Initialization, Finalization, and Threads¶
See Python Initialization Configuration for details on how to configure the interpreter prior to initialization.
Before Python Initialization¶
In an application embedding Python, the Py_Initialize()
function must
be called before using any other Python/C API functions; with the exception of
a few functions and the global configuration variables.
The following functions can be safely called before Python is initialized:
Functions that initialize the interpreter:
the runtime pre-initialization functions covered in Python Initialization Configuration
Configuration functions:
PyInitFrozenExtensions()
the configuration functions covered in Python Initialization Configuration
Informative functions:
Utilities:
the status reporting and utility functions covered in Python Initialization Configuration
Memory allocators:
Synchronization:
Note
Despite their apparent similarity to some of the functions listed above,
the following functions should not be called before the interpreter has
been initialized: Py_EncodeLocale()
, Py_GetPath()
,
Py_GetPrefix()
, Py_GetExecPrefix()
,
Py_GetProgramFullPath()
, Py_GetPythonHome()
,
Py_GetProgramName()
, PyEval_InitThreads()
, and
Py_RunMain()
.
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.
-
int Py_BytesWarningFlag¶
This API is kept for backward compatibility: setting
PyConfig.bytes_warning
should be used instead, see Python Initialization Configuration.Issue a warning when comparing
bytes
orbytearray
withstr
orbytes
withint
. Issue an error if greater or equal to2
.Set by the
-b
option.Deprecated since version 3.12, will be removed in version 3.15.
-
int Py_DebugFlag¶
This API is kept for backward compatibility: setting
PyConfig.parser_debug
should be used instead, see Python Initialization Configuration.Turn on parser debugging output (for expert only, depending on compilation options).
Set by the
-d
option and thePYTHONDEBUG
environment variable.Deprecated since version 3.12, will be removed in version 3.15.
-
int Py_DontWriteBytecodeFlag¶
This API is kept for backward compatibility: setting
PyConfig.write_bytecode
should be used instead, see Python Initialization Configuration.If set to non-zero, Python won’t try to write
.pyc
files on the import of source modules.Set by the
-B
option and thePYTHONDONTWRITEBYTECODE
environment variable.Deprecated since version 3.12, will be removed in version 3.15.
-
int Py_FrozenFlag¶
This API is kept for backward compatibility: setting
PyConfig.pathconfig_warnings
should be used instead, see Python Initialization Configuration.Suppress error messages when calculating the module search path in
Py_GetPath()
.Private flag used by
_freeze_module
andfrozenmain
programs.Deprecated since version 3.12, will be removed in version 3.15.
-
int Py_HashRandomizationFlag¶
This API is kept for backward compatibility: setting
PyConfig.hash_seed
andPyConfig.use_hash_seed
should be used instead, see Python Initialization Configuration.Set to
1
if thePYTHONHASHSEED
environment variable is set to a non-empty string.If the flag is non-zero, read the
PYTHONHASHSEED
environment variable to initialize the secret hash seed.Deprecated since version 3.12, will be removed in version 3.15.
-
int Py_IgnoreEnvironmentFlag¶
This API is kept for backward compatibility: setting
PyConfig.use_environment
should be used instead, see Python Initialization Configuration.Ignore all
PYTHON*
environment variables, e.g.PYTHONPATH
andPYTHONHOME
, that might be set.Deprecated since version 3.12, will be removed in version 3.15.
-
int Py_InspectFlag¶
This API is kept for backward compatibility: setting
PyConfig.inspect
should be used instead, see Python Initialization Configuration.When a script is passed as first argument or the
-c
option is used, enter interactive mode after executing the script or the command, even whensys.stdin
does not appear to be a terminal.Set by the
-i
option and thePYTHONINSPECT
environment variable.Deprecated since version 3.12, will be removed in version 3.15.
-
int Py_InteractiveFlag¶
This API is kept for backward compatibility: setting
PyConfig.interactive
should be used instead, see Python Initialization Configuration.Set by the
-i
option.Deprecated since version 3.12, will be removed in version 3.15.
-
int Py_IsolatedFlag¶
This API is kept for backward compatibility: setting
PyConfig.isolated
should be used instead, see Python Initialization Configuration.Run Python in isolated mode. In isolated mode
sys.path
contains neither the script’s directory nor the user’s site-packages directory.Set by the
-I
option.Added in version 3.4.
Deprecated since version 3.12, will be removed in version 3.15.
-
int Py_LegacyWindowsFSEncodingFlag¶
This API is kept for backward compatibility: setting
PyPreConfig.legacy_windows_fs_encoding
should be used instead, see Python Initialization Configuration.If the flag is non-zero, use the
mbcs
encoding withreplace
error handler, instead of the UTF-8 encoding withsurrogatepass
error handler, for the filesystem encoding and error handler.Set to
1
if thePYTHONLEGACYWINDOWSFSENCODING
environment variable is set to a non-empty string.See PEP 529 for more details.
Availability: Windows.
Deprecated since version 3.12, will be removed in version 3.15.
-
int Py_LegacyWindowsStdioFlag¶
This API is kept for backward compatibility: setting
PyConfig.legacy_windows_stdio
should be used instead, see Python Initialization Configuration.If the flag is non-zero, use
io.FileIO
instead ofio._WindowsConsoleIO
forsys
standard streams.Set to
1
if thePYTHONLEGACYWINDOWSSTDIO
environment variable is set to a non-empty string.See PEP 528 for more details.
Availability: Windows.
Deprecated since version 3.12, will be removed in version 3.15.
-
int Py_NoSiteFlag¶
This API is kept for backward compatibility: setting
PyConfig.site_import
should be used instead, see Python Initialization Configuration.Disable the import of the module
site
and the site-dependent manipulations ofsys.path
that it entails. Also disable these manipulations ifsite
is explicitly imported later (callsite.main()
if you want them to be triggered).Set by the
-S
option.Deprecated since version 3.12, will be removed in version 3.15.
-
int Py_NoUserSiteDirectory¶
This API is kept for backward compatibility: setting
PyConfig.user_site_directory
should be used instead, see Python Initialization Configuration.Don’t add the
user site-packages directory
tosys.path
.Set by the
-s
and-I
options, and thePYTHONNOUSERSITE
environment variable.Deprecated since version 3.12, will be removed in version 3.15.
-
int Py_OptimizeFlag¶
This API is kept for backward compatibility: setting
PyConfig.optimization_level
should be used instead, see Python Initialization Configuration.Set by the
-O
option and thePYTHONOPTIMIZE
environment variable.Deprecated since version 3.12, will be removed in version 3.15.
-
int Py_QuietFlag¶
This API is kept for backward compatibility: setting
PyConfig.quiet
should be used instead, see Python Initialization Configuration.Don’t display the copyright and version messages even in interactive mode.
Set by the
-q
option.Added in version 3.2.
Deprecated since version 3.12, will be removed in version 3.15.
-
int Py_UnbufferedStdioFlag¶
This API is kept for backward compatibility: setting
PyConfig.buffered_stdio
should be used instead, see Python Initialization Configuration.Force the stdout and stderr streams to be unbuffered.
Set by the
-u
option and thePYTHONUNBUFFERED
environment variable.Deprecated since version 3.12, will be removed in version 3.15.
-
int Py_VerboseFlag¶
This API is kept for backward compatibility: setting
PyConfig.verbose
should be used instead, see Python Initialization Configuration.Print a message each time a module is initialized, showing the place (filename or built-in module) from which it is loaded. If greater or equal to
2
, print a message for each file that is checked for when searching for a module. Also provides information on module cleanup at exit.Set by the
-v
option and thePYTHONVERBOSE
environment variable.Deprecated since version 3.12, will be removed in version 3.15.
Initializing and finalizing the interpreter¶
-
void Py_Initialize()¶
- Part of the Stable ABI.
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 modulesbuiltins
,__main__
andsys
. It also initializes the module search path (sys.path
). It does not setsys.argv
; use the Python Initialization Configuration API for that. This is a no-op when called for a second time (without callingPy_FinalizeEx()
first). There is no return value; it is a fatal error if the initialization fails.Use
Py_InitializeFromConfig()
to customize the Python Initialization Configuration.Note
On Windows, changes the console mode from
O_TEXT
toO_BINARY
, which will also affect non-Python uses of the console using the C Runtime.
-
void Py_InitializeEx(int initsigs)¶
- Part of the Stable ABI.
This function works like
Py_Initialize()
if initsigs is1
. If initsigs is0
, it skips initialization registration of signal handlers, which may be useful when CPython is embedded as part of a larger application.Use
Py_InitializeFromConfig()
to customize the Python Initialization Configuration.
-
PyStatus Py_InitializeFromConfig(const PyConfig *config)¶
Initialize Python from config configuration, as described in Initialization with PyConfig.
See the Python Initialization Configuration section for details on pre-initializing the interpreter, populating the runtime configuration structure, and querying the returned status structure.
-
int Py_IsInitialized()¶
- Part of the Stable ABI.
Return true (nonzero) when the Python interpreter has been initialized, false (zero) if not. After
Py_FinalizeEx()
is called, this returns false untilPy_Initialize()
is called again.
-
int Py_IsFinalizing()¶
- Part of the Stable ABI since version 3.13.
Return true (non-zero) if the main Python interpreter is shutting down. Return false (zero) otherwise.
Added in version 3.13.
-
int Py_FinalizeEx()¶
- Part of the Stable ABI since version 3.6.
Undo all initializations made by
Py_Initialize()
and subsequent use of Python/C API functions, and destroy all sub-interpreters (seePy_NewInterpreter()
below) that were created and not yet destroyed since the last call toPy_Initialize()
. This is a no-op when called for a second time (without callingPy_Initialize()
again first).Since this is the reverse of
Py_Initialize()
, it should be called in the same thread with the same interpreter active. That means the main thread and the main interpreter. This should never be called whilePy_RunMain()
is running.Normally the return value is
0
. If there were errors during finalization (flushing buffered data),-1
is returned.Note that Python will do a best effort at freeing all memory allocated by the Python interpreter. Therefore, any C-Extension should make sure to correctly clean up all of the previously allocated PyObjects before using them in subsequent calls to
Py_Initialize()
. Otherwise it could introduce vulnerabilities and incorrect behavior.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. Interned strings will all be deallocated regardless of their reference count. 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 callsPy_Initialize()
andPy_FinalizeEx()
more than once.Py_FinalizeEx()
must not be called recursively from within itself. Therefore, it must not be called by any code that may be run as part of the interpreter shutdown process, such asatexit
handlers, object finalizers, or any code that may be run while flushing the stdout and stderr files.Raises an auditing event
cpython._PySys_ClearAuditHooks
with no arguments.Added in version 3.6.
-
void Py_Finalize()¶
- Part of the Stable ABI.
This is a backwards-compatible version of
Py_FinalizeEx()
that disregards the return value.
-
int Py_BytesMain(int argc, char **argv)¶
- Part of the Stable ABI since version 3.8.
Similar to
Py_Main()
but argv is an array of bytes strings, allowing the calling application to delegate the text decoding step to the CPython runtime.Added in version 3.8.
-
int Py_Main(int argc, wchar_t **argv)¶
- Part of the Stable ABI.
The main program for the standard interpreter, encapsulating a full initialization/finalization cycle, as well as additional behaviour to implement reading configurations settings from the environment and command line, and then executing
__main__
in accordance with Command line.This is made available for programs which wish to support the full CPython command line interface, rather than just embedding a Python runtime in a larger application.
The argc and argv parameters are similar to those which are passed to a C program’s
main()
function, except that the argv entries are first converted towchar_t
usingPy_DecodeLocale()
. It is also important to note that the argument list entries may be modified to point to strings other than those passed in (however, the contents of the strings pointed to by the argument list are not modified).The return value is
2
if the argument list does not represent a valid Python command line, and otherwise the same asPy_RunMain()
.In terms of the CPython runtime configuration APIs documented in the runtime configuration section (and without accounting for error handling),
Py_Main
is approximately equivalent to:PyConfig config; PyConfig_InitPythonConfig(&config); PyConfig_SetArgv(&config, argc, argv); Py_InitializeFromConfig(&config); PyConfig_Clear(&config); Py_RunMain();
In normal usage, an embedding application will call this function instead of calling
Py_Initialize()
,Py_InitializeEx()
orPy_InitializeFromConfig()
directly, and all settings will be applied as described elsewhere in this documentation. If this function is instead called after a preceding runtime initialization API call, then exactly which environmental and command line configuration settings will be updated is version dependent (as it depends on which settings correctly support being modified after they have already been set once when the runtime was first initialized).
-
int Py_RunMain(void)¶
Executes the main module in a fully configured CPython runtime.
Executes the command (
PyConfig.run_command
), the script (PyConfig.run_filename
) or the module (PyConfig.run_module
) specified on the command line or in the configuration. If none of these values are set, runs the interactive Python prompt (REPL) using the__main__
module’s global namespace.If
PyConfig.inspect
is not set (the default), the return value will be0
if the interpreter exits normally (that is, without raising an exception), the exit status of an unhandledSystemExit
, or1
for any other unhandled exception.If
PyConfig.inspect
is set (such as when the-i
option is used), rather than returning when the interpreter exits, execution will instead resume in an interactive Python prompt (REPL) using the__main__
module’s global namespace. If the interpreter exited with an exception, it is immediately raised in the REPL session. The function return value is then determined by the way the REPL session terminates:0
,1
, or the status of aSystemExit
, as specified above.This function always finalizes the Python interpreter before it returns.
See Python Configuration for an example of a customized Python that always runs in isolated mode using
Py_RunMain()
.
-
int PyUnstable_AtExit(PyInterpreterState *interp, void (*func)(void*), void *data)¶
- This is Unstable API. It may change without warning in minor releases.
Register an
atexit
callback for the target interpreter interp. This is similar toPy_AtExit()
, but takes an explicit interpreter and data pointer for the callback.There must be an attached thread state for interp.
Added in version 3.13.
Process-wide parameters¶
-
void Py_SetProgramName(const wchar_t *name)¶
- Part of the Stable ABI.
This API is kept for backward compatibility: setting
PyConfig.program_name
should be used instead, see Python Initialization Configuration.This function should be called before
Py_Initialize()
is called for the first time, if it is called at all. It tells the interpreter the value of theargv[0]
argument to themain()
function of the program (converted to wide characters). This is used byPy_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_t* string.Deprecated since version 3.11, will be removed in version 3.15.
-
wchar_t *Py_GetProgramName()¶
- Part of the Stable ABI.
Return the program name set with
PyConfig.program_name
, or the default. The returned string points into static storage; the caller should not modify its value.This function should not be called before
Py_Initialize()
, otherwise it returnsNULL
.Changed in version 3.10: It now returns
NULL
if called beforePy_Initialize()
.Deprecated since version 3.13, will be removed in version 3.15: Use
PyConfig_Get("executable")
(sys.executable
) instead.
-
wchar_t *Py_GetPrefix()¶
- Part of the Stable ABI.
Return the prefix for installed platform-independent files. This is derived through a number of complicated rules from the program name set with
PyConfig.program_name
and some environment variables; for example, if the program name is'/usr/local/bin/python'
, the prefix is'/usr/local'
. The returned string points into static storage; the caller should not modify its value. This corresponds to the prefix variable in the top-levelMakefile
and the--prefix
argument to the configure script at build time. The value is available to Python code assys.base_prefix
. It is only useful on Unix. See also the next function.This function should not be called before
Py_Initialize()
, otherwise it returnsNULL
.Changed in version 3.10: It now returns
NULL
if called beforePy_Initialize()
.Deprecated since version 3.13, will be removed in version 3.15: Use
PyConfig_Get("base_prefix")
(sys.base_prefix
) instead. UsePyConfig_Get("prefix")
(sys.prefix
) if virtual environments need to be handled.
-
wchar_t *Py_GetExecPrefix()¶
- Part of the Stable ABI.
Return the exec-prefix for installed platform-dependent files. This is derived through a number of complicated rules from the program name set with
PyConfig.program_name
and some environment variables; for example, if the program name is'/usr/local/bin/python'
, the exec-prefix is'/usr/local'
. The returned string points into static storage; the caller should not modify its value. This corresponds to the exec_prefix variable in the top-levelMakefile
and the--exec-prefix
argument to the configure script at build time. The value is available to Python code assys.base_exec_prefix
. It is only useful on Unix.Background: The exec-prefix differs from the prefix when platform dependent files (such as executables and shared libraries) are installed in a different directory tree. In a typical installation, platform dependent files may be installed in the
/usr/local/plat
subtree while platform independent may be installed in/usr/local
.Generally speaking, a platform is a combination of hardware and software families, e.g. Sparc machines running the Solaris 2.x operating system are considered the same platform, but Intel machines running Solaris 2.x are another platform, and Intel machines running Linux are yet another platform. Different major revisions of the same operating system generally also form different platforms. Non-Unix operating systems are a different story; the installation strategies on those systems are so different that the prefix and exec-prefix are meaningless, and set to the empty string. Note that compiled Python bytecode files are platform independent (but not independent from the Python version by which they were compiled!).
System administrators will know how to configure the mount or automount programs to share
/usr/local
between platforms while having/usr/local/plat
be a different filesystem for each platform.This function should not be called before
Py_Initialize()
, otherwise it returnsNULL
.Changed in version 3.10: It now returns
NULL
if called beforePy_Initialize()
.Deprecated since version 3.13, will be removed in version 3.15: Use
PyConfig_Get("base_exec_prefix")
(sys.base_exec_prefix
) instead. UsePyConfig_Get("exec_prefix")
(sys.exec_prefix
) if virtual environments need to be handled.
-
wchar_t *Py_GetProgramFullPath()¶
- Part of the Stable ABI.
Return the full program name of the Python executable; this is computed as a side-effect of deriving the default module search path from the program name (set by
PyConfig.program_name
). The returned string points into static storage; the caller should not modify its value. The value is available to Python code assys.executable
.This function should not be called before
Py_Initialize()
, otherwise it returnsNULL
.Changed in version 3.10: It now returns
NULL
if called beforePy_Initialize()
.Deprecated since version 3.13, will be removed in version 3.15: Use
PyConfig_Get("executable")
(sys.executable
) instead.
-
wchar_t *Py_GetPath()¶
- Part of the Stable ABI.
Return the default module search path; this is computed from the program name (set by
PyConfig.program_name
) and some environment variables. The returned string consists of a series of directory names separated by a platform dependent delimiter character. The delimiter character is':'
on Unix and macOS,';'
on Windows. The returned string points into static storage; the caller should not modify its value. The listsys.path
is initialized with this value on interpreter startup; it can be (and usually is) modified later to change the search path for loading modules.This function should not be called before
Py_Initialize()
, otherwise it returnsNULL
.Changed in version 3.10: It now returns
NULL
if called beforePy_Initialize()
.Deprecated since version 3.13, will be removed in version 3.15: Use
PyConfig_Get("module_search_paths")
(sys.path
) instead.
-
const char *Py_GetVersion()¶
- Part of the Stable ABI.
Return the version of this Python interpreter. This is a string that looks something like
"3.0a5+ (py3k:63103M, May 12 2008, 00:53:55) \n[GCC 4.2.3]"
The first word (up to the first space character) is the current Python version; the first characters are the major and minor version separated by a period. The returned string points into static storage; the caller should not modify its value. The value is available to Python code as
sys.version
.See also the
Py_Version
constant.
-
const char *Py_GetPlatform()¶
- Part of the Stable ABI.
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 macOS, it is'darwin'
. On Windows, it is'win'
. The returned string points into static storage; the caller should not modify its value. The value is available to Python code assys.platform
.
-
const char *Py_GetCopyright()¶
- Part of the Stable ABI.
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()¶
- Part of the Stable ABI.
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()¶
- Part of the Stable ABI.
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)¶
- Part of the Stable ABI.
This API is kept for backward compatibility: setting
PyConfig.argv
,PyConfig.parse_argv
andPyConfig.safe_path
should be used instead, see Python Initialization Configuration.Set
sys.argv
based on argc and argv. These parameters are similar to those passed to the program’smain()
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 initializesys.argv
, a fatal condition is signalled usingPy_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 tosys.path
.Otherwise (that is, if argc is
0
orargv[0]
doesn’t point to an existing file name), an empty string is prepended tosys.path
, which is the same as prepending the current working directory ("."
).
Use
Py_DecodeLocale()
to decode a bytes string to get a wchar_t* string.See also
PyConfig.orig_argv
andPyConfig.argv
members of the Python Initialization Configuration.Note
It is recommended that applications embedding the Python interpreter for purposes other than executing a single script pass
0
as updatepath, and updatesys.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 calledPySys_SetArgv()
, for example using:PyRun_SimpleString("import sys; sys.path.pop(0)\n");
Added in version 3.1.3.
Deprecated since version 3.11, will be removed in version 3.15.
-
void PySys_SetArgv(int argc, wchar_t **argv)¶
- Part of the Stable ABI.
This API is kept for backward compatibility: setting
PyConfig.argv
andPyConfig.parse_argv
should be used instead, see Python Initialization Configuration.This function works like
PySys_SetArgvEx()
with updatepath set to1
unless the python interpreter was started with the-I
.Use
Py_DecodeLocale()
to decode a bytes string to get a wchar_t* string.See also
PyConfig.orig_argv
andPyConfig.argv
members of the Python Initialization Configuration.Changed in version 3.4: The updatepath value depends on
-I
.Deprecated since version 3.11, will be removed in version 3.15.
-
void Py_SetPythonHome(const wchar_t *home)¶
- Part of the Stable ABI.
This API is kept for backward compatibility: setting
PyConfig.home
should be used instead, see Python Initialization Configuration.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_t* string.Deprecated since version 3.11, will be removed in version 3.15.
-
wchar_t *Py_GetPythonHome()¶
- Part of the Stable ABI.
Return the default “home”, that is, the value set by
PyConfig.home
, or the value of thePYTHONHOME
environment variable if it is set.This function should not be called before
Py_Initialize()
, otherwise it returnsNULL
.Changed in version 3.10: It now returns
NULL
if called beforePy_Initialize()
.Deprecated since version 3.13, will be removed in version 3.15: Use
PyConfig_Get("home")
or thePYTHONHOME
environment variable instead.
Thread State and the Global Interpreter Lock¶
Unless on a free-threaded build of CPython, 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
, known as a thread state.
Each OS thread has a thread-local pointer to a PyThreadState
; a thread state
referenced by this pointer is considered to be attached.
A thread can only have one attached thread state at a time. An attached thread state is typically analogous with holding the GIL, except on free-threaded builds. On builds with the GIL enabled, attaching a thread state will block until the GIL can be acquired. However, even on builds with the GIL disabled, it is still required to have an attached thread state to call most of the C API.
In general, there will always be an attached thread state when using Python’s C API.
Only in some specific cases (such as in a Py_BEGIN_ALLOW_THREADS
block) will the
thread not have an attached thread state. If uncertain, check if PyThreadState_GetUnchecked()
returns
NULL
.
Detaching the thread state from extension code¶
Most extension code manipulating the thread state has the following simple structure:
Save the thread state in a local variable.
... Do some blocking I/O operation ...
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 attached thread state holds the GIL for the entire interpreter. When detaching
the attached thread state, the GIL is released, allowing other threads to attach
a thread state to their own thread, thus getting the GIL and can start executing.
The pointer to the prior attached thread state is stored as a local variable.
Upon reaching Py_END_ALLOW_THREADS
, the thread state that was
previously attached is passed to PyEval_RestoreThread()
.
This function will block until another releases its thread state,
thus allowing the old thread state to get re-attached and the
C API can be called again.
For free-threaded builds, the GIL is normally out of the question, but detaching the thread state is still required for blocking I/O and long operations. The difference is that threads don’t have to wait for the GIL to be released to attach their thread state, allowing true multi-core parallelism.
Note
Calling system I/O functions is the most common use case for detaching
the thread state, 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 detach the
thread state 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, because they don’t have an
attached thread state.
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 an attached thread state before you can start using the Python/C API. When you are done, you should detach the thread state, and finally free it.
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. This is because PyGILState_Ensure()
and similar functions default to attaching a
thread state for the main interpreter, meaning that the thread can’t safely
interact with the calling subinterpreter.
Supporting subinterpreters in non-Python threads¶
If you would like to support subinterpreters with non-Python created threads, you
must use the PyThreadState_*
API instead of the traditional PyGILState_*
API.
In particular, you must store the interpreter state from the calling
function and pass it to PyThreadState_New()
, which will ensure that
the thread state is targeting the correct interpreter:
/* The return value of PyInterpreterState_Get() from the
function that created this thread. */
PyInterpreterState *interp = ThreadData->interp;
PyThreadState *tstate = PyThreadState_New(interp);
PyThreadState_Swap(tstate);
/* GIL of the subinterpreter is now held.
Perform Python actions here. */
result = CallSomeFunction();
/* evaluate result or handle exception */
/* Destroy the thread state. No Python API allowed beyond this point. */
PyThreadState_Clear(tstate);
PyThreadState_DeleteCurrent();
Cautions about fork()¶
Another important thing to note about threads is their behaviour in the face
of the C fork()
call. On most systems with fork()
, after a
process forks only the thread that issued the fork will exist. This has a
concrete impact both on how locks must be handled and on all stored state
in CPython’s runtime.
The fact that only the “current” thread remains
means any locks held by other threads will never be released. Python solves
this for os.fork()
by acquiring the locks it uses internally before
the fork, and releasing them afterwards. In addition, it resets any
Lock objects in the child. When extending or embedding Python, there
is no way to inform Python of additional (non-Python) locks that need to be
acquired before or reset after a fork. OS facilities such as
pthread_atfork()
would need to be used to accomplish the same thing.
Additionally, when extending or embedding Python, calling fork()
directly rather than through os.fork()
(and returning to or calling
into Python) may result in a deadlock by one of Python’s internal locks
being held by a thread that is defunct after the fork.
PyOS_AfterFork_Child()
tries to reset the necessary locks, but is not
always able to.
The fact that all other threads go away also means that CPython’s
runtime state there must be cleaned up properly, which os.fork()
does. This means finalizing all other PyThreadState
objects
belonging to the current interpreter and all other
PyInterpreterState
objects. Due to this and the special
nature of the “main” interpreter,
fork()
should only be called in that interpreter’s “main”
thread, where the CPython global runtime was originally initialized.
The only exception is if exec()
will be called immediately
after.
Cautions regarding runtime finalization¶
In the late stage of interpreter shutdown, after attempting to wait for
non-daemon threads to exit (though this can be interrupted by
KeyboardInterrupt
) and running the atexit
functions, the runtime
is marked as finalizing: Py_IsFinalizing()
and
sys.is_finalizing()
return true. At this point, only the finalization
thread that initiated finalization (typically the main thread) is allowed to
acquire the GIL.
If any thread, other than the finalization thread, attempts to attach a thread state during finalization, either explicitly or implicitly, the thread enters a permanently blocked state where it remains until the program exits. In most cases this is harmless, but this can result in deadlock if a later stage of finalization attempts to acquire a lock owned by the blocked thread, or otherwise waits on the blocked thread.
Gross? Yes. This prevents random crashes and/or unexpectedly skipped C++ finalizations further up the call stack when such threads were forcibly exited here in CPython 3.13 and earlier. The CPython runtime thread state C APIs have never had any error reporting or handling expectations at thread state attachment time that would’ve allowed for graceful exit from this situation. Changing that would require new stable C APIs and rewriting the majority of C code in the CPython ecosystem to use those with error handling.
High-level API¶
These are the most commonly used types and functions when writing C extension code, or when embedding the Python interpreter:
-
type PyInterpreterState¶
- Part of the Limited API (as an opaque struct).
This data structure represents the state shared by a number of cooperating threads. Threads belonging to the same interpreter share their module administration and a few other internal items. There are no public members in this structure.
Threads belonging to different interpreters initially share nothing, except process state like available memory, open file descriptors and such. The global interpreter lock is also shared by all threads, regardless of to which interpreter they belong.
Changed in version 3.12: PEP 684 introduced the possibility of a per-interpreter GIL. See
Py_NewInterpreterFromConfig()
.
-
type PyThreadState¶
- Part of the Limited API (as an opaque struct).
This data structure represents the state of a single thread. The only public data member is:
-
PyInterpreterState *interp¶
This thread’s interpreter state.
-
PyInterpreterState *interp¶
-
void PyEval_InitThreads()¶
- Part of the Stable ABI.
Deprecated function which does nothing.
In Python 3.6 and older, this function created the GIL if it didn’t exist.
Changed in version 3.9: The function now does nothing.
Changed in version 3.7: This function is now called by
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.Deprecated since version 3.9.
-
PyThreadState *PyEval_SaveThread()¶
- Part of the Stable ABI.
Detach the attached thread state and return it. The thread will have no thread state upon returning.
-
void PyEval_RestoreThread(PyThreadState *tstate)¶
- Part of the Stable ABI.
Set the attached thread state to tstate. The passed thread state should not be attached, otherwise deadlock ensues. tstate will be attached upon returning.
Note
Calling this function from a thread when the runtime is finalizing will hang the thread until the program exits, even if the thread was not created by Python. Refer to Cautions regarding runtime finalization for more details.
Changed in version 3.14: Hangs the current thread, rather than terminating it, if called while the interpreter is finalizing.
-
PyThreadState *PyThreadState_Get()¶
- Part of the Stable ABI.
Return the attached thread state. If the thread has no attached thread state, (such as when inside of
Py_BEGIN_ALLOW_THREADS
block), then this issues a fatal error (so that the caller needn’t check forNULL
).See also
PyThreadState_GetUnchecked()
.
-
PyThreadState *PyThreadState_GetUnchecked()¶
Similar to
PyThreadState_Get()
, but don’t kill the process with a fatal error if it is NULL. The caller is responsible to check if the result is NULL.Added in version 3.13: In Python 3.5 to 3.12, the function was private and known as
_PyThreadState_UncheckedGet()
.
-
PyThreadState *PyThreadState_Swap(PyThreadState *tstate)¶
- Part of the Stable ABI.
Set the attached thread state to tstate, and return the thread state that was attached prior to calling.
This function is safe to call without an attached thread state; it will simply return
NULL
indicating that there was no prior thread state.See also
Note
Similar to
PyGILState_Ensure()
, this function will hang the thread if the runtime is finalizing.
The following functions use thread-local storage, and are not compatible with sub-interpreters:
-
type PyGILState_STATE¶
- Part of the Stable ABI.
The type of the value returned by
PyGILState_Ensure()
and passed toPyGILState_Release()
.-
enumerator PyGILState_LOCKED¶
The GIL was already held when
PyGILState_Ensure()
was called.
-
enumerator PyGILState_UNLOCKED¶
The GIL was not held when
PyGILState_Ensure()
was called.
-
enumerator PyGILState_LOCKED¶
-
PyGILState_STATE PyGILState_Ensure()¶
- Part of the Stable ABI.
Ensure that the current thread is ready to call the Python C API regardless of the current state of Python, or of the attached thread state. 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 betweenPyGILState_Ensure()
andPyGILState_Release()
calls as long as the thread state is restored to its previous state before the Release(). For example, normal usage of thePy_BEGIN_ALLOW_THREADS
andPy_END_ALLOW_THREADS
macros is acceptable.The return value is an opaque “handle” to the attached thread state when
PyGILState_Ensure()
was called, and must be passed toPyGILState_Release()
to ensure Python is left in the same state. Even though recursive calls are allowed, these handles cannot be shared - each unique call toPyGILState_Ensure()
must save the handle for its call toPyGILState_Release()
.When the function returns, there will be an attached thread state and the thread will be able to call arbitrary Python code. Failure is a fatal error.
Warning
Calling this function when the runtime is finalizing is unsafe. Doing so will either hang the thread until the program ends, or fully crash the interpreter in rare cases. Refer to Cautions regarding runtime finalization for more details.
Changed in version 3.14: Hangs the current thread, rather than terminating it, if called while the interpreter is finalizing.
-
void PyGILState_Release(PyGILState_STATE)¶
- Part of the Stable ABI.
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 toPyGILState_Release()
on the same thread.
-
PyThreadState *PyGILState_GetThisThreadState()¶
- Part of the Stable ABI.
Get the attached 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.Note
This function may return non-
NULL
even when the thread state is detached. PreferPyThreadState_Get()
orPyThreadState_GetUnchecked()
for most cases.See also
-
int PyGILState_Check()¶
Return
1
if the current thread is holding the GIL and0
otherwise. This function can be called from any thread at any time. Only if it has had its thread state initialized viaPyGILState_Ensure()
will it return1
. 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.Note
If the current Python process has ever created a subinterpreter, this function will always return
1
. PreferPyThreadState_GetUnchecked()
for most cases.Added in version 3.4.
The following macros are normally used without a trailing semicolon; look for example usage in the Python source distribution.
-
Py_BEGIN_ALLOW_THREADS¶
- Part of the Stable ABI.
This macro expands to
{ PyThreadState *_save; _save = PyEval_SaveThread();
. Note that it contains an opening brace; it must be matched with a followingPy_END_ALLOW_THREADS
macro. See above for further discussion of this macro.
-
Py_END_ALLOW_THREADS¶
- Part of the Stable ABI.
This macro expands to
PyEval_RestoreThread(_save); }
. Note that it contains a closing brace; it must be matched with an earlierPy_BEGIN_ALLOW_THREADS
macro. See above for further discussion of this macro.
-
Py_BLOCK_THREADS¶
- Part of the Stable ABI.
This macro expands to
PyEval_RestoreThread(_save);
: it is equivalent toPy_END_ALLOW_THREADS
without the closing brace.
-
Py_UNBLOCK_THREADS¶
- Part of the Stable ABI.
This macro expands to
_save = PyEval_SaveThread();
: it is equivalent toPy_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
and sets an attached thread state.
-
PyInterpreterState *PyInterpreterState_New()¶
- Part of the Stable ABI.
Create a new interpreter state object. An attached thread state is not needed, but may optionally exist if it is necessary to serialize calls to this function.
Raises an auditing event
cpython.PyInterpreterState_New
with no arguments.
-
void PyInterpreterState_Clear(PyInterpreterState *interp)¶
- Part of the Stable ABI.
Reset all information in an interpreter state object. There must be an attached thread state for the interpreter.
Raises an auditing event
cpython.PyInterpreterState_Clear
with no arguments.
-
void PyInterpreterState_Delete(PyInterpreterState *interp)¶
- Part of the Stable ABI.
Destroy an interpreter state object. There should not be an attached thread state for the target interpreter. The interpreter state must have been reset with a previous call to
PyInterpreterState_Clear()
.
-
PyThreadState *PyThreadState_New(PyInterpreterState *interp)¶
- Part of the Stable ABI.
Create a new thread state object belonging to the given interpreter object. An attached thread state is not needed.
-
void PyThreadState_Clear(PyThreadState *tstate)¶
- Part of the Stable ABI.
Reset all information in a thread state object. tstate must be attached
Changed in version 3.9: This function now calls the
PyThreadState.on_delete
callback. Previously, that happened inPyThreadState_Delete()
.Changed in version 3.13: The
PyThreadState.on_delete
callback was removed.
-
void PyThreadState_Delete(PyThreadState *tstate)¶
- Part of the Stable ABI.
Destroy a thread state object. tstate should not be attached to any thread. tstate must have been reset with a previous call to
PyThreadState_Clear()
.
-
void PyThreadState_DeleteCurrent(void)¶
Detach the attached thread state (which must have been reset with a previous call to
PyThreadState_Clear()
) and then destroy it.No thread state will be attached upon returning.
-
PyFrameObject *PyThreadState_GetFrame(PyThreadState *tstate)¶
- Part of the Stable ABI since version 3.10.
Get the current frame of the Python thread state tstate.
Return a strong reference. Return
NULL
if no frame is currently executing.See also
PyEval_GetFrame()
.tstate must not be
NULL
, and must be attached.Added in version 3.9.
-
uint64_t PyThreadState_GetID(PyThreadState *tstate)¶
- Part of the Stable ABI since version 3.10.
Get the unique thread state identifier of the Python thread state tstate.
tstate must not be
NULL
, and must be attached.Added in version 3.9.
-
PyInterpreterState *PyThreadState_GetInterpreter(PyThreadState *tstate)¶
- Part of the Stable ABI since version 3.10.
Get the interpreter of the Python thread state tstate.
tstate must not be
NULL
, and must be attached.Added in version 3.9.
-
void PyThreadState_EnterTracing(PyThreadState *tstate)¶
Suspend tracing and profiling in the Python thread state tstate.
Resume them using the
PyThreadState_LeaveTracing()
function.Added in version 3.11.
-
void PyThreadState_LeaveTracing(PyThreadState *tstate)¶
Resume tracing and profiling in the Python thread state tstate suspended by the
PyThreadState_EnterTracing()
function.See also
PyEval_SetTrace()
andPyEval_SetProfile()
functions.Added in version 3.11.
-
int PyUnstable_ThreadState_SetStackProtection(PyThreadState *tstate, void *stack_start_addr, size_t stack_size)¶
- This is Unstable API. It may change without warning in minor releases.
Set the stack protection start address and stack protection size of a Python thread state.
On success, return
0
. On failure, set an exception and return-1
.CPython implements recursion control for C code by raising
RecursionError
when it notices that the machine execution stack is close to overflow. See for example thePy_EnterRecursiveCall()
function. For this, it needs to know the location of the current thread’s stack, which it normally gets from the operating system. When the stack is changed, for example using context switching techniques like the Boost library’sboost::context
, you must callPyUnstable_ThreadState_SetStackProtection()
to inform CPython of the change.Call
PyUnstable_ThreadState_SetStackProtection()
either before or after changing the stack. Do not call any other Python C API between the call and the stack change.See
PyUnstable_ThreadState_ResetStackProtection()
for undoing this operation.Added in version 3.14.1:
Warning
This function was added in a bugfix release, and extensions that use it will be incompatible with Python 3.14.0. Most packaging tools for Python are not able to handle this incompatibility automatically, and will need explicit configuration. When using PyPA standards (wheels and source distributions), specify
Requires-Python: != 3.14.0.*
in core metadata.
-
void PyUnstable_ThreadState_ResetStackProtection(PyThreadState *tstate)¶
- This is Unstable API. It may change without warning in minor releases.
Reset the stack protection start address and stack protection size of a Python thread state to the operating system defaults.
See
PyUnstable_ThreadState_SetStackProtection()
for an explanation.Added in version 3.14.1:
Warning
This function was added in a bugfix release, and extensions that use it will be incompatible with Python 3.14.0. Most packaging tools for Python are not able to handle this incompatibility automatically, and will need explicit configuration. When using PyPA standards (wheels and source distributions), specify
Requires-Python: != 3.14.0.*
in core metadata.
-
PyInterpreterState *PyInterpreterState_Get(void)¶
- Part of the Stable ABI since version 3.9.
Get the current interpreter.
Issue a fatal error if there is no attached thread state. It cannot return NULL.
Added in version 3.9.
-
int64_t PyInterpreterState_GetID(PyInterpreterState *interp)¶
- Part of the Stable ABI since version 3.7.
Return the interpreter’s unique ID. If there was any error in doing so then
-1
is returned and an error is set.The caller must have an attached thread state.
Added in version 3.7.
-
PyObject *PyInterpreterState_GetDict(PyInterpreterState *interp)¶
- Return value: Borrowed reference. Part of the Stable ABI since version 3.8.
Return a dictionary in which interpreter-specific data may be stored. If this function returns
NULL
then no exception has been raised and the caller should assume no interpreter-specific dict is available.This is not a replacement for
PyModule_GetState()
, which extensions should use to store interpreter-specific state information.The returned dictionary is borrowed from the interpreter and is valid until interpreter shutdown.
Added in version 3.8.
-
typedef PyObject *(*_PyFrameEvalFunction)(PyThreadState *tstate, _PyInterpreterFrame *frame, int throwflag)¶
Type of a frame evaluation function.
The throwflag parameter is used by the
throw()
method of generators: if non-zero, handle the current exception.Changed in version 3.9: The function now takes a tstate parameter.
Changed in version 3.11: The frame parameter changed from
PyFrameObject*
to_PyInterpreterFrame*
.
-
_PyFrameEvalFunction _PyInterpreterState_GetEvalFrameFunc(PyInterpreterState *interp)¶
Get the frame evaluation function.
See the PEP 523 “Adding a frame evaluation API to CPython”.
Added in version 3.9.
-
void _PyInterpreterState_SetEvalFrameFunc(PyInterpreterState *interp, _PyFrameEvalFunction eval_frame)¶
Set the frame evaluation function.
See the PEP 523 “Adding a frame evaluation API to CPython”.
Added in version 3.9.
-
PyObject *PyThreadState_GetDict()¶
- Return value: Borrowed reference. Part of the Stable ABI.
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 thread state is attached. If this function returns
NULL
, no exception has been raised and the caller should assume no thread state is attached.
-
int PyThreadState_SetAsyncExc(unsigned long id, PyObject *exc)¶
- Part of the Stable ABI.
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 an attached thread state. 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)¶
- Part of the Stable ABI.
Attach tstate to the current thread, which must not be
NULL
or already attached.The calling thread must not already have an attached thread state.
Note
Calling this function from a thread when the runtime is finalizing will hang the thread until the program exits, even if the thread was not created by Python. Refer to Cautions regarding runtime finalization for more details.
Changed in version 3.8: Updated to be consistent with
PyEval_RestoreThread()
,Py_END_ALLOW_THREADS()
, andPyGILState_Ensure()
, and terminate the current thread if called while the interpreter is finalizing.Changed in version 3.14: Hangs the current thread, rather than terminating it, if called while the interpreter is finalizing.
PyEval_RestoreThread()
is a higher-level function which is always available (even when threads have not been initialized).
-
void PyEval_ReleaseThread(PyThreadState *tstate)¶
- Part of the Stable ABI.
Detach the attached thread state. The tstate argument, which must not be
NULL
, is only used to check that it represents the attached 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).
Sub-interpreter support¶
While in most uses, you will only embed a single Python interpreter, there are cases where you need to create several independent interpreters in the same process and perhaps even in the same thread. Sub-interpreters allow you to do that.
The “main” interpreter is the first one created when the runtime initializes.
It is usually the only Python interpreter in a process. Unlike sub-interpreters,
the main interpreter has unique process-global responsibilities like signal
handling. It is also responsible for execution during runtime initialization and
is usually the active interpreter during runtime finalization. The
PyInterpreterState_Main()
function returns a pointer to its state.
You can switch between sub-interpreters using the PyThreadState_Swap()
function. You can create and destroy them using the following functions:
-
type PyInterpreterConfig¶
Structure containing most parameters to configure a sub-interpreter. Its values are used only in
Py_NewInterpreterFromConfig()
and never modified by the runtime.Added in version 3.12.
Structure fields:
-
int use_main_obmalloc¶
If this is
0
then the sub-interpreter will use its own “object” allocator state. Otherwise it will use (share) the main interpreter’s.If this is
0
thencheck_multi_interp_extensions
must be1
(non-zero). If this is1
thengil
must not bePyInterpreterConfig_OWN_GIL
.
-
int allow_fork¶
If this is
0
then the runtime will not support forking the process in any thread where the sub-interpreter is currently active. Otherwise fork is unrestricted.Note that the
subprocess
module still works when fork is disallowed.
-
int allow_exec¶
If this is
0
then the runtime will not support replacing the current process via exec (e.g.os.execv()
) in any thread where the sub-interpreter is currently active. Otherwise exec is unrestricted.Note that the
subprocess
module still works when exec is disallowed.
-
int allow_threads¶
If this is
0
then the sub-interpreter’sthreading
module won’t create threads. Otherwise threads are allowed.
-
int allow_daemon_threads¶
If this is
0
then the sub-interpreter’sthreading
module won’t create daemon threads. Otherwise daemon threads are allowed (as long asallow_threads
is non-zero).
-
int check_multi_interp_extensions¶
If this is
0
then all extension modules may be imported, including legacy (single-phase init) modules, in any thread where the sub-interpreter is currently active. Otherwise only multi-phase init extension modules (see PEP 489) may be imported. (Also seePy_mod_multiple_interpreters
.)This must be
1
(non-zero) ifuse_main_obmalloc
is0
.
-
int gil¶
This determines the operation of the GIL for the sub-interpreter. It may be one of the following:
-
PyInterpreterConfig_DEFAULT_GIL¶
Use the default selection (
PyInterpreterConfig_SHARED_GIL
).
-
PyInterpreterConfig_SHARED_GIL¶
Use (share) the main interpreter’s GIL.
-
PyInterpreterConfig_OWN_GIL¶
Use the sub-interpreter’s own GIL.
If this is
PyInterpreterConfig_OWN_GIL
thenPyInterpreterConfig.use_main_obmalloc
must be0
.-
PyInterpreterConfig_DEFAULT_GIL¶
-
int use_main_obmalloc¶
-
PyStatus Py_NewInterpreterFromConfig(PyThreadState **tstate_p, const PyInterpreterConfig *config)¶
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__
andsys
. The table of loaded modules (sys.modules
) and the module search path (sys.path
) are also separate. The new environment has nosys.argv
variable. It has new standard I/O stream file objectssys.stdin
,sys.stdout
andsys.stderr
(however these refer to the same underlying file descriptors).The given config controls the options with which the interpreter is initialized.
Upon success, tstate_p will be set to the first thread state created in the new sub-interpreter. This thread state is attached. Note that no actual thread is created; see the discussion of thread states below. If creation of the new interpreter is unsuccessful, tstate_p is set to
NULL
; no exception is set since the exception state is stored in the attached thread state, which might not exist.Like all other Python/C API functions, an attached thread state must be present before calling this function, but it might be detached upon returning. On success, the returned thread state will be attached. If the sub-interpreter is created with its own GIL then the attached thread state of the calling interpreter will be detached. When the function returns, the new interpreter’s thread state will be attached to the current thread and the previous interpreter’s attached thread state will remain detached.
Added in version 3.12.
Sub-interpreters are most effective when isolated from each other, with certain functionality restricted:
PyInterpreterConfig config = { .use_main_obmalloc = 0, .allow_fork = 0, .allow_exec = 0, .allow_threads = 1, .allow_daemon_threads = 0, .check_multi_interp_extensions = 1, .gil = PyInterpreterConfig_OWN_GIL, }; PyThreadState *tstate = NULL; PyStatus status = Py_NewInterpreterFromConfig(&tstate, &config); if (PyStatus_Exception(status)) { Py_ExitStatusException(status); }
Note that the config is used only briefly and does not get modified. During initialization the config’s values are converted into various
PyInterpreterState
values. A read-only copy of the config may be stored internally on thePyInterpreterState
.Extension modules are shared between (sub-)interpreters as follows:
For modules using multi-phase initialization, e.g.
PyModule_FromDefAndSpec()
, a separate module object is created and initialized for each interpreter. Only C-level static and global variables are shared between these module objects.For modules using single-phase initialization, e.g.
PyModule_Create()
, the first time a particular extension is imported, it is initialized normally, and a (shallow) copy of its 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’sinit
function is not called. Objects in the module’s dictionary thus end up shared across (sub-)interpreters, which might cause unwanted behavior (see Bugs and caveats below).Note that this is different from what happens when an extension is imported after the interpreter has been completely re-initialized by calling
Py_FinalizeEx()
andPy_Initialize()
; in that case, the extension’sinitmodule
function is called again. As with multi-phase initialization, this means that only C-level static and global variables are shared between these modules.
-
PyThreadState *Py_NewInterpreter(void)¶
- Part of the Stable ABI.
Create a new sub-interpreter. This is essentially just a wrapper around
Py_NewInterpreterFromConfig()
with a config that preserves the existing behavior. The result is an unisolated sub-interpreter that shares the main interpreter’s GIL, allows fork/exec, allows daemon threads, and allows single-phase init modules.
-
void Py_EndInterpreter(PyThreadState *tstate)¶
- Part of the Stable ABI.
Destroy the (sub-)interpreter represented by the given thread state. The given thread state must be attached. When the call returns, there will be no attached thread state. All thread states associated with this interpreter are destroyed.
Py_FinalizeEx()
will destroy all sub-interpreters that haven’t been explicitly destroyed at that point.
A Per-Interpreter GIL¶
Using Py_NewInterpreterFromConfig()
you can create
a sub-interpreter that is completely isolated from other interpreters,
including having its own GIL. The most important benefit of this
isolation is that such an interpreter can execute Python code without
being blocked by other interpreters or blocking any others. Thus a
single Python process can truly take advantage of multiple CPU cores
when running Python code. The isolation also encourages a different
approach to concurrency than that of just using threads.
(See PEP 554 and PEP 684.)
Using an isolated interpreter requires vigilance in preserving that
isolation. That especially means not sharing any objects or mutable
state without guarantees about thread-safety. Even objects that are
otherwise immutable (e.g. None
, (1, 5)
) can’t normally be shared
because of the refcount. One simple but less-efficient approach around
this is to use a global lock around all use of some state (or object).
Alternately, effectively immutable objects (like integers or strings)
can be made safe in spite of their refcounts by making them immortal.
In fact, this has been done for the builtin singletons, small integers,
and a number of other builtin objects.
If you preserve isolation then you will have access to proper multi-core computing without the complications that come with free-threading. Failure to preserve isolation will expose you to the full consequences of free-threading, including races and hard-to-debug crashes.
Aside from that, one of the main challenges of using multiple isolated interpreters is how to communicate between them safely (not break isolation) and efficiently. The runtime and stdlib do not provide any standard approach to this yet. A future stdlib module would help mitigate the effort of preserving isolation and expose effective tools for communicating (and sharing) data between interpreters.
Added in version 3.12.
Bugs and caveats¶
Because sub-interpreters (and the main interpreter) are part of the same
process, the insulation between them isn’t perfect — for example, using
low-level file operations like os.close()
they can
(accidentally or maliciously) affect each other’s open files. Because of the
way extensions are shared between (sub-)interpreters, some extensions may not
work properly; this is especially likely when using single-phase initialization
or (static) global variables.
It is possible to insert objects created in one sub-interpreter into
a namespace of another (sub-)interpreter; this should be avoided if possible.
Special care should be taken to avoid sharing user-defined functions, methods, instances or classes between sub-interpreters, since import operations executed by such objects may affect the wrong (sub-)interpreter’s dictionary of loaded modules. It is equally important to avoid sharing objects from which the above are reachable.
Also note that combining this functionality with PyGILState_*
APIs
is delicate, because these APIs assume a bijection between Python thread states
and OS-level threads, an assumption broken by the presence of sub-interpreters.
It is highly recommended that you don’t switch sub-interpreters between a pair
of matching 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)¶
- Part of the Stable ABI.
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 an attached thread state (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 thread state is detached.This function doesn’t need an attached thread state. However, to call this function in a subinterpreter, the caller must have an attached thread state. Otherwise, the function func can be scheduled to be called from the wrong interpreter.
Warning
This is a low-level function, only useful for very special cases. There is no guarantee that func will be called as quick as possible. If the main thread is busy executing a system call, func won’t be called before the system call returns. This function is generally not suitable for calling Python code from arbitrary C threads. Instead, use the PyGILState API.
Added in version 3.1.
Changed in version 3.9: If this function is called in a subinterpreter, the function func is now scheduled to be called from the subinterpreter, rather than being called from the main interpreter. Each subinterpreter now has its own list of scheduled calls.
Changed in version 3.12: This function now always schedules func to be run in the main interpreter.
-
int Py_MakePendingCalls(void)¶
- Part of the Stable ABI.
Execute all pending calls. This is usually executed automatically by the interpreter.
This function returns
0
on success, and returns-1
with an exception set on failure.If this is not called in the main thread of the main interpreter, this function does nothing and returns
0
. The caller must hold an attached thread state.Added in version 3.1.
Changed in version 3.12: This function only runs pending calls in the main interpreter.
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.
-
typedef int (*Py_tracefunc)(PyObject *obj, PyFrameObject *frame, int what, PyObject *arg)¶
The type of the trace function registered using
PyEval_SetProfile()
andPyEval_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 constantsPyTrace_CALL
,PyTrace_EXCEPTION
,PyTrace_LINE
,PyTrace_RETURN
,PyTrace_C_CALL
,PyTrace_C_EXCEPTION
,PyTrace_C_RETURN
, orPyTrace_OPCODE
, and arg depends on the value of what:Value of what
Meaning of arg
Always
Py_None
.Exception information as returned by
sys.exc_info()
.Always
Py_None
.Value being returned to the caller, or
NULL
if caused by an exception.Function object being called.
Function object being called.
Function object being called.
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 receive 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 settingf_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 settingf_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 exceptPyTrace_LINE
PyTrace_OPCODE
andPyTrace_EXCEPTION
.See also the
sys.setprofile()
function.The caller must have an attached thread state.
-
void PyEval_SetProfileAllThreads(Py_tracefunc func, PyObject *obj)¶
Like
PyEval_SetProfile()
but sets the profile function in all running threads belonging to the current interpreter instead of the setting it only on the current thread.The caller must have an attached thread state.
As
PyEval_SetProfile()
, this function ignores any exceptions raised while setting the profile functions in all threads.
Added in version 3.12.
-
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 usingPyEval_SetTrace()
will not receivePyTrace_C_CALL
,PyTrace_C_EXCEPTION
orPyTrace_C_RETURN
as a value for the what parameter.See also the
sys.settrace()
function.The caller must have an attached thread state.
-
void PyEval_SetTraceAllThreads(Py_tracefunc func, PyObject *obj)¶
Like
PyEval_SetTrace()
but sets the tracing function in all running threads belonging to the current interpreter instead of the setting it only on the current thread.The caller must have an attached thread state.
As
PyEval_SetTrace()
, this function ignores any exceptions raised while setting the trace functions in all threads.
Added in version 3.12.
Reference tracing¶
Added in version 3.13.
-
typedef int (*PyRefTracer)(PyObject*, int event, void *data)¶
The type of the trace function registered using
PyRefTracer_SetTracer()
. The first parameter is a Python object that has been just created (when event is set toPyRefTracer_CREATE
) or about to be destroyed (when event is set toPyRefTracer_DESTROY
). The data argument is the opaque pointer that was provided whenPyRefTracer_SetTracer()
was called.
Added in version 3.13.
-
int PyRefTracer_CREATE¶
The value for the event parameter to
PyRefTracer
functions when a Python object has been created.
-
int PyRefTracer_DESTROY¶
The value for the event parameter to
PyRefTracer
functions when a Python object has been destroyed.
-
int PyRefTracer_SetTracer(PyRefTracer tracer, void *data)¶
Register a reference tracer function. The function will be called when a new Python has been created or when an object is going to be destroyed. If data is provided it must be an opaque pointer that will be provided when the tracer function is called. Return
0
on success. Set an exception and return-1
on error.Note that tracer functions must not create Python objects inside or otherwise the call will be re-entrant. The tracer also must not clear any existing exception or set an exception. A thread state will be active every time the tracer function is called.
There must be an attached thread state when calling this function.
Added in version 3.13.
-
PyRefTracer PyRefTracer_GetTracer(void **data)¶
Get the registered reference tracer function and the value of the opaque data pointer that was registered when
PyRefTracer_SetTracer()
was called. If no tracer was registered this function will return NULL and will set the data pointer to NULL.There must be an attached thread state when calling this function.
Added in version 3.13.
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.
A thread state does not need to be attached when calling these functions; they suppl 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.
Added in version 3.7.
See also
“A New C-API for Thread-Local Storage in CPython” (PEP 539)
-
type 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()¶
- Part of the Stable ABI since version 3.7.
Return a value which is the same state as a value initialized with
Py_tss_NEEDS_INIT
, orNULL
in the case of dynamic allocation failure.
-
void PyThread_tss_free(Py_tss_t *key)¶
- Part of the Stable ABI since version 3.7.
Free the given key allocated by
PyThread_tss_alloc()
, after first callingPyThread_tss_delete()
to ensure any associated thread locals have been unassigned. This is a no-op if the key argument isNULL
.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)¶
- Part of the Stable ABI since version 3.7.
Return a non-zero value if the given
Py_tss_t
has been initialized byPyThread_tss_create()
.
-
int PyThread_tss_create(Py_tss_t *key)¶
- Part of the Stable ABI since version 3.7.
Return a zero value on successful initialization of a TSS key. The behavior is undefined if the value pointed to by the key argument is not initialized by
Py_tss_NEEDS_INIT
. This function can be called repeatedly on the same key – calling it on an already initialized key is a no-op and immediately returns success.
-
void PyThread_tss_delete(Py_tss_t *key)¶
- Part of the Stable ABI since version 3.7.
Destroy a TSS key to forget the values associated with the key across all threads, and change the key’s initialization state to uninitialized. A destroyed key is able to be initialized again by
PyThread_tss_create()
. This function can be called repeatedly on the same key – calling it on an already destroyed key is a no-op.
-
int PyThread_tss_set(Py_tss_t *key, void *value)¶
- Part of the Stable ABI since version 3.7.
Return a zero value to indicate successfully associating a void* value with a TSS key in the current thread. Each thread has a distinct mapping of the key to a void* value.
-
void *PyThread_tss_get(Py_tss_t *key)¶
- Part of the Stable ABI since version 3.7.
Return the void* value associated with a TSS key in the current thread. This returns
NULL
if no value is associated with the key in the current thread.
Thread Local Storage (TLS) API¶
Deprecated since version 3.7: This API is superseded by 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()¶
- Part of the Stable ABI.
-
void PyThread_delete_key(int key)¶
- Part of the Stable ABI.
-
int PyThread_set_key_value(int key, void *value)¶
- Part of the Stable ABI.
-
void *PyThread_get_key_value(int key)¶
- Part of the Stable ABI.
-
void PyThread_delete_key_value(int key)¶
- Part of the Stable ABI.
-
void PyThread_ReInitTLS()¶
- Part of the Stable ABI.
Synchronization Primitives¶
The C-API provides a basic mutual exclusion lock.
-
type PyMutex¶
A mutual exclusion lock. The
PyMutex
should be initialized to zero to represent the unlocked state. For example:PyMutex mutex = {0};
Instances of
PyMutex
should not be copied or moved. Both the contents and address of aPyMutex
are meaningful, and it must remain at a fixed, writable location in memory.Note
A
PyMutex
currently occupies one byte, but the size should be considered unstable. The size may change in future Python releases without a deprecation period.Added in version 3.13.
-
void PyMutex_Lock(PyMutex *m)¶
Lock mutex m. If another thread has already locked it, the calling thread will block until the mutex is unlocked. While blocked, the thread will temporarily detach the thread state if one exists.
Added in version 3.13.
-
void PyMutex_Unlock(PyMutex *m)¶
Unlock mutex m. The mutex must be locked — otherwise, the function will issue a fatal error.
Added in version 3.13.
-
int PyMutex_IsLocked(PyMutex *m)¶
Returns non-zero if the mutex m is currently locked, zero otherwise.
Note
This function is intended for use in assertions and debugging only and should not be used to make concurrency control decisions, as the lock state may change immediately after the check.
Added in version 3.14.
Python Critical Section API¶
The critical section API provides a deadlock avoidance layer on top of per-object locks for free-threaded CPython. They are intended to replace reliance on the global interpreter lock, and are no-ops in versions of Python with the global interpreter lock.
Critical sections are intended to be used for custom types implemented
in C-API extensions. They should generally not be used with built-in types like
list
and dict
because their public C-APIs
already use critical sections internally, with the notable
exception of PyDict_Next()
, which requires critical section
to be acquired externally.
Critical sections avoid deadlocks by implicitly suspending active critical
sections, hence, they do not provide exclusive access such as provided by
traditional locks like PyMutex
. When a critical section is started,
the per-object lock for the object is acquired. If the code executed inside the
critical section calls C-API functions then it can suspend the critical section thereby
releasing the per-object lock, so other threads can acquire the per-object lock
for the same object.
Variants that accept PyMutex
pointers rather than Python objects are also
available. Use these variants to start a critical section in a situation where
there is no PyObject
– for example, when working with a C type that
does not extend or wrap PyObject
but still needs to call into the C
API in a manner that might lead to deadlocks.
The functions and structs used by the macros are exposed for cases where C macros are not available. They should only be used as in the given macro expansions. Note that the sizes and contents of the structures may change in future Python versions.
Note
Operations that need to lock two objects at once must use
Py_BEGIN_CRITICAL_SECTION2
. You cannot use nested critical
sections to lock more than one object at once, because the inner critical
section may suspend the outer critical sections. This API does not provide
a way to lock more than two objects at once.
Example usage:
static PyObject *
set_field(MyObject *self, PyObject *value)
{
Py_BEGIN_CRITICAL_SECTION(self);
Py_SETREF(self->field, Py_XNewRef(value));
Py_END_CRITICAL_SECTION();
Py_RETURN_NONE;
}
In the above example, Py_SETREF
calls Py_DECREF
, which
can call arbitrary code through an object’s deallocation function. The critical
section API avoids potential deadlocks due to reentrancy and lock ordering
by allowing the runtime to temporarily suspend the critical section if the
code triggered by the finalizer blocks and calls PyEval_SaveThread()
.
-
Py_BEGIN_CRITICAL_SECTION(op)¶
Acquires the per-object lock for the object op and begins a critical section.
In the free-threaded build, this macro expands to:
{ PyCriticalSection _py_cs; PyCriticalSection_Begin(&_py_cs, (PyObject*)(op))
In the default build, this macro expands to
{
.Added in version 3.13.
-
Py_BEGIN_CRITICAL_SECTION_MUTEX(m)¶
Locks the mutex m and begins a critical section.
In the free-threaded build, this macro expands to:
{ PyCriticalSection _py_cs; PyCriticalSection_BeginMutex(&_py_cs, m)
Note that unlike
Py_BEGIN_CRITICAL_SECTION
, there is no cast for the argument of the macro - it must be aPyMutex
pointer.On the default build, this macro expands to
{
.Added in version 3.14.
-
Py_END_CRITICAL_SECTION()¶
Ends the critical section and releases the per-object lock.
In the free-threaded build, this macro expands to:
PyCriticalSection_End(&_py_cs); }
In the default build, this macro expands to
}
.Added in version 3.13.
-
Py_BEGIN_CRITICAL_SECTION2(a, b)¶
Acquires the per-objects locks for the objects a and b and begins a critical section. The locks are acquired in a consistent order (lowest address first) to avoid lock ordering deadlocks.
In the free-threaded build, this macro expands to:
{ PyCriticalSection2 _py_cs2; PyCriticalSection2_Begin(&_py_cs2, (PyObject*)(a), (PyObject*)(b))
In the default build, this macro expands to
{
.Added in version 3.13.
-
Py_BEGIN_CRITICAL_SECTION2_MUTEX(m1, m2)¶
Locks the mutexes m1 and m2 and begins a critical section.
In the free-threaded build, this macro expands to:
{ PyCriticalSection2 _py_cs2; PyCriticalSection2_BeginMutex(&_py_cs2, m1, m2)
Note that unlike
Py_BEGIN_CRITICAL_SECTION2
, there is no cast for the arguments of the macro - they must bePyMutex
pointers.On the default build, this macro expands to
{
.Added in version 3.14.
-
Py_END_CRITICAL_SECTION2()¶
Ends the critical section and releases the per-object locks.
In the free-threaded build, this macro expands to:
PyCriticalSection2_End(&_py_cs2); }
In the default build, this macro expands to
}
.Added in version 3.13.
Legacy Locking APIs¶
These APIs are obsolete since Python 3.13 with the introduction of
PyMutex
.
Changed in version 3.15: These APIs are now a simple wrapper around PyMutex
.
-
type PyThread_type_lock¶
A pointer to a mutual exclusion lock.
-
type PyLockStatus¶
The result of acquiring a lock with a timeout.
-
enumerator PY_LOCK_FAILURE¶
Failed to acquire the lock.
-
enumerator PY_LOCK_ACQUIRED¶
The lock was successfully acquired.
-
enumerator PY_LOCK_INTR¶
The lock was interrupted by a signal.
-
enumerator PY_LOCK_FAILURE¶
-
PyThread_type_lock PyThread_allocate_lock(void)¶
- Part of the Stable ABI.
Allocate a new lock.
On success, this function returns a lock; on failure, this function returns
0
without an exception set.The caller does not need to hold an attached thread state.
Changed in version 3.15: This function now always uses
PyMutex
. In prior versions, this would use a lock provided by the operating system.
-
void PyThread_free_lock(PyThread_type_lock lock)¶
- Part of the Stable ABI.
Destroy lock. The lock should not be held by any thread when calling this.
The caller does not need to hold an attached thread state.
-
PyLockStatus PyThread_acquire_lock_timed(PyThread_type_lock lock, long long microseconds, int intr_flag)¶
- Part of the Stable ABI.
Acquire lock with a timeout.
This will wait for microseconds microseconds to acquire the lock. If the timeout expires, this function returns
PY_LOCK_FAILURE
. If microseconds is-1
, this will wait indefinitely until the lock has been released.If intr_flag is
1
, acquiring the lock may be interrupted by a signal, in which case this function returnsPY_LOCK_INTR
. Upon interruption, it’s generally expected that the caller makes a call toPy_MakePendingCalls()
to propagate an exception to Python code.If the lock is successfully acquired, this function returns
PY_LOCK_ACQUIRED
.The caller does not need to hold an attached thread state.
-
int PyThread_acquire_lock(PyThread_type_lock lock, int waitflag)¶
- Part of the Stable ABI.
Acquire lock.
If waitflag is
1
and another thread currently holds the lock, this function will wait until the lock can be acquired and will always return1
.If waitflag is
0
and another thread holds the lock, this function will not wait and instead return0
. If the lock is not held by any other thread, then this function will acquire it and return1
.Unlike
PyThread_acquire_lock_timed()
, acquiring the lock cannot be interrupted by a signal.The caller does not need to hold an attached thread state.
-
int PyThread_release_lock(PyThread_type_lock lock)¶
- Part of the Stable ABI.
Release lock. If lock is not held, then this function issues a fatal error.
The caller does not need to hold an attached thread state.
Operating System Thread APIs¶
-
PYTHREAD_INVALID_THREAD_ID¶
Sentinel value for an invalid thread ID.
This is currently equivalent to
(unsigned long)-1
.
-
unsigned long PyThread_start_new_thread(void (*func)(void*), void *arg)¶
- Part of the Stable ABI.
Start function func in a new thread with argument arg. The resulting thread is not intended to be joined.
func must not be
NULL
, but arg may beNULL
.On success, this function returns the identifier of the new thread; on failure, this returns
PYTHREAD_INVALID_THREAD_ID
.The caller does not need to hold an attached thread state.
-
unsigned long PyThread_get_thread_ident(void)¶
- Part of the Stable ABI.
Return the identifier of the current thread, which will never be zero.
This function cannot fail, and the caller does not need to hold an attached thread state.
See also
-
PyObject *PyThread_GetInfo(void)¶
- Part of the Stable ABI since version 3.3.
Get general information about the current thread in the form of a struct sequence object. This information is accessible as
sys.thread_info
in Python.On success, this returns a new strong reference to the thread information; on failure, this returns
NULL
with an exception set.The caller must hold an attached thread state.
-
PY_HAVE_THREAD_NATIVE_ID¶
This macro is defined when the system supports native thread IDs.
-
unsigned long PyThread_get_thread_native_id(void)¶
- Part of the Stable ABI on platforms with native thread IDs.
Get the native identifier of the current thread as it was assigned by the operating system’s kernel, which will never be less than zero.
This function is only available when
PY_HAVE_THREAD_NATIVE_ID
is defined.This function cannot fail, and the caller does not need to hold an attached thread state.
See also
-
void PyThread_exit_thread(void)¶
- Part of the Stable ABI.
Terminate the current thread. This function is generally considered unsafe and should be avoided. It is kept solely for backwards compatibility.
This function is only safe to call if all functions in the full call stack are written to safely allow it.
Warning
If the current system uses POSIX threads (also known as “pthreads”), this calls pthread_exit(3), which attempts to unwind the stack and call C++ destructors on some libc implementations. However, if a
noexcept
function is reached, it may terminate the process. Other systems, such as macOS, do unwinding.On Windows, this function calls
_endthreadex()
, which kills the thread without calling C++ destructors.In any case, there is a risk of corruption on the thread’s stack.
Deprecated since version 3.14.
-
void PyThread_init_thread(void)¶
- Part of the Stable ABI.
Initialize
PyThread*
APIs. Python executes this function automatically, so there’s little need to call it from an extension module.
-
int PyThread_set_stacksize(size_t size)¶
- Part of the Stable ABI.
Set the stack size of the current thread to size bytes.
This function returns
0
on success,-1
if size is invalid, or-2
if the system does not support changing the stack size. This function does not set exceptions.The caller does not need to hold an attached thread state.
-
size_t PyThread_get_stacksize(void)¶
- Part of the Stable ABI.
Return the stack size of the current thread in bytes, or
0
if the system’s default stack size is in use.The caller does not need to hold an attached thread state.


— Interface to shell pipelines

pipes
— Interface to shell pipelines¶
Deprecated since version 3.11, removed in version 3.13.
This module is no longer part of the Python standard library. It was removed in Python 3.13 after being deprecated in Python 3.11. The removal was decided in PEP 594.
Applications should use the subprocess
module instead.
The last version of Python that provided the pipes
module was
Python 3.12.