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Python 3.6.5 Documentation > Initialization, Finalization, and Threads

Initialization, Finalization, and Threads
*****************************************

Initializing and finalizing the interpreter
===========================================

void Py_Initialize()

Initialize the Python interpreter. In an application embedding
Python, this should be called before using any other Python/C API
functions; with the exception of "Py_SetProgramName()",
"Py_SetPythonHome()" and "Py_SetPath()". This initializes the
table of loaded modules ("sys.modules"), and creates the
fundamental modules "builtins", "__main__" and "sys". It also
initializes the module search path ("sys.path"). It does not set
"sys.argv"; use "PySys_SetArgvEx()" for that. This is a no-op when
called for a second time (without calling "Py_FinalizeEx()" first).
There is no return value; it is a fatal error if the initialization
fails.

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

void Py_InitializeEx(int initsigs)

This function works like "Py_Initialize()" if *initsigs* is "1". If
*initsigs* is "0", it skips initialization registration of signal
handlers, which might be useful when Python is embedded.

int Py_IsInitialized()

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

int Py_FinalizeEx()

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). 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.

New in version 3.6.

void Py_Finalize()

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

Process-wide parameters
=======================

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

This function should be called before "Py_Initialize()", if it is
called at all. It specifies which encoding and error handling to
use with standard IO, with the same meanings as in "str.encode()".

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

"encoding" and/or "errors" may be NULL to use "PYTHONIOENCODING"
and/or default values (depending on other settings).

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

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

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

New in version 3.4.

void Py_SetProgramName(wchar_t *name)

This function should be called before "Py_Initialize()" is called
for the first time, if it is called at all. It tells the
interpreter the value of the "argv[0]" argument to the "main()"
function of the program (converted to wide characters). This is
used by "Py_GetPath()" and some other functions below to find the
Python run-time libraries relative to the interpreter executable.
The default value is "'python'". The argument should point to a
zero-terminated wide character string in static storage whose
contents will not change for the duration of the program’s
execution. No code in the Python interpreter will change the
contents of this storage.

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

wchar* Py_GetProgramName()

Return the program name set with "Py_SetProgramName()", or the
default. The returned string points into static storage; the caller
should not modify its value.

wchar_t* Py_GetPrefix()

Return the *prefix* for installed platform-independent files. This
is derived through a number of complicated rules from the program
name set with "Py_SetProgramName()" and some environment variables;
for example, if the program name is "'/usr/local/bin/python'", the
prefix is "'/usr/local'". The returned string points into static
storage; the caller should not modify its value. This corresponds
to the **prefix** variable in the top-level "Makefile" and the "--
prefix" argument to the **configure** script at build time. The
value is available to Python code as "sys.prefix". It is only
useful on Unix. See also the next function.

wchar_t* Py_GetExecPrefix()

Return the *exec-prefix* for installed platform-*dependent* files.
This is derived through a number of complicated rules from the
program name set with "Py_SetProgramName()" and some environment
variables; for example, if the program name is
"'/usr/local/bin/python'", the exec-prefix is "'/usr/local'". The
returned string points into static storage; the caller should not
modify its value. This corresponds to the **exec_prefix** variable
in the top-level "Makefile" and the "--exec-prefix" argument to the
**configure** script at build time. The value is available to
Python code as "sys.exec_prefix". It is only useful on Unix.

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

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

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

wchar_t* Py_GetProgramFullPath()

Return the full program name of the Python executable; this is
computed as a side-effect of deriving the default module search
path from the program name (set by "Py_SetProgramName()" above).
The returned string points into static storage; the caller should
not modify its value. The value is available to Python code as
"sys.executable".

wchar_t* Py_GetPath()

Return the default module search path; this is computed from the
program name (set by "Py_SetProgramName()" above) and some
environment variables. The returned string consists of a series of
directory names separated by a platform dependent delimiter
character. The delimiter character is "':'" on Unix and Mac OS X,
"';'" 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.

void Py_SetPath(const wchar_t *)

Set the default module search path. If this function is called
before "Py_Initialize()", then "Py_GetPath()" won’t attempt to
compute a default search path but uses the one provided instead.
This is useful if Python is embedded by an application that has
full knowledge of the location of all modules. The path components
should be separated by the platform dependent delimiter character,
which is "':'" on Unix and Mac OS X, "';'" on Windows.

This also causes "sys.executable" to be set only to the raw program
name (see "Py_SetProgramName()") and for "sys.prefix" and
"sys.exec_prefix" to be empty. It is up to the caller to modify
these if required after calling "Py_Initialize()".

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

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

const char* Py_GetVersion()

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 three 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".

const char* Py_GetPlatform()

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

const char* Py_GetCopyright()

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

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

const char* Py_GetCompiler()

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

"[GCC 2.7.2.2]"

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

const char* Py_GetBuildInfo()

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

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

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

void PySys_SetArgvEx(int argc, wchar_t **argv, int updatepath)

Set "sys.argv" based on *argc* and *argv*. These parameters are
similar to those passed to the program’s "main()" function with the
difference that the first entry should refer to the script file to
be executed rather than the executable hosting the Python
interpreter. If there isn’t a script that will be run, the first
entry in *argv* can be an empty string. If this function fails to
initialize "sys.argv", a fatal condition is signalled using
"Py_FatalError()".

If *updatepath* is zero, this is all the function does. If
*updatepath* is non-zero, the function also modifies "sys.path"
according to the following algorithm:

* If the name of an existing script is passed in "argv[0]", the
absolute path of the directory where the script is located is
prepended to "sys.path".

* Otherwise (that is, if *argc* is "0" or "argv[0]" doesn’t point
to an existing file name), an empty string is prepended to
"sys.path", which is the same as prepending the current working
directory (""."").

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

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.

void PySys_SetArgv(int argc, wchar_t **argv)

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

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

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

void Py_SetPythonHome(wchar_t *home)

Set the default “home” directory, that is, the location of the
standard Python libraries. See "PYTHONHOME" for the meaning of the
argument string.

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

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

w_char* Py_GetPythonHome()

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.

Thread State and the Global Interpreter Lock
============================================

The Python interpreter is not fully thread-safe. In order to support
multi-threaded Python programs, there’s a global lock, called the
*global interpreter lock* or *GIL*, that must be held by the current
thread before it can safely access Python objects. Without the lock,
even the simplest operations could cause problems in a multi-threaded
program: for example, when two threads simultaneously increment the
reference count of the same object, the reference count could end up
being incremented only once instead of twice.

Therefore, the rule exists that only the thread that has acquired the
*GIL* may operate on Python objects or call Python/C API functions. In
order to emulate concurrency of execution, the interpreter regularly
tries to switch threads (see "sys.setswitchinterval()"). The lock is
also released around potentially blocking I/O operations like reading
or writing a file, so that other Python threads can run in the
meantime.

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

Releasing the GIL from extension code
-------------------------------------

Most extension code manipulating the *GIL* has the following simple
structure:

Save the thread state in a local variable.
Release the global interpreter lock.
... Do some blocking I/O operation ...
Reacquire the global interpreter lock.
Restore the thread state from the local variable.

This is so common that a pair of macros exists to simplify it:

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

The "Py_BEGIN_ALLOW_THREADS" macro opens a new block and declares a
hidden local variable; the "Py_END_ALLOW_THREADS" macro closes the
block. These two macros are still available when Python is compiled
without thread 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();
...Do some blocking I/O operation...
PyEval_RestoreThread(_save);

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

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

Non-Python created threads
--------------------------

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

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

The "PyGILState_Ensure()" and "PyGILState_Release()" functions do all
of the above automatically. The typical idiom for calling into Python
from a C thread is:

PyGILState_STATE gstate;
gstate = PyGILState_Ensure();

/* Perform Python actions here. */
result = CallSomeFunction();
/* evaluate result or handle exception */

/* Release the thread. No Python API allowed beyond this point. */
PyGILState_Release(gstate);

Note that the "PyGILState_*()" functions assume there is only one
global interpreter (created automatically by "Py_Initialize()").
Python supports the creation of additional interpreters (using
"Py_NewInterpreter()"), but mixing multiple interpreters and the
"PyGILState_*()" API is unsupported.

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. That
also 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()" tries to reset the necessary locks,
but is not always able to.

High-level API
--------------

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

PyInterpreterState

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.

PyThreadState

This data structure represents the state of a single thread. The
only public data member is "PyInterpreterState *""interp", which
points to this thread’s interpreter state.

void PyEval_InitThreads()

Initialize and acquire the global interpreter lock. It should be
called in the main thread before creating a second thread or
engaging in any other thread operations such as
"PyEval_ReleaseThread(tstate)". It is not needed before calling
"PyEval_SaveThread()" or "PyEval_RestoreThread()".

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

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

Note: When only the main thread exists, no GIL operations are
needed. This is a common situation (most Python programs do not
use threads), and the lock operations slow the interpreter down a
bit. Therefore, the lock is not created 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 acquires it. Before the Python "_thread" module creates a
new thread, knowing that either it has the lock or the lock
hasn’t been created yet, it calls "PyEval_InitThreads()". When
this call returns, it is guaranteed that the lock has been
created and that the calling thread has acquired it.It is **not**
safe to call this function when it is unknown which thread (if
any) currently has the global interpreter lock.This function is
not available when thread support is disabled at compile time.

int PyEval_ThreadsInitialized()

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

PyThreadState* PyEval_SaveThread()

Release the global interpreter lock (if it has been created and
thread support is enabled) 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.
(This function is available even when thread support is disabled at
compile time.)

void PyEval_RestoreThread(PyThreadState *tstate)

Acquire the global interpreter lock (if it has been created and
thread support is enabled) 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. (This function is available even when 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 is *NULL*, this issues a
fatal error (so that the caller needn’t check for *NULL*).

PyThreadState* PyThreadState_Swap(PyThreadState *tstate)

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.

void PyEval_ReInitThreads()

This function is called from "PyOS_AfterFork()" to ensure that
newly created child processes don’t hold locks referring to threads
which are not running in the child process.

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

PyGILState_STATE PyGILState_Ensure()

Ensure that the current thread is ready to call the Python C API
regardless of the current state of Python, or of the global
interpreter lock. This may be called as many times as desired by a
thread as long as each call is matched with a call to
"PyGILState_Release()". In general, other thread-related APIs may
be used between "PyGILState_Ensure()" and "PyGILState_Release()"
calls as long as the thread state is restored to its previous state
before the Release(). For example, normal usage of the
"Py_BEGIN_ALLOW_THREADS" and "Py_END_ALLOW_THREADS" macros is
acceptable.

The return value is an opaque “handle” to the thread state when
"PyGILState_Ensure()" was called, and must be passed to
"PyGILState_Release()" to ensure Python is left in the same state.
Even though recursive calls are allowed, these handles *cannot* be
shared - each unique call to "PyGILState_Ensure()" must save the
handle for its call to "PyGILState_Release()".

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

void PyGILState_Release(PyGILState_STATE)

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

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

PyThreadState* PyGILState_GetThisThreadState()

Get the current thread state for this thread. May return "NULL" if
no GILState API has been used on the current thread. Note that the
main thread always has such a thread-state, even if no auto-thread-
state call has been made on the main thread. This is mainly a
helper/diagnostic function.

int PyGILState_Check()

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

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

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. It is a no-op when
thread support is disabled at compile time.

Py_END_ALLOW_THREADS

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. It is a no-op when thread support is disabled at
compile time.

Py_BLOCK_THREADS

This macro expands to "PyEval_RestoreThread(_save);": it is
equivalent to "Py_END_ALLOW_THREADS" without the closing brace. It
is a no-op when thread support is disabled at compile time.

Py_UNBLOCK_THREADS

This macro expands to "_save = PyEval_SaveThread();": it is
equivalent to "Py_BEGIN_ALLOW_THREADS" without the opening brace
and variable declaration. 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 enabled at compile time, and must be called only when the global
interpreter lock has been created.

PyInterpreterState* PyInterpreterState_New()

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.

void PyInterpreterState_Clear(PyInterpreterState *interp)

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

void PyInterpreterState_Delete(PyInterpreterState *interp)

Destroy an interpreter state object. The global interpreter lock
need not be held. The interpreter state must have been reset with
a previous call to "PyInterpreterState_Clear()".

PyThreadState* PyThreadState_New(PyInterpreterState *interp)

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

void PyThreadState_Clear(PyThreadState *tstate)

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

void PyThreadState_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 to "PyThreadState_Clear()".

PyObject* PyThreadState_GetDict()
*Return value: Borrowed reference.*

Return a dictionary in which extensions can store thread-specific
state information. Each extension should use a unique key to use
to store state in the dictionary. It is okay to call this function
when no current thread state is available. If this function returns
*NULL*, no exception has been raised and the caller should assume
no current thread state is available.

int PyThreadState_SetAsyncExc(long id, PyObject *exc)

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.

void PyEval_AcquireThread(PyThreadState *tstate)

Acquire the global interpreter lock and set the current thread
state to *tstate*, which should not be *NULL*. 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 always
available (even when thread support isn’t enabled or when threads
have not been initialized).

void PyEval_ReleaseThread(PyThreadState *tstate)

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 thread support isn’t enabled or when threads
have not been initialized).

void PyEval_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 use "PyEval_RestoreThread()" or
"PyEval_AcquireThread()" instead.

void PyEval_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 use "PyEval_SaveThread()" or
"PyEval_ReleaseThread()" instead.

Sub-interpreter support
=======================

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

PyThreadState* Py_NewInterpreter()

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:
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. Note that this is different from what
happens when an extension is imported after the interpreter has
been completely re-initialized by calling "Py_FinalizeEx()" and
"Py_Initialize()"; in that case, the extension’s "initmodule"
function *is* called again.

void Py_EndInterpreter(PyThreadState *tstate)

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.) "Py_FinalizeEx()" 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 the
extension makes use of (static) global variables, or when the
extension manipulates its module’s dictionary after its
initialization. It is possible to insert objects created in one sub-
interpreter into a namespace of another sub-interpreter; this should
be done with great care 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.

Also note that combining this functionality with "PyGILState_*()" APIs
is delicate, because these APIs assume a bijection between Python
thread states and OS-level threads, an assumption broken by the
presence of sub-interpreters. It is highly recommended that you don’t
switch sub-interpreters between a pair of matching
"PyGILState_Ensure()" and "PyGILState_Release()" calls. Furthermore,
extensions (such as "ctypes") using these APIs to allow calling of
Python code from non-Python created threads will probably be broken
when using sub-interpreters.

Asynchronous Notifications
==========================

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

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

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.

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.

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.

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

The type of the trace function registered using
"PyEval_SetProfile()" and "PyEval_SetTrace()". The first parameter
is the object passed to the registration function as *obj*, *frame*
is the frame object to which the event pertains, *what* is one of
the constants "PyTrace_CALL", "PyTrace_EXCEPTION", "PyTrace_LINE",
"PyTrace_RETURN", "PyTrace_C_CALL", "PyTrace_C_EXCEPTION", or
"PyTrace_C_RETURN", and *arg* depends on the value of *what*:

int PyTrace_CALL

The value of the *what* parameter to a "Py_tracefunc" function when
a new call to a function or method is being reported, or a new
entry into a generator. Note that the creation of the iterator for
a generator function is not reported as there is no control
transfer to the Python bytecode in the corresponding frame.

int PyTrace_EXCEPTION

The value of the *what* parameter to a "Py_tracefunc" function when
an exception has been raised. The callback function is called with
this value for *what* when after any bytecode is processed after
which the exception becomes set within the frame being executed.
The effect of this is that as exception propagation causes the
Python stack to unwind, the callback is called upon return to each
frame as the exception propagates. Only trace functions receives
these events; they are not needed by the profiler.

int PyTrace_LINE

The value passed as the *what* parameter to a trace function (but
not a profiling function) when a line-number event is being
reported.

int PyTrace_RETURN

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

int PyTrace_C_CALL

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

int PyTrace_C_EXCEPTION

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

int PyTrace_C_RETURN

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

void PyEval_SetProfile(Py_tracefunc func, PyObject *obj)

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

void PyEval_SetTrace(Py_tracefunc func, PyObject *obj)

Set the tracing function to *func*. This is similar to
"PyEval_SetProfile()", except the tracing function does receive
line-number events and 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.

PyObject* PyEval_GetCallStats(PyObject *self)

Return a tuple of function call counts. There are constants
defined for the positions 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
changing the argument tuple and calling the function directly, it
gets recorded twice.

This function is only present if Python is compiled with
"CALL_PROFILE" defined.

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 after *interp* from the
list of all such objects.

PyThreadState * PyInterpreterState_ThreadHead(PyInterpreterState *interp)

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

PyThreadState* PyThreadState_Next(PyThreadState *tstate)

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

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