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


Before Python Initialization
============================

In an application embedding  Python, the "Py_Initialize()" function
must be called before using any other Python/C API functions; with the
exception of a few functions and the global configuration variables.

The following functions can be safely called before Python is
initialized:

* Configuration functions:

  * "PyImport_AppendInittab()"

  * "PyImport_ExtendInittab()"

  * "PyInitFrozenExtensions()"

  * "PyMem_SetAllocator()"

  * "PyMem_SetupDebugHooks()"

  * "PyObject_SetArenaAllocator()"

  * "Py_SetPath()"

  * "Py_SetProgramName()"

  * "Py_SetPythonHome()"

  * "Py_SetStandardStreamEncoding()"

  * "PySys_AddWarnOption()"

  * "PySys_AddXOption()"

  * "PySys_ResetWarnOptions()"

* Informative functions:

  * "Py_IsInitialized()"

  * "PyMem_GetAllocator()"

  * "PyObject_GetArenaAllocator()"

  * "Py_GetBuildInfo()"

  * "Py_GetCompiler()"

  * "Py_GetCopyright()"

  * "Py_GetPlatform()"

  * "Py_GetVersion()"

* Utilities:

  * "Py_DecodeLocale()"

* Memory allocators:

  * "PyMem_RawMalloc()"

  * "PyMem_RawRealloc()"

  * "PyMem_RawCalloc()"

  * "PyMem_RawFree()"

Note: The following functions **should not be called** before
  "Py_Initialize()": "Py_EncodeLocale()", "Py_GetPath()",
  "Py_GetPrefix()", "Py_GetExecPrefix()", "Py_GetProgramFullPath()",
  "Py_GetPythonHome()", "Py_GetProgramName()" and
  "PyEval_InitThreads()".


Global configuration variables
==============================

Python has variables for the global configuration to control different
features and options. By default, these flags are controlled by
command line options.

When a flag is set by an option, the value of the flag is the number
of times that the option was set. For example, "-b" sets
"Py_BytesWarningFlag" to 1 and "-bb" sets "Py_BytesWarningFlag" to 2.

Py_BytesWarningFlag

   Issue a warning when comparing "bytes" or "bytearray" with "str" or
   "bytes" with "int".  Issue an error if greater or equal to "2".

   Set by the "-b" option.

Py_DebugFlag

   Turn on parser debugging output (for expert only, depending on
   compilation options).

   Set by the "-d" option and the "PYTHONDEBUG" environment variable.

Py_DontWriteBytecodeFlag

   If set to non-zero, Python won’t try to write ".pyc" files on the
   import of source modules.

   Set by the "-B" option and the "PYTHONDONTWRITEBYTECODE"
   environment variable.

Py_FrozenFlag

   Suppress error messages when calculating the module search path in
   "Py_GetPath()".

   Private flag used by "_freeze_importlib" and "frozenmain" programs.

Py_HashRandomizationFlag

   Set to "1" if the "PYTHONHASHSEED" environment variable is set to a
   non-empty string.

   If the flag is non-zero, read the "PYTHONHASHSEED" environment
   variable to initialize the secret hash seed.

Py_IgnoreEnvironmentFlag

   Ignore all "PYTHON*" environment variables, e.g. "PYTHONPATH" and
   "PYTHONHOME", that might be set.

   Set by the "-E" and "-I" options.

Py_InspectFlag

   When a script is passed as first argument or the "-c" option is
   used, enter interactive mode after executing the script or the
   command, even when "sys.stdin" does not appear to be a terminal.

   Set by the "-i" option and the "PYTHONINSPECT" environment
   variable.

Py_InteractiveFlag

   Set by the "-i" option.

Py_IsolatedFlag

   Run Python in isolated mode. In isolated mode "sys.path" contains
   neither the script’s directory nor the user’s site-packages
   directory.

   Set by the "-I" option.

   New in version 3.4.

Py_LegacyWindowsFSEncodingFlag

   If the flag is non-zero, use the "mbcs" encoding instead of the
   UTF-8 encoding for the filesystem encoding.

   Set to "1" if the "PYTHONLEGACYWINDOWSFSENCODING" environment
   variable is set to a non-empty string.

   See **PEP 529** for more details.

   Availability: Windows.

Py_LegacyWindowsStdioFlag

   If the flag is non-zero, use "io.FileIO" instead of
   "WindowsConsoleIO" for "sys" standard streams.

   Set to "1" if the "PYTHONLEGACYWINDOWSSTDIO" environment variable
   is set to a non-empty string.

   See **PEP 528** for more details.

   Availability: Windows.

Py_NoSiteFlag

   Disable the import of the module "site" and the site-dependent
   manipulations of "sys.path" that it entails.  Also disable these
   manipulations if "site" is explicitly imported later (call
   "site.main()" if you want them to be triggered).

   Set by the "-S" option.

Py_NoUserSiteDirectory

   Don’t add the "user site-packages directory" to "sys.path".

   Set by the "-s" and "-I" options, and the "PYTHONNOUSERSITE"
   environment variable.

Py_OptimizeFlag

   Set by the "-O" option and the "PYTHONOPTIMIZE" environment
   variable.

Py_QuietFlag

   Don’t display the copyright and version messages even in
   interactive mode.

   Set by the "-q" option.

   New in version 3.2.

Py_UnbufferedStdioFlag

   Force the stdout and stderr streams to be unbuffered.

   Set by the "-u" option and the "PYTHONUNBUFFERED" environment
   variable.

Py_VerboseFlag

   Print a message each time a module is initialized, showing the
   place (filename or built-in module) from which it is loaded.  If
   greater or equal to "2", print a message for each file that is
   checked for when searching for a module. Also provides information
   on module cleanup at exit.

   Set by the "-v" option and the "PYTHONVERBOSE" environment
   variable.


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; see Before Python Initialization for the few exceptions.

   This initializes the table of loaded modules ("sys.modules"), and
   creates the fundamental modules "builtins", "__main__" and "sys".
   It also initializes the module search path ("sys.path"). It does
   not set "sys.argv"; use "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(const 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

   "'Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam'"

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

const char* Py_GetCompiler()

   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(const 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.

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_Child()" 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 3.7: This function is now called by
   "Py_Initialize()", so you don’t have to call it yourself anymore.

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

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.

   Changed in version 3.7: The *GIL* is now initialized by
   "Py_Initialize()".

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.

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.

   Note: Calling this function from a thread when the runtime is
     finalizing will terminate the thread, even if the thread was not
     created by Python. You can use "_Py_IsFinalizing()" or
     "sys.is_finalizing()" to check if the interpreter is in process
     of being finalized before calling this function to avoid unwanted
     termination.

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_Child()" 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.

   Note: Calling this function from a thread when the runtime is
     finalizing will terminate the thread, even if the thread was not
     created by Python. You can use "_Py_IsFinalizing()" or
     "sys.is_finalizing()" to check if the interpreter is in process
     of being finalized before calling this function to avoid unwanted
     termination.

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.

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.

Py_BLOCK_THREADS

   This macro expands to "PyEval_RestoreThread(_save);": it is
   equivalent to "Py_END_ALLOW_THREADS" without the closing brace.

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.


Low-level API
-------------

All of the following functions must be called after "Py_Initialize()".

Changed in version 3.7: "Py_Initialize()" now initializes the *GIL*.

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()".

PY_INT64_T PyInterpreterState_GetID(PyInterpreterState *interp)

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

   New in version 3.7.

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(unsigned 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.

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

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 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 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",
   "PyTrace_C_RETURN", or "PyTrace_OPCODE", 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   |
   |                                | "sys.exc_info()".                      |
   +--------------------------------+----------------------------------------+
   | "PyTrace_LINE"                 | Always "Py_None".                      |
   +--------------------------------+----------------------------------------+
   | "PyTrace_RETURN"               | 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".                      |
   +--------------------------------+----------------------------------------+

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 "Py_tracefunc"
   function (but not a profiling function) when a line-number event is
   being reported. It may be disabled for a frame by setting
   "f_trace_lines" to *0* on that frame.

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.

int PyTrace_OPCODE

   The value for the *what* parameter to "Py_tracefunc" functions (but
   not profiling functions) when a new opcode is about to be executed.
   This event is not emitted by default: it must be explicitly
   requested by setting "f_trace_opcodes" to *1* on the frame.

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"
   "PyTrace_OPCODE" 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 per-opcode events, but does not receive any
   event related to C function objects being called.  Any trace
   function registered using "PyEval_SetTrace()" will not receive
   "PyTrace_C_CALL", "PyTrace_C_EXCEPTION" or "PyTrace_C_RETURN" as a
   value for the *what* parameter.


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_Main()

   Return the main interpreter state object.

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.


Thread Local Storage Support
============================

The Python interpreter provides low-level support for thread-local
storage (TLS) which wraps the underlying native TLS implementation to
support the Python-level thread local storage API ("threading.local").
The CPython C level APIs are similar to those offered by pthreads and
Windows: use a thread key and functions to associate a "void*" value
per thread.

The GIL does *not* need to be held when calling these functions; they
supply their own locking.

Note that "Python.h" does not include the declaration of the TLS APIs,
you need to include "pythread.h" to use thread-local storage.

Note: None of these API functions handle memory management on behalf
  of the "void*" values.  You need to allocate and deallocate them
  yourself. If the "void*" values happen to be "PyObject*", these
  functions don’t do refcount operations on them either.


Thread Specific Storage (TSS) API
---------------------------------

TSS API is introduced to supersede the use of the existing TLS API
within the CPython interpreter.  This API uses a new type "Py_tss_t"
instead of "int" to represent thread keys.

New in version 3.7.

See also: “A New C-API for Thread-Local Storage in CPython” (**PEP
  539**)

Py_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 "Py_tss_t" variables.
   Note that this macro won’t be defined with Py_LIMITED_API.


Dynamic Allocation
~~~~~~~~~~~~~~~~~~

Dynamic allocation of the "Py_tss_t", required in extension modules
built with Py_LIMITED_API, where static allocation of this type is not
possible due to its implementation being opaque at build time.

Py_tss_t* PyThread_tss_alloc()

   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.

void PyThread_tss_free(Py_tss_t *key)

   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 "Py_tss_t" has not been initialized by
"PyThread_tss_create()".

int PyThread_tss_is_created(Py_tss_t *key)

   Return a non-zero value if the given "Py_tss_t" has been
   initialized by "PyThread_tss_create()".

int PyThread_tss_create(Py_tss_t *key)

   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.

void PyThread_tss_delete(Py_tss_t *key)

   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.

int PyThread_tss_set(Py_tss_t *key, void *value)

   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(Py_tss_t *key)

   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 Thread
Specific Storage (TSS) API.

Note: This version of the API does not support platforms where the
  native TLS key is defined in a way that cannot be safely cast to
  "int".  On such platforms, "PyThread_create_key()" will return
  immediately with a failure status, and the other TLS functions will
  all be no-ops on such platforms.

Due to the compatibility problem noted above, this version of the API
should not be used in new code.

int PyThread_create_key()

void PyThread_delete_key(int key)

int PyThread_set_key_value(int key, void *value)

void* PyThread_get_key_value(int key)

void PyThread_delete_key_value(int key)

void PyThread_ReInitTLS()
