Initialization, Finalization, and Threads

Initializing and finalizing the interpreter

voidPy_Initialize()

Initialize the Python interpreter. In an application embedding Python, thisshould be called before using any other Python/C API functions; with theexception ofPy_SetProgramName(),Py_SetPythonHome() andPy_SetPath(). This initializesthe table of loaded modules (sys.modules), and creates the fundamentalmodulesbuiltins,__main__ andsys. It also initializesthe module search path (sys.path). It does not setsys.argv; usePySys_SetArgvEx() for that. This is a no-op when called for a second time(without callingPy_FinalizeEx() first). There is no return value; it is afatal error if the initialization fails.

Note

On Windows, changes the console mode fromO_TEXT toO_BINARY, which willalso affect non-Python uses of the console using the C Runtime.

voidPy_InitializeEx(int initsigs)

This function works likePy_Initialize() ifinitsigs is1. Ifinitsigs is0, it skips initialization registration of signal handlers, whichmight be useful when Python is embedded.

intPy_IsInitialized()

Return true (nonzero) when the Python interpreter has been initialized, false(zero) if not. AfterPy_FinalizeEx() is called, this returns false untilPy_Initialize() is called again.

intPy_FinalizeEx()

Undo all initializations made byPy_Initialize() and subsequent use ofPython/C API functions, and destroy all sub-interpreters (seePy_NewInterpreter() below) that were created and not yet destroyed sincethe last call toPy_Initialize(). Ideally, this frees all memoryallocated by the Python interpreter. This is a no-op when called for a secondtime (without callingPy_Initialize() again first). Normally thereturn value is0. If there were errors during finalization(flushing buffered data),-1 is returned.

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

Bugs and caveats: The destruction of modules and objects in modules is donein random order; this may cause destructors (__del__() methods) to failwhen they depend on other objects (even functions) or modules. Dynamicallyloaded extension modules loaded by Python are not unloaded. Small amounts ofmemory 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 notfreed. Some memory allocated by extension modules may not be freed. Someextensions may not work properly if their initialization routine is called morethan once; this can happen if an application callsPy_Initialize() andPy_FinalizeEx() more than once.

New in version 3.6.

voidPy_Finalize()

This is a backwards-compatible version ofPy_FinalizeEx() thatdisregards the return value.

Process-wide parameters

intPy_SetStandardStreamEncoding(const char *encoding, const char *errors)

This function should be called beforePy_Initialize(), if it iscalled at all. It specifies which encoding and error handling to usewith standard IO, with the same meanings as instr.encode().

It overridesPYTHONIOENCODING values, and allows embedding codeto control IO encoding when the environment variable does not work.

encoding and/orerrors may be NULL to usePYTHONIOENCODING and/or default values (depending on othersettings).

Note thatsys.stderr always uses the “backslashreplace” errorhandler, regardless of this (or any other) setting.

IfPy_FinalizeEx() is called, this function will need to be calledagain in order to affect subsequent calls toPy_Initialize().

Returns0 if successful, a nonzero value on error (e.g. calling after theinterpreter has already been initialized).

New in version 3.4.

voidPy_SetProgramName(wchar_t *name)

This function should be called beforePy_Initialize() is called forthe first time, if it is called at all. It tells the interpreter the valueof theargv[0] argument to themain() function of the program(converted to wide characters).This is used byPy_GetPath() and some other functions below to findthe Python run-time libraries relative to the interpreter executable. Thedefault value is'python'. The argument should point to azero-terminated wide character string in static storage whose contents will notchange for the duration of the program’s execution. No code in the Pythoninterpreter will change the contents of this storage.

UsePy_DecodeLocale() to decode a bytes string to get awchar_* string.

wchar*Py_GetProgramName()

Return the program name set withPy_SetProgramName(), or the default.The returned string points into static storage; the caller should not modify itsvalue.

wchar_t*Py_GetPrefix()

Return theprefix for installed platform-independent files. This is derivedthrough a number of complicated rules from the program name set withPy_SetProgramName() and some environment variables; for example, if theprogram name is'/usr/local/bin/python', the prefix is'/usr/local'. Thereturned string points into static storage; the caller should not modify itsvalue. This corresponds to theprefix variable in the top-levelMakefile and the--prefix argument to theconfigurescript at build time. The value is available to Python code assys.prefix.It is only useful on Unix. See also the next function.

wchar_t*Py_GetExecPrefix()

Return theexec-prefix for installed platform-dependent files. This isderived through a number of complicated rules from the program name set withPy_SetProgramName() and some environment variables; for example, if theprogram name is'/usr/local/bin/python', the exec-prefix is'/usr/local'. The returned string points into static storage; the callershould not modify its value. This corresponds to theexec_prefixvariable in the top-levelMakefile and the--exec-prefixargument to theconfigure script at build time. The value isavailable to Python code assys.exec_prefix. It is only useful on Unix.

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

Generally speaking, a platform is a combination of hardware and softwarefamilies, e.g. Sparc machines running the Solaris 2.x operating system areconsidered the same platform, but Intel machines running Solaris 2.x are anotherplatform, and Intel machines running Linux are yet another platform. Differentmajor revisions of the same operating system generally also form differentplatforms. Non-Unix operating systems are a different story; the installationstrategies on those systems are so different that the prefix and exec-prefix aremeaningless, and set to the empty string. Note that compiled Python bytecodefiles are platform independent (but not independent from the Python version bywhich they were compiled!).

System administrators will know how to configure themount orautomount programs to share/usr/local between platformswhile having/usr/local/plat be a different filesystem for eachplatform.

wchar_t*Py_GetProgramFullPath()

Return the full program name of the Python executable; this is computed as aside-effect of deriving the default module search path from the program name(set byPy_SetProgramName() above). The returned string points intostatic storage; the caller should not modify its value. The value is availableto Python code assys.executable.

wchar_t*Py_GetPath()

Return the default module search path; this is computed from the program name(set byPy_SetProgramName() above) and some environment variables.The returned string consists of a series of directory names separated by aplatform dependent delimiter character. The delimiter character is':'on Unix and Mac OS X,';' on Windows. The returned string points intostatic storage; the caller should not modify its value. The listsys.path is initialized with this value on interpreter startup; itcan be (and usually is) modified later to change the search path for loadingmodules.

voidPy_SetPath(const wchar_t *)

Set the default module search path. If this function is called beforePy_Initialize(), thenPy_GetPath() won’t attempt to compute adefault search path but uses the one provided instead. This is useful ifPython is embedded by an application that has full knowledge of the locationof all modules. The path components should be separated by the platformdependent delimiter character, which is':' on Unix and Mac OS X,';'on Windows.

This also causessys.executable to be set only to the raw programname (seePy_SetProgramName()) and forsys.prefix andsys.exec_prefix to be empty. It is up to the caller to modify theseif required after callingPy_Initialize().

UsePy_DecodeLocale() to decode a bytes string to get awchar_* string.

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

const char*Py_GetVersion()

Return the version of this Python interpreter. This is a string that lookssomething 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 three characters are the major and minor version separated by aperiod. The returned string points into static storage; the caller should notmodify its value. The value is available to Python code assys.version.

const char*Py_GetPlatform()

Return the platform identifier for the current platform. On Unix, this isformed from the “official” name of the operating system, converted to lowercase, followed by the major revision number; e.g., for Solaris 2.x, which isalso known as SunOS 5.x, the value is'sunos5'. On Mac OS X, it is'darwin'. On Windows, it is'win'. The returned string points intostatic storage; the caller should not modify its value. The value is availableto Python code assys.platform.

const char*Py_GetCopyright()

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

'Copyright1991-1995StichtingMathematischCentrum,Amsterdam'

The returned string points into static storage; the caller should not modify itsvalue. The value is available to Python code assys.copyright.

const char*Py_GetCompiler()

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

"[GCC 2.7.2.2]"

The returned string points into static storage; the caller should not modify itsvalue. The value is available to Python code as part of the variablesys.version.

const char*Py_GetBuildInfo()

Return information about the sequence number and build date and time of thecurrent Python interpreter instance, for example

"#67, Aug  1 1997, 22:34:28"

The returned string points into static storage; the caller should not modify itsvalue. The value is available to Python code as part of the variablesys.version.

voidPySys_SetArgvEx(int argc, wchar_t **argv, int updatepath)

Setsys.argv based onargc andargv. These parameters aresimilar to those passed to the program’smain() function with thedifference that the first entry should refer to the script file to beexecuted rather than the executable hosting the Python interpreter. If thereisn’t a script that will be run, the first entry inargv can be an emptystring. If this function fails to initializesys.argv, a fatalcondition is signalled usingPy_FatalError().

Ifupdatepath is zero, this is all the function does. Ifupdatepathis non-zero, the function also modifiessys.path according to thefollowing algorithm:

  • If the name of an existing script is passed inargv[0], the absolutepath of the directory where the script is located is prepended tosys.path.

  • Otherwise (that is, ifargc is0 orargv[0] doesn’t pointto an existing file name), an empty string is prepended tosys.path, which is the same as prepending the current workingdirectory (".").

UsePy_DecodeLocale() to decode a bytes string to get awchar_* string.

Note

It is recommended that applications embedding the Python interpreterfor purposes other than executing a single script pass0 asupdatepath,and updatesys.path themselves if desired.SeeCVE-2008-5983.

On versions before 3.1.3, you can achieve the same effect by manuallypopping the firstsys.path element after having calledPySys_SetArgv(), for example using:

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

New in version 3.1.3.

voidPySys_SetArgv(int argc, wchar_t **argv)

This function works likePySys_SetArgvEx() withupdatepath setto1 unless thepython interpreter was started with the-I.

UsePy_DecodeLocale() to decode a bytes string to get awchar_* string.

Changed in version 3.4:Theupdatepath value depends on-I.

voidPy_SetPythonHome(wchar_t *home)

Set the default “home” directory, that is, the location of the standardPython libraries. SeePYTHONHOME for the meaning of theargument string.

The argument should point to a zero-terminated character string in staticstorage whose contents will not change for the duration of the program’sexecution. No code in the Python interpreter will change the contents ofthis storage.

UsePy_DecodeLocale() to decode a bytes string to get awchar_* string.

w_char*Py_GetPythonHome()

Return the default “home”, that is, the value set by a previous call toPy_SetPythonHome(), or the value of thePYTHONHOMEenvironment variable if it is set.

Thread State and the Global Interpreter Lock

The Python interpreter is not fully thread-safe. In order to supportmulti-threaded Python programs, there’s a global lock, called theglobalinterpreter lock orGIL, that must be held by the current thread beforeit can safely access Python objects. Without the lock, even the simplestoperations could cause problems in a multi-threaded program: for example, whentwo threads simultaneously increment the reference count of the same object, thereference count could end up being incremented only once instead of twice.

Therefore, the rule exists that only the thread that has acquired theGIL may operate on Python objects or call Python/C API functions.In order to emulate concurrency of execution, the interpreter regularlytries to switch threads (seesys.setswitchinterval()). The lock is alsoreleased around potentially blocking I/O operations like reading or writinga file, so that other Python threads can run in the meantime.

The Python interpreter keeps some thread-specific bookkeeping informationinside a data structure calledPyThreadState. There’s also oneglobal variable pointing to the currentPyThreadState: it canbe retrieved usingPyThreadState_Get().

Releasing the GIL from extension code

Most extension code manipulating theGIL has the following simplestructure:

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_THREADS

ThePy_BEGIN_ALLOW_THREADS macro opens a new block and declares ahidden local variable; thePy_END_ALLOW_THREADS macro closes theblock. These two macros are still available when Python is compiled withoutthread support (they simply have an empty expansion).

When thread support is enabled, 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 thecurrent 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 threadstate in the global variable). Conversely, when acquiring the lock and restoringthe thread state, the lock must be acquired before storing the thread statepointer.

Note

Calling system I/O functions is the most common use case for releasingthe GIL, but it can also be useful before calling long-running computationswhich don’t need access to Python objects, such as compression orcryptographic functions operating over memory buffers. For example, thestandardzlib andhashlib modules release the GIL whencompressing or hashing data.

Non-Python created threads

When threads are created using the dedicated Python APIs (such as thethreading module), a thread state is automatically associated to themand the code showed above is therefore correct. However, when threads arecreated from C (for example by a third-party library with its own threadmanagement), they don’t hold the GIL, nor is there a thread state structurefor them.

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

ThePyGILState_Ensure() andPyGILState_Release() functions doall of the above automatically. The typical idiom for calling into Pythonfrom 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 thePyGILState_*() functions assume there is only one globalinterpreter (created automatically byPy_Initialize()). Pythonsupports the creation of additional interpreters (usingPy_NewInterpreter()), but mixing multiple interpreters and thePyGILState_*() API is unsupported.

Another important thing to note about threads is their behaviour in the faceof the Cfork() call. On most systems withfork(), after aprocess forks only the thread that issued the fork will exist. That alsomeans any locks held by other threads will never be released. Python solvesthis foros.fork() by acquiring the locks it uses internally beforethe fork, and releasing them afterwards. In addition, it resets anyLock Objects in the child. When extending or embedding Python, thereis no way to inform Python of additional (non-Python) locks that need to beacquired before or reset after a fork. OS facilities such aspthread_atfork() would need to be used to accomplish the same thing.Additionally, when extending or embedding Python, callingfork()directly rather than throughos.fork() (and returning to or callinginto Python) may result in a deadlock by one of Python’s internal locksbeing held by a thread that is defunct after the fork.PyOS_AfterFork() tries to reset the necessary locks, but is notalways able to.

High-level API

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

PyInterpreterState

This data structure represents the state shared by a number of cooperatingthreads. Threads belonging to the same interpreter share their moduleadministration and a few other internal items. There are no public members inthis structure.

Threads belonging to different interpreters initially share nothing, exceptprocess state like available memory, open file descriptors and such. The globalinterpreter lock is also shared by all threads, regardless of to whichinterpreter they belong.

PyThreadState

This data structure represents the state of a single thread. The only publicdata member isPyInterpreterState*interp, which points tothis thread’s interpreter state.

voidPyEval_InitThreads()

Initialize and acquire the global interpreter lock. It should be called in themain thread before creating a second thread or engaging in any other threadoperations such asPyEval_ReleaseThread(tstate). It is not needed beforecallingPyEval_SaveThread() orPyEval_RestoreThread().

This is a no-op when called for a second time.

Changed in version 3.2:This function cannot be called beforePy_Initialize() anymore.

Note

When only the main thread exists, no GIL operations are needed. This is acommon situation (most Python programs do not use threads), and the lockoperations slow the interpreter down a bit. Therefore, the lock is notcreated initially. This situation is equivalent to having acquired the lock:when there is only a single thread, all object accesses are safe. Therefore,when this function initializes the global interpreter lock, it also acquiresit. Before the Python_thread module creates a new thread, knowingthat either it has the lock or the lock hasn’t been created yet, it callsPyEval_InitThreads(). When this call returns, it is guaranteed thatthe lock has been created and that the calling thread has acquired it.

It isnot safe to call this function when it is unknown which thread (ifany) currently has the global interpreter lock.

This function is not available when thread support is disabled at compile time.

intPyEval_ThreadsInitialized()

Returns a non-zero value ifPyEval_InitThreads() has been called. Thisfunction can be called without holding the GIL, and therefore can be used toavoid calls to the locking API when running single-threaded. This function isnot available when thread support is disabled at compile time.

PyThreadState*PyEval_SaveThread()

Release the global interpreter lock (if it has been created and threadsupport is enabled) and reset the thread state toNULL, returning theprevious thread state (which is notNULL). If the lock has been created,the current thread must have acquired it. (This function is available evenwhen thread support is disabled at compile time.)

voidPyEval_RestoreThread(PyThreadState *tstate)

Acquire the global interpreter lock (if it has been created and threadsupport is enabled) and set the thread state totstate, which must not beNULL. If the lock has been created, the current thread must not haveacquired it, otherwise deadlock ensues. (This function is available evenwhen thread support is disabled at compile time.)

PyThreadState*PyThreadState_Get()

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

PyThreadState*PyThreadState_Swap(PyThreadState *tstate)

Swap the current thread state with the thread state given by the argumenttstate, which may beNULL. The global interpreter lock must be heldand is not released.

voidPyEval_ReInitThreads()

This function is called fromPyOS_AfterFork() to ensure that newlycreated child processes don’t hold locks referring to threads whichare not running in the child process.

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

PyGILState_STATEPyGILState_Ensure()

Ensure that the current thread is ready to call the Python C API regardlessof the current state of Python, or of the global interpreter lock. This maybe called as many times as desired by a thread as long as each call ismatched with a call toPyGILState_Release(). In general, otherthread-related APIs may be used betweenPyGILState_Ensure() andPyGILState_Release() calls as long as the thread state is restored toits previous state before the Release(). For example, normal usage of thePy_BEGIN_ALLOW_THREADS andPy_END_ALLOW_THREADS macros isacceptable.

The return value is an opaque “handle” to the thread state whenPyGILState_Ensure() was called, and must be passed toPyGILState_Release() to ensure Python is left in the same state. Eventhough recursive calls are allowed, these handlescannot be shared - eachunique call toPyGILState_Ensure() must save the handle for its calltoPyGILState_Release().

When the function returns, the current thread will hold the GIL and be ableto call arbitrary Python code. Failure is a fatal error.

voidPyGILState_Release(PyGILState_STATE)

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

Every call toPyGILState_Ensure() must be matched by a call toPyGILState_Release() on the same thread.

PyThreadState*PyGILState_GetThisThreadState()

Get the current thread state for this thread. May returnNULL if noGILState API has been used on the current thread. Note that the main threadalways has such a thread-state, even if no auto-thread-state call has beenmade on the main thread. This is mainly a helper/diagnostic function.

intPyGILState_Check()

Return1 if the current thread is holding the GIL and0 otherwise.This function can be called from any thread at any time.Only if it has had its Python thread state initialized and currently isholding the GIL will it return1.This is mainly a helper/diagnostic function. It can be usefulfor example in callback contexts or memory allocation functions whenknowing that the GIL is locked can allow the caller to perform sensitiveactions or otherwise behave differently.

New in version 3.4.

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

Py_BEGIN_ALLOW_THREADS

This macro expands to{PyThreadState*_save;_save=PyEval_SaveThread();.Note that it contains an opening brace; it must be matched with a followingPy_END_ALLOW_THREADS macro. See above for further discussion of thismacro. It is a no-op when thread support is disabled at compile time.

Py_END_ALLOW_THREADS

This macro expands toPyEval_RestoreThread(_save);}. Note that it containsa closing brace; it must be matched with an earlierPy_BEGIN_ALLOW_THREADS macro. See above for further discussion ofthis macro. It is a no-op when thread support is disabled at compile time.

Py_BLOCK_THREADS

This macro expands toPyEval_RestoreThread(_save);: it is equivalent toPy_END_ALLOW_THREADS without the closing brace. It is a no-op whenthread support is disabled at compile time.

Py_UNBLOCK_THREADS

This macro expands to_save=PyEval_SaveThread();: it is equivalent toPy_BEGIN_ALLOW_THREADS without the opening brace and variabledeclaration. It is a no-op when thread support is disabled at compile time.

Low-level API

All of the following functions are only available when thread support is enabledat compile time, and must be called only when the global interpreter lock hasbeen created.

PyInterpreterState*PyInterpreterState_New()

Create a new interpreter state object. The global interpreter lock need notbe held, but may be held if it is necessary to serialize calls to thisfunction.

voidPyInterpreterState_Clear(PyInterpreterState *interp)

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

voidPyInterpreterState_Delete(PyInterpreterState *interp)

Destroy an interpreter state object. The global interpreter lock need not beheld. The interpreter state must have been reset with a previous call toPyInterpreterState_Clear().

PyThreadState*PyThreadState_New(PyInterpreterState *interp)

Create a new thread state object belonging to the given interpreter object.The global interpreter lock need not be held, but may be held if it isnecessary to serialize calls to this function.

voidPyThreadState_Clear(PyThreadState *tstate)

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

voidPyThreadState_Delete(PyThreadState *tstate)

Destroy a thread state object. The global interpreter lock need not be held.The thread state must have been reset with a previous call toPyThreadState_Clear().

PyObject*PyThreadState_GetDict()
Return value: Borrowed reference.

Return a dictionary in which extensions can store thread-specific stateinformation. Each extension should use a unique key to use to store state inthe dictionary. It is okay to call this function when no current thread stateis available. If this function returnsNULL, no exception has been raised andthe caller should assume no current thread state is available.

intPyThreadState_SetAsyncExc(long id,PyObject *exc)

Asynchronously raise an exception in a thread. Theid argument is the threadid of the target thread;exc is the exception object to be raised. Thisfunction does not steal any references toexc. To prevent naive misuse, youmust 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 bezero if the thread id isn’t found. Ifexc isNULL, the pendingexception (if any) for the thread is cleared. This raises no exceptions.

voidPyEval_AcquireThread(PyThreadState *tstate)

Acquire the global interpreter lock and set the current thread state totstate, which should not beNULL. The lock must have been created earlier.If this thread already has the lock, deadlock ensues.

PyEval_RestoreThread() is a higher-level function which is alwaysavailable (even when thread support isn’t enabled or when threads havenot been initialized).

voidPyEval_ReleaseThread(PyThreadState *tstate)

Reset the current thread state toNULL and release the global interpreterlock. The lock must have been created earlier and must be held by the currentthread. Thetstate argument, which must not beNULL, is only used to checkthat it represents the current thread state — if it isn’t, a fatal error isreported.

PyEval_SaveThread() is a higher-level function which is alwaysavailable (even when thread support isn’t enabled or when threads havenot been initialized).

voidPyEval_AcquireLock()

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 usePyEval_RestoreThread() orPyEval_AcquireThread()instead.

voidPyEval_ReleaseLock()

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 usePyEval_SaveThread() orPyEval_ReleaseThread()instead.

Sub-interpreter support

While in most uses, you will only embed a single Python interpreter, thereare cases where you need to create several independent interpreters in thesame process and perhaps even in the same thread. Sub-interpreters allowyou to do that. You can switch between sub-interpreters using thePyThreadState_Swap() function. You can create and destroy themusing the following functions:

PyThreadState*Py_NewInterpreter()

Create a new sub-interpreter. This is an (almost) totally separate environmentfor the execution of Python code. In particular, the new interpreter hasseparate, independent versions of all imported modules, including thefundamental modulesbuiltins,__main__ andsys. Thetable of loaded modules (sys.modules) and the module search path(sys.path) are also separate. The new environment has nosys.argvvariable. It has new standard I/O stream file objectssys.stdin,sys.stdout andsys.stderr (however these refer to the same underlyingfile descriptors).

The return value points to the first thread state created in the newsub-interpreter. This thread state is made in the current thread state.Note that no actual thread is created; see the discussion of thread statesbelow. If creation of the new interpreter is unsuccessful,NULL isreturned; no exception is set since the exception state is stored in thecurrent thread state and there may not be a current thread state. (Like allother Python/C API functions, the global interpreter lock must be held beforecalling this function and is still held when it returns; however, unlike mostother Python/C API functions, there needn’t be a current thread state onentry.)

Extension modules are shared between (sub-)interpreters as follows: the firsttime a particular extension is imported, it is initialized normally, and a(shallow) copy of its module’s dictionary is squirreled away. When the sameextension is imported by another (sub-)interpreter, a new module is initializedand filled with the contents of this copy; the extension’sinit function isnot called. Note that this is different from what happens when an extension isimported after the interpreter has been completely re-initialized by callingPy_FinalizeEx() andPy_Initialize(); in that case, the extension’sinitmodule functionis called again.

voidPy_EndInterpreter(PyThreadState *tstate)

Destroy the (sub-)interpreter represented by the given thread state. The giventhread state must be the current thread state. See the discussion of threadstates below. When the call returns, the current thread state isNULL. Allthread states associated with this interpreter are destroyed. (The globalinterpreter lock must be held before calling this function and is still heldwhen it returns.)Py_FinalizeEx() will destroy all sub-interpreters thathaven’t been explicitly destroyed at that point.

Bugs and caveats

Because sub-interpreters (and the main interpreter) are part of the sameprocess, the insulation between them isn’t perfect — for example, usinglow-level file operations likeos.close() they can(accidentally or maliciously) affect each other’s open files. Because of theway extensions are shared between (sub-)interpreters, some extensions may notwork properly; this is especially likely when the extension makes use of(static) global variables, or when the extension manipulates its module’sdictionary after its initialization. It is possible to insert objects createdin one sub-interpreter into a namespace of another sub-interpreter; this shouldbe done with great care to avoid sharing user-defined functions, methods,instances or classes between sub-interpreters, since import operations executedby such objects may affect the wrong (sub-)interpreter’s dictionary of loadedmodules.

Also note that combining this functionality withPyGILState_*() APIsis delicate, because these APIs assume a bijection between Python thread statesand 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 pairof matchingPyGILState_Ensure() andPyGILState_Release() calls.Furthermore, extensions (such asctypes) using these APIs to allow callingof Python code from non-Python created threads will probably be broken when usingsub-interpreters.

Asynchronous Notifications

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

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

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

When successfully queued,func will beeventually called from themain interpreter thread with the argumentarg. It will be calledasynchronously with respect to normally running Python code, but withboth these conditions met:

func must return0 on success, or-1 on failure with an exceptionset.func won’t be interrupted to perform another asynchronousnotification recursively, but it can still be interrupted to switchthreads if the global interpreter lock is released.

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

Warning

This is a low-level function, only useful for very special cases.There is no guarantee thatfunc will be called as quick aspossible. If the main thread is busy executing a system call,func won’t be called before the system call returns. Thisfunction is generallynot suitable for calling Python code fromarbitrary C threads. Instead, use thePyGILState API.

New in version 3.1.

Profiling and Tracing

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

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

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

The type of the trace function registered usingPyEval_SetProfile() andPyEval_SetTrace(). The first parameter is the object passed to theregistration function asobj,frame is the frame object to which the eventpertains,what is one of the constantsPyTrace_CALL,PyTrace_EXCEPTION,PyTrace_LINE,PyTrace_RETURN,PyTrace_C_CALL,PyTrace_C_EXCEPTION, orPyTrace_C_RETURN, andarg depends on the value ofwhat:

Value ofwhat

Meaning ofarg

PyTrace_CALL

AlwaysPy_None.

PyTrace_EXCEPTION

Exception information as returned bysys.exc_info().

PyTrace_LINE

AlwaysPy_None.

PyTrace_RETURN

Value being returned to the caller,orNULL 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.

intPyTrace_CALL

The value of thewhat parameter to aPy_tracefunc function when a newcall 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 reportedas there is no control transfer to the Python bytecode in the correspondingframe.

intPyTrace_EXCEPTION

The value of thewhat parameter to aPy_tracefunc function when anexception has been raised. The callback function is called with this value forwhat when after any bytecode is processed after which the exception becomesset within the frame being executed. The effect of this is that as exceptionpropagation causes the Python stack to unwind, the callback is called uponreturn to each frame as the exception propagates. Only trace functions receivesthese events; they are not needed by the profiler.

intPyTrace_LINE

The value passed as thewhat parameter to a trace function (but not aprofiling function) when a line-number event is being reported.

intPyTrace_RETURN

The value for thewhat parameter toPy_tracefunc functions when acall is about to return.

intPyTrace_C_CALL

The value for thewhat parameter toPy_tracefunc functions when a Cfunction is about to be called.

intPyTrace_C_EXCEPTION

The value for thewhat parameter toPy_tracefunc functions when a Cfunction has raised an exception.

intPyTrace_C_RETURN

The value for thewhat parameter toPy_tracefunc functions when a Cfunction has returned.

voidPyEval_SetProfile(Py_tracefunc func,PyObject *obj)

Set the profiler function tofunc. Theobj parameter is passed to thefunction as its first parameter, and may be any Python object, orNULL. Ifthe profile function needs to maintain state, using a different value forobjfor each thread provides a convenient and thread-safe place to store it. Theprofile function is called for all monitored events exceptPyTrace_LINEandPyTrace_EXCEPTION.

voidPyEval_SetTrace(Py_tracefunc func,PyObject *obj)

Set the tracing function tofunc. This is similar toPyEval_SetProfile(), except the tracing function does receive line-numberevents and does not receive any event related to C function objects being called. Anytrace function registered usingPyEval_SetTrace() will not receivePyTrace_C_CALL,PyTrace_C_EXCEPTION orPyTrace_C_RETURNas a value for thewhat parameter.

PyObject*PyEval_GetCallStats(PyObject *self)

Return a tuple of function call counts. There are constants defined for thepositions within the tuple:

Name

Value

PCALL_ALL

0

PCALL_FUNCTION

1

PCALL_FAST_FUNCTION

2

PCALL_FASTER_FUNCTION

3

PCALL_METHOD

4

PCALL_BOUND_METHOD

5

PCALL_CFUNCTION

6

PCALL_TYPE

7

PCALL_GENERATOR

8

PCALL_OTHER

9

PCALL_POP

10

PCALL_FAST_FUNCTION means no argument tuple needs to be created.PCALL_FASTER_FUNCTION means that the fast-path frame setup code is used.

If there is a method call where the call can be optimized by changingthe argument tuple and calling the function directly, it gets recordedtwice.

This function is only present if Python is compiled withCALL_PROFILEdefined.

Advanced Debugger Support

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

PyInterpreterState*PyInterpreterState_Head()

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

PyInterpreterState*PyInterpreterState_Next(PyInterpreterState *interp)

Return the next interpreter state object afterinterp from the list of allsuch objects.

PyThreadState *PyInterpreterState_ThreadHead(PyInterpreterState *interp)

Return the pointer to the firstPyThreadState object in the list ofthreads associated with the interpreterinterp.

PyThreadState*PyThreadState_Next(PyThreadState *tstate)

Return the next thread state object aftertstate from the list of all suchobjects belonging to the samePyInterpreterState object.