s also one global variable pointing to the current

: it can be retrieved using

PyThreadState_Get()

Releasing the GIL from extension code

Most extension code manipulating the

has the following simple structure:

Savethethreadstateinalocalvariable.Releasetheglobalinterpreterlock....DosomeblockingI/Ooperation...Reacquiretheglobalinterpreterlock.Restorethethreadstatefromthelocalvariable.This is so common that a pair of macros exists to simplify it:

Py_BEGIN_ALLOW_THREADS...DosomeblockingI/Ooperation...Py_END_ALLOW_THREADSThe

macro opens a new block and declares a hidden local variable; the

macro closes the block.

The block above expands to the following code:

PyThreadState*_save;_save=PyEval_SaveThread();...DosomeblockingI/Ooperation...PyEval_RestoreThread(_save);Here is how these functions work: the global interpreter lock is used to protect the pointer to the current thread state. When releasing the lock and saving the thread state, the current thread state pointer must be retrieved before the lock is released (since another thread could immediately acquire the lock and store its own thread state in the global variable). Conversely, when acquiring the lock and restoring the thread state, the lock must be acquired before storing the thread state pointer.

Note

Calling system I/O functions is the most common use case for releasing the GIL, but it can also be useful before calling long-running computations which don’t need access to Python objects, such as compression or cryptographic functions operating over memory buffers. For example, the standard

zlib

and

hashlib

modules release the GIL when compressing or hashing data.

Non-Python created threads

When threads are created using the dedicated Python APIs (such as the

threading

module), a thread state is automatically associated to them and the code showed above is therefore correct. However, when threads are created from C (for example by a third-party library with its own thread management), they don’t hold the GIL, nor is there a thread state structure for them.

If you need to call Python code from these threads (often this will be part of a callback API provided by the aforementioned third-party library), you must first register these threads with the interpreter by creating a thread state data structure, then acquiring the GIL, and finally storing their thread state pointer, before you can start using the Python/C API. When you are done, you should reset the thread state pointer, release the GIL, and finally free the thread state data structure.

The

and

functions do all of the above automatically. The typical idiom for calling into Python from a C thread is:

PyGILState_STATEgstate;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

). Python supports the creation of additional interpreters (using

Py_NewInterpreter()

), but mixing multiple interpreters and the PyGILState_* API is unsupported.

Cautions about fork()

Another important thing to note about threads is their behaviour in the face of the C fork() call. On most systems with fork(), after a process forks only the thread that issued the fork will exist. This has a concrete impact both on how locks must be handled and on all stored state in CPython’s runtime.

The fact that only the “current” thread remains means any locks held by other threads will never be released. Python solves this for

os.fork()

by acquiring the locks it uses internally before the fork, and releasing them afterwards. In addition, it resets any

Lock Objects

in the child. When extending or embedding Python, there is no way to inform Python of additional (non-Python) locks that need to be acquired before or reset after a fork. OS facilities such as pthread_atfork() would need to be used to accomplish the same thing. Additionally, when extending or embedding Python, calling fork() directly rather than through

os.fork()

(and returning to or calling into Python) may result in a deadlock by one of Python’s internal locks being held by a thread that is defunct after the fork.

PyOS_AfterFork_Child()

tries to reset the necessary locks, but is not always able to.

The fact that all other threads go away also means that CPython’s runtime state there must be cleaned up properly, which

os.fork()

does. This means finalizing all other

objects belonging to the current interpreter and all other

objects. Due to this and the special nature of the

“main” interpreter

, fork() should only be called in that interpreter’s “main” thread, where the CPython global runtime was originally initialized. The only exception is if exec() will be called immediately after.

High-level API

These are the most commonly used types and functions when writing C extension code, or when embedding the Python interpreter:

typePyInterpreterState

Part of the

Limited API

(as an opaque struct).This data structure represents the state shared by a number of cooperating threads. Threads belonging to the same interpreter share their module administration and a few other internal items. There are no public members in this structure.

Threads belonging to different interpreters initially share nothing, except process state like available memory, open file descriptors and such. The global interpreter lock is also shared by all threads, regardless of to which interpreter they belong.

typePyThreadState

Part of the

Limited API

(as an opaque struct).This data structure represents the state of a single thread. The only public data member is:

*interp

This thread’s interpreter state.

voidPyEval_InitThreads()

Part of the

.Deprecated function which does nothing.

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

Changed in version 3.9: The function now does nothing.

Changed in version 3.7: This function is now called by

, so you don’t have to call it yourself anymore.

Changed in version 3.2: This function cannot be called before

anymore.

Deprecated since version 3.9.

intPyEval_ThreadsInitialized()

Part of the

.Returns a non-zero value if

PyEval_InitThreads()

has been called. This function can be called without holding the GIL, and therefore can be used to avoid calls to the locking API when running single-threaded.

Changed in version 3.7: The

is now initialized by

Deprecated since version 3.9.

*PyEval_SaveThread()

Part of the

.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.

voidPyEval_RestoreThread(

*tstate)

Part of the

.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

to check if the interpreter is in process of being finalized before calling this function to avoid unwanted termination.

*PyThreadState_Get()

Part of the

.Return the current thread state. The global interpreter lock must be held. When the current thread state is NULL, this issues a fatal error (so that the caller needn’t check for NULL).

*PyThreadState_Swap(

*tstate)

Part of the

.Swap the current thread state with the thread state given by the argument tstate, which may be NULL. The global interpreter lock must be held and is not released.

The following functions use thread-local storage, and are not compatible with sub-interpreters:

PyGILState_STATEPyGILState_Ensure()

Part of the

.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

. In general, other thread-related APIs may be used between

and

calls as long as the thread state is restored to its previous state before the Release(). For example, normal usage of the

and

macros is acceptable.

The return value is an opaque “handle” to the thread state when

was called, and must be passed to

to ensure Python is left in the same state. Even though recursive calls are allowed, these handles cannot be shared - each unique call to

must save the handle for its call to

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

to check if the interpreter is in process of being finalized before calling this function to avoid unwanted termination.

voidPyGILState_Release(PyGILState_STATE)

Part of the

.Release any resources previously acquired. After this call, Python’s state will be the same as it was prior to the corresponding

call (but generally this state will be unknown to the caller, hence the use of the GILState API).

Every call to

must be matched by a call to

on the same thread.

*PyGILState_GetThisThreadState()

Part of the

.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.

intPyGILState_Check()

Return 1 if the current thread is holding the GIL and 0 otherwise. This function can be called from any thread at any time. Only if it has had its Python thread state initialized and currently is holding the GIL will it return 1. This is mainly a helper/diagnostic function. It can be useful for example in callback contexts or memory allocation functions when knowing that the GIL is locked can allow the caller to perform sensitive actions or otherwise behave differently.

New in version 3.4.

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

Py_BEGIN_ALLOW_THREADS

Part of the

.This macro expands to {PyThreadState*_save;_save=PyEval_SaveThread();. Note that it contains an opening brace; it must be matched with a following

macro. See above for further discussion of this macro.

Py_END_ALLOW_THREADS

Part of the

.This macro expands to PyEval_RestoreThread(_save);}. Note that it contains a closing brace; it must be matched with an earlier

macro. See above for further discussion of this macro.

Py_BLOCK_THREADS

Part of the

.This macro expands to PyEval_RestoreThread(_save);: it is equivalent to

without the closing brace.

Py_UNBLOCK_THREADS

Part of the

.This macro expands to _save=PyEval_SaveThread();: it is equivalent to

without the opening brace and variable declaration.

Low-level API

All of the following functions must be called after

Changed in version 3.7:

now initializes the

*PyInterpreterState_New()

Part of the

.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.

voidPyInterpreterState_Clear(

*interp)

Part of the

.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.

voidPyInterpreterState_Delete(

*interp)

Part of the

PyInterpreterState_Clear()

.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

*PyThreadState_New(

*interp)

Part of the

.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.

voidPyThreadState_Clear(

*tstate)

Part of the

.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()

voidPyThreadState_Delete(

*tstate)

Part of the

.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()

voidPyThreadState_DeleteCurrent(void)

Destroy the current thread state and release the global interpreter lock. Like

PyThreadState_Delete()

, the global interpreter lock need not be held. The thread state must have been reset with a previous call to

PyThreadState_Clear()

PyFrameObject

*PyThreadState_GetFrame(

*tstate)

Part of the

since version 3.10.Get the current frame of the Python thread state tstate.

Return a

strong reference

. Return NULL if no frame is currently executing.

See also

PyEval_GetFrame()

tstate must not be NULL.

New in version 3.9.

uint64_tPyThreadState_GetID(

*tstate)

Part of the

since version 3.10.Get the unique thread state identifier of the Python thread state tstate.

tstate must not be NULL.

New in version 3.9.

*PyThreadState_GetInterpreter(

*tstate)

Part of the

since version 3.10.Get the interpreter of the Python thread state tstate.

tstate must not be NULL.

New in version 3.9.

voidPyThreadState_EnterTracing(

PyThreadState_LeaveTracing()

*tstate)

Suspend tracing and profiling in the Python thread state tstate.

Resume them using the

function.

New in version 3.11.

voidPyThreadState_LeaveTracing(

PyThreadState_EnterTracing()

*tstate)

Resume tracing and profiling in the Python thread state tstate suspended by the

function.

See also

PyEval_SetTrace()

and

PyEval_SetProfile()

functions.

New in version 3.11.

*PyInterpreterState_Get(void)

Part of the

since version 3.9.Get the current interpreter.

Issue a fatal error if there no current Python thread state or no current interpreter. It cannot return NULL.

The caller must hold the GIL.

New in version 3.9.

int64_tPyInterpreterState_GetID(

*interp)

Part of the

since version 3.7.Return the interpreter’s unique ID. If there was any error in doing so then -1 is returned and an error is set.

The caller must hold the GIL.

New in version 3.7.

*PyInterpreterState_GetDict(

*interp)

Part of the

since version 3.8.Return a dictionary in which interpreter-specific data may be stored. If this function returns NULL then no exception has been raised and the caller should assume no interpreter-specific dict is available.

This is not a replacement for

PyModule_GetState()

, which extensions should use to store interpreter-specific state information.

New in version 3.8.

typedef

*(*_PyFrameEvalFunction)(

*tstate,_PyInterpreterFrame*frame,intthrowflag)

Type of a frame evaluation function.

The throwflag parameter is used by the throw() method of generators: if non-zero, handle the current exception.

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

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

_PyFrameEvalFunction

_PyInterpreterState_GetEvalFrameFunc(

*interp)

Get the frame evaluation function.

See the

PEP 523

“Adding a frame evaluation API to CPython”.

New in version 3.9.

void_PyInterpreterState_SetEvalFrameFunc(

*interp,

_PyFrameEvalFunction

eval_frame)

Set the frame evaluation function.

See the

PEP 523

“Adding a frame evaluation API to CPython”.

New in version 3.9.

*PyThreadState_GetDict()

Return value: Borrowed reference. Part of the

.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.

intPyThreadState_SetAsyncExc(unsignedlongid,

*exc)

Part of the

.Asynchronously raise an exception in a thread. The id argument is the thread id of the target thread; exc is the exception object to be raised. This function does not steal any references to exc. To prevent naive misuse, you must write your own C extension to call this. Must be called with the GIL held. Returns the number of thread states modified; this is normally one, but will be zero if the thread id isn’t found. If exc is NULL, the pending exception (if any) for the thread is cleared. This raises no exceptions.

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

voidPyEval_AcquireThread(

*tstate)

Part of the

.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

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

Py_END_ALLOW_THREADS()

, and

, and terminate the current thread if called while the interpreter is finalizing.

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

voidPyEval_ReleaseThread(

*tstate)

Part of the

.Reset the current thread state to NULL and release the global interpreter lock. The lock must have been created earlier and must be held by the current thread. The tstate argument, which must not be NULL, is only used to check that it represents the current thread state — if it isn’t, a fatal error is reported.

PyEval_SaveThread()

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

voidPyEval_AcquireLock()

Part of the

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

Deprecated since version 3.2: This function does not update the current thread state. Please use

PyEval_AcquireThread()

instead.

Note

Calling this function from a thread when the runtime is finalizing will terminate the thread, even if the thread was not created by Python. You can use _Py_IsFinalizing() or

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

Py_END_ALLOW_THREADS()

, and

, and terminate the current thread if called while the interpreter is finalizing.

voidPyEval_ReleaseLock()

Part of the

PyInterpreterState_Main()

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

Deprecated since version 3.2: This function does not update the current thread state. Please use

PyEval_SaveThread()

PyEval_ReleaseThread()

instead.

Sub-interpreter support

While in most uses, you will only embed a single Python interpreter, there are cases where you need to create several independent interpreters in the same process and perhaps even in the same thread. Sub-interpreters allow you to do that.

The “main” interpreter is the first one created when the runtime initializes. It is usually the only Python interpreter in a process. Unlike sub-interpreters, the main interpreter has unique process-global responsibilities like signal handling. It is also responsible for execution during runtime initialization and is usually the active interpreter during runtime finalization. The

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:

*Py_NewInterpreter()

Part of the

PyModule_FromDefAndSpec()

.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 return value points to the first thread state created in the new sub-interpreter. This thread state is made in the current thread state. Note that no actual thread is created; see the discussion of thread states below. If creation of the new interpreter is unsuccessful, NULL is returned; no exception is set since the exception state is stored in the current thread state and there may not be a current thread state. (Like all other Python/C API functions, the global interpreter lock must be held before calling this function and is still held when it returns; however, unlike most other Python/C API functions, there needn’t be a current thread state on entry.)

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

For modules using multi-phase initialization, e.g.

, a separate module object is created and initialized for each interpreter. Only C-level static and global variables are shared between these module objects.

For modules using single-phase initialization, e.g.

PyModule_Create()

, the first time a particular extension is imported, it is initialized normally, and a (shallow) copy of its module’s dictionary is squirreled away. When the same extension is imported by another (sub-)interpreter, a new module is initialized and filled with the contents of this copy; the extension’s init function is not called. Objects in the module’s dictionary thus end up shared across (sub-)interpreters, which might cause unwanted behavior (see

Bugs and caveats

below).

Note that this is different from what happens when an extension is imported after the interpreter has been completely re-initialized by calling

and

; in that case, the extension’s initmodule function is called again. As with multi-phase initialization, this means that only C-level static and global variables are shared between these modules.

voidPy_EndInterpreter(

*tstate)

Part of the

.Destroy the (sub-)interpreter represented by the given thread state. The given thread state must be the current thread state. See the discussion of thread states below. When the call returns, the current thread state is NULL. All thread states associated with this interpreter are destroyed. (The global interpreter lock must be held before calling this function and is still held when it returns.)

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

Bugs and caveats

Because sub-interpreters (and the main interpreter) are part of the same process, the insulation between them isn’t perfect — for example, using low-level file operations like

os.close()

they can (accidentally or maliciously) affect each other’s open files. Because of the way extensions are shared between (sub-)interpreters, some extensions may not work properly; this is especially likely when using single-phase initialization or (static) global variables. It is possible to insert objects created in one sub-interpreter into a namespace of another (sub-)interpreter; this should be avoided if possible.

Special care should be taken to avoid sharing user-defined functions, methods, instances or classes between sub-interpreters, since import operations executed by such objects may affect the wrong (sub-)interpreter’s dictionary of loaded modules. It is equally important to avoid sharing objects from which the above are reachable.

Also note that combining this functionality with PyGILState_* APIs is delicate, because these APIs assume a bijection between Python thread states and OS-level threads, an assumption broken by the presence of sub-interpreters. It is highly recommended that you don’t switch sub-interpreters between a pair of matching

and

calls. Furthermore, extensions (such as

ctypes

) using these APIs to allow calling of Python code from non-Python created threads will probably be broken when using sub-interpreters.

Asynchronous Notifications

A mechanism is provided to make asynchronous notifications to the main interpreter thread. These notifications take the form of a function pointer and a void pointer argument.

intPy_AddPendingCall(int(*func)(void*), void*arg)

Part of the

.Schedule a function to be called from the main interpreter thread. On success, 0 is returned and func is queued for being called in the main thread. On failure, -1 is returned without setting any exception.

When successfully queued, func will be eventually called from the main interpreter thread with the argument arg. It will be called asynchronously with respect to normally running Python code, but with both these conditions met:

on a

bytecode

boundary;

with the main thread holding the

global interpreter lock

(func can therefore use the full C API).

func must return 0 on success, or -1 on failure with an exception set. func won’t be interrupted to perform another asynchronous notification recursively, but it can still be interrupted to switch threads if the global interpreter lock is released.

This function doesn’t need a current thread state to run, and it doesn’t need the global interpreter lock.

To call this function in a subinterpreter, the caller must hold the GIL. Otherwise, the function func can be scheduled to be called from the wrong interpreter.

Warning

This is a low-level function, only useful for very special cases. There is no guarantee that func will be called as quick as possible. If the main thread is busy executing a system call, func won’t be called before the system call returns. This function is generally not suitable for calling Python code from arbitrary C threads. Instead, use the

PyGILState API

New in version 3.1.

Changed in version 3.9: If this function is called in a subinterpreter, the function func is now scheduled to be called from the subinterpreter, rather than being called from the main interpreter. Each subinterpreter now has its own list of scheduled calls.

Profiling and Tracing

The Python interpreter provides some low-level support for attaching profiling and execution tracing facilities. These are used for profiling, debugging, and coverage analysis tools.

This C interface allows the profiling or tracing code to avoid the overhead of calling through Python-level callable objects, making a direct C function call instead. The essential attributes of the facility have not changed; the interface allows trace functions to be installed per-thread, and the basic events reported to the trace function are the same as had been reported to the Python-level trace functions in previous versions.

typedefint(*Py_tracefunc)(

*obj,

PyFrameObject

*frame,intwhat,

*arg)

The type of the trace function registered using

PyEval_SetProfile()

and

PyEval_SetTrace()

. The first parameter is the object passed to the registration function as obj, frame is the frame object to which the event pertains, what is one of the constants

, or

, and arg depends on the value of what:

Value of what

Meaning of arg

PyTrace_CALL

Always

Py_None

PyTrace_EXCEPTION

Exception information as returned by

Always

Value being returned to the caller, or NULL if caused by an exception.

PyTrace_C_CALL

Function object being called.

PyTrace_C_EXCEPTION

Function object being called.

PyTrace_C_RETURN

Function object being called.

PyTrace_OPCODE

Always

Py_None

intPyTrace_CALL

The value of the what parameter to a

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.

intPyTrace_EXCEPTION

The value of the what parameter to a

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.

intPyTrace_LINE

The value passed as the what parameter to a

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.

intPyTrace_RETURN

The value for the what parameter to

functions when a call is about to return.

intPyTrace_C_CALL

The value for the what parameter to

functions when a C function is about to be called.

intPyTrace_C_EXCEPTION

The value for the what parameter to

functions when a C function has raised an exception.

intPyTrace_C_RETURN

The value for the what parameter to

functions when a C function has returned.

intPyTrace_OPCODE

The value for the what parameter to

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.

voidPyEval_SetProfile(

func,

*obj)

Set the profiler function to func. The obj parameter is passed to the function as its first parameter, and may be any Python object, or NULL. If the profile function needs to maintain state, using a different value for obj for each thread provides a convenient and thread-safe place to store it. The profile function is called for all monitored events except

PyTrace_LINE

PyTrace_OPCODE

and

PyTrace_EXCEPTION

See also

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

PEP 539

)

typePy_tss_t

This data structure represents the state of a thread key, the definition of which may depend on the underlying TLS implementation, and it has an internal field representing the key’s initialization state. There are no public members in this structure.

When

Py_LIMITED_API

is not defined, static allocation of this type by

Py_tss_NEEDS_INIT

is allowed.

Py_tss_NEEDS_INIT

This macro expands to the initializer for

variables. Note that this macro won’t be defined with

Py_LIMITED_API

Dynamic Allocation

Dynamic allocation of the

, 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.

*PyThread_tss_alloc()

Part of the

since version 3.7.Return a value which is the same state as a value initialized with

Py_tss_NEEDS_INIT

, or NULL in the case of dynamic allocation failure.

voidPyThread_tss_free(

*key)

Part of the

since version 3.7.Free the given key allocated by

PyThread_tss_alloc()

, after first calling

PyThread_tss_delete()

to ensure any associated thread locals have been unassigned. This is a no-op if the key argument is NULL.

Note

A freed key becomes a dangling pointer. You should reset the key to NULL.

Methods

The parameter key of these functions must not be NULL. Moreover, the behaviors of

PyThread_tss_set()

and

PyThread_tss_get()

are undefined if the given

has not been initialized by

PyThread_tss_create()

intPyThread_tss_is_created(

*key)

Part of the

since version 3.7.Return a non-zero value if the given

has been initialized by

PyThread_tss_create()

intPyThread_tss_create(

*key)

Part of the

since version 3.7.Return a zero value on successful initialization of a TSS key. The behavior is undefined if the value pointed to by the key argument is not initialized by

Py_tss_NEEDS_INIT

. This function can be called repeatedly on the same key – calling it on an already initialized key is a no-op and immediately returns success.

voidPyThread_tss_delete(

*key)

Part of the

since version 3.7.Destroy a TSS key to forget the values associated with the key across all threads, and change the key’s initialization state to uninitialized. A destroyed key is able to be initialized again by

PyThread_tss_create()

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

intPyThread_tss_set(

*key, void*value)

Part of the

since version 3.7.Return a zero value to indicate successfully associating a void* value with a TSS key in the current thread. Each thread has a distinct mapping of the key to a void* value.

void*PyThread_tss_get(

*key)

Part of the

Thread Specific Storage (TSS) API

since version 3.7.Return the void* value associated with a TSS key in the current thread. This returns NULL if no value is associated with the key in the current thread.

Thread Local Storage (TLS) API

Deprecated since version 3.7: This API is superseded by

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.

intPyThread_create_key()

Part of the