Doc/c-api/interp-lifecycle.rst
.. highlight:: c
.. _initialization:
See :ref:Python Initialization Configuration <init-config> for details
on how to configure the interpreter prior to initialization.
.. _pre-init-safe:
In an application embedding Python, the :c:func:Py_Initialize function must
be called before using any other Python/C API functions; with the exception of
a few functions and the :ref:global configuration variables <global-conf-vars>.
The following functions can be safely called before Python is initialized:
Functions that initialize the interpreter:
Py_InitializePy_InitializeExPy_InitializeFromConfigPy_BytesMainPy_Maininit-configConfiguration functions:
PyImport_AppendInittabPyImport_ExtendInittab!PyInitFrozenExtensionsPyMem_SetAllocatorPyMem_SetupDebugHooksPyObject_SetArenaAllocatorPy_SetProgramNamePy_SetPythonHomeinit-configInformative functions:
Py_IsInitializedPyMem_GetAllocatorPyObject_GetArenaAllocatorPy_GetBuildInfoPy_GetCompilerPy_GetCopyrightPy_GetPlatformPy_GetVersionPy_IsInitializedUtilities:
Py_DecodeLocaleinit-configMemory allocators:
PyMem_RawMallocPyMem_RawReallocPyMem_RawCallocPyMem_RawFreeSynchronization:
PyMutex_LockPyMutex_Unlock.. 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: :c:func:Py_EncodeLocale, :c:func:PyEval_InitThreads, and
:c:func:Py_RunMain.
.. _global-conf-vars:
Python has variables for the global configuration to control different features
and options. By default, these flags are controlled by :ref:command line options <using-on-interface-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 :c:data:Py_BytesWarningFlag
to 1 and -bb sets :c:data:Py_BytesWarningFlag to 2.
.. c:var:: int Py_BytesWarningFlag
This API is kept for backward compatibility: setting
:c:member:PyConfig.bytes_warning should be used instead, see :ref:Python Initialization Configuration <init-config>.
Issue a warning when comparing :class:bytes or :class:bytearray with
:class:str or :class:bytes with :class:int. Issue an error if greater
or equal to 2.
Set by the :option:-b option.
.. deprecated-removed:: 3.12 3.15
.. c:var:: int Py_DebugFlag
This API is kept for backward compatibility: setting
:c:member:PyConfig.parser_debug should be used instead, see :ref:Python Initialization Configuration <init-config>.
Turn on parser debugging output (for expert only, depending on compilation options).
Set by the :option:-d option and the :envvar:PYTHONDEBUG environment
variable.
.. deprecated-removed:: 3.12 3.15
.. c:var:: int Py_DontWriteBytecodeFlag
This API is kept for backward compatibility: setting
:c:member:PyConfig.write_bytecode should be used instead, see :ref:Python Initialization Configuration <init-config>.
If set to non-zero, Python won't try to write .pyc files on the
import of source modules.
Set by the :option:-B option and the :envvar:PYTHONDONTWRITEBYTECODE
environment variable.
.. deprecated-removed:: 3.12 3.15
.. c:var:: int Py_FrozenFlag
This API is kept for backward compatibility: setting
:c:member:PyConfig.pathconfig_warnings should be used instead, see
:ref:Python Initialization Configuration <init-config>.
Private flag used by _freeze_module and frozenmain programs.
.. deprecated-removed:: 3.12 3.15
.. c:var:: int Py_HashRandomizationFlag
This API is kept for backward compatibility: setting
:c:member:PyConfig.hash_seed and :c:member:PyConfig.use_hash_seed should
be used instead, see :ref:Python Initialization Configuration <init-config>.
Set to 1 if the :envvar:PYTHONHASHSEED environment variable is set to
a non-empty string.
If the flag is non-zero, read the :envvar:PYTHONHASHSEED environment
variable to initialize the secret hash seed.
.. deprecated-removed:: 3.12 3.15
.. c:var:: int Py_IgnoreEnvironmentFlag
This API is kept for backward compatibility: setting
:c:member:PyConfig.use_environment should be used instead, see
:ref:Python Initialization Configuration <init-config>.
Ignore all :envvar:!PYTHON* environment variables, e.g.
:envvar:PYTHONPATH and :envvar:PYTHONHOME, that might be set.
Set by the :option:-E and :option:-I options.
.. deprecated-removed:: 3.12 3.15
.. c:var:: int Py_InspectFlag
This API is kept for backward compatibility: setting
:c:member:PyConfig.inspect should be used instead, see
:ref:Python Initialization Configuration <init-config>.
When a script is passed as first argument or the :option:-c option is used,
enter interactive mode after executing the script or the command, even when
:data:sys.stdin does not appear to be a terminal.
Set by the :option:-i option and the :envvar:PYTHONINSPECT environment
variable.
.. deprecated-removed:: 3.12 3.15
.. c:var:: int Py_InteractiveFlag
This API is kept for backward compatibility: setting
:c:member:PyConfig.interactive should be used instead, see
:ref:Python Initialization Configuration <init-config>.
Set by the :option:-i option.
.. deprecated-removed:: 3.12 3.15
.. c:var:: int Py_IsolatedFlag
This API is kept for backward compatibility: setting
:c:member:PyConfig.isolated should be used instead, see
:ref:Python Initialization Configuration <init-config>.
Run Python in isolated mode. In isolated mode :data:sys.path contains
neither the script's directory nor the user's site-packages directory.
Set by the :option:-I option.
.. versionadded:: 3.4
.. deprecated-removed:: 3.12 3.15
.. c:var:: int Py_LegacyWindowsFSEncodingFlag
This API is kept for backward compatibility: setting
:c:member:PyPreConfig.legacy_windows_fs_encoding should be used instead, see
:ref:Python Initialization Configuration <init-config>.
If the flag is non-zero, use the mbcs encoding with replace error
handler, instead of the UTF-8 encoding with surrogatepass error handler,
for the :term:filesystem encoding and error handler.
Set to 1 if the :envvar:PYTHONLEGACYWINDOWSFSENCODING environment
variable is set to a non-empty string.
See :pep:529 for more details.
.. availability:: Windows.
.. deprecated-removed:: 3.12 3.15
.. c:var:: int Py_LegacyWindowsStdioFlag
This API is kept for backward compatibility: setting
:c:member:PyConfig.legacy_windows_stdio should be used instead, see
:ref:Python Initialization Configuration <init-config>.
If the flag is non-zero, use :class:io.FileIO instead of
:class:!io._WindowsConsoleIO for :mod:sys standard streams.
Set to 1 if the :envvar:PYTHONLEGACYWINDOWSSTDIO environment
variable is set to a non-empty string.
See :pep:528 for more details.
.. availability:: Windows.
.. deprecated-removed:: 3.12 3.15
.. c:var:: int Py_NoSiteFlag
This API is kept for backward compatibility: setting
:c:member:PyConfig.site_import should be used instead, see
:ref:Python Initialization Configuration <init-config>.
Disable the import of the module :mod:site and the site-dependent
manipulations of :data:sys.path that it entails. Also disable these
manipulations if :mod:site is explicitly imported later (call
:func:site.main if you want them to be triggered).
Set by the :option:-S option.
.. deprecated-removed:: 3.12 3.15
.. c:var:: int Py_NoUserSiteDirectory
This API is kept for backward compatibility: setting
:c:member:PyConfig.user_site_directory should be used instead, see
:ref:Python Initialization Configuration <init-config>.
Don't add the :data:user site-packages directory <site.USER_SITE> to
:data:sys.path.
Set by the :option:-s and :option:-I options, and the
:envvar:PYTHONNOUSERSITE environment variable.
.. deprecated-removed:: 3.12 3.15
.. c:var:: int Py_OptimizeFlag
This API is kept for backward compatibility: setting
:c:member:PyConfig.optimization_level should be used instead, see
:ref:Python Initialization Configuration <init-config>.
Set by the :option:-O option and the :envvar:PYTHONOPTIMIZE environment
variable.
.. deprecated-removed:: 3.12 3.15
.. c:var:: int Py_QuietFlag
This API is kept for backward compatibility: setting
:c:member:PyConfig.quiet should be used instead, see :ref:Python Initialization Configuration <init-config>.
Don't display the copyright and version messages even in interactive mode.
Set by the :option:-q option.
.. versionadded:: 3.2
.. deprecated-removed:: 3.12 3.15
.. c:var:: int Py_UnbufferedStdioFlag
This API is kept for backward compatibility: setting
:c:member:PyConfig.buffered_stdio should be used instead, see :ref:Python Initialization Configuration <init-config>.
Force the stdout and stderr streams to be unbuffered.
Set by the :option:-u option and the :envvar:PYTHONUNBUFFERED
environment variable.
.. deprecated-removed:: 3.12 3.15
.. c:var:: int Py_VerboseFlag
This API is kept for backward compatibility: setting
:c:member:PyConfig.verbose should be used instead, see :ref:Python Initialization Configuration <init-config>.
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 :option:-v option and the :envvar:PYTHONVERBOSE environment
variable.
.. deprecated-removed:: 3.12 3.15
.. c:function:: void Py_Initialize()
.. index:: single: PyEval_InitThreads() single: modules (in module sys) single: path (in module sys) pair: module; builtins pair: module; main pair: module; sys triple: module; search; path single: Py_FinalizeEx (C function)
Initialize the Python interpreter. In an application embedding Python,
this should be called before using any other Python/C API functions; see
:ref:Before Python Initialization <pre-init-safe> for the few exceptions.
This initializes the table of loaded modules (sys.modules), and creates
the fundamental modules :mod:builtins, :mod:__main__ and :mod:sys.
It also initializes the module search path (sys.path). It does not set
sys.argv; use the :ref:Python Initialization Configuration <init-config>
API for that. This is a no-op when called for a second time (without calling
:c:func:Py_FinalizeEx first). There is no return value; it is a fatal
error if the initialization fails.
Use :c:func:Py_InitializeFromConfig to customize the
:ref:Python Initialization Configuration <init-config>.
.. note::
On Windows, changes the console mode from O_TEXT to O_BINARY,
which will also affect non-Python uses of the console using the C Runtime.
.. c:function:: void Py_InitializeEx(int initsigs)
This function works like :c:func:Py_Initialize if initsigs is 1. If
initsigs is 0, it skips initialization registration of signal handlers,
which may be useful when CPython is embedded as part of a larger application.
Use :c:func:Py_InitializeFromConfig to customize the
:ref:Python Initialization Configuration <init-config>.
.. c:function:: PyStatus Py_InitializeFromConfig(const PyConfig *config)
Initialize Python from config configuration, as described in
:ref:init-from-config.
See the :ref:init-config section for details on pre-initializing the
interpreter, populating the runtime configuration structure, and querying
the returned status structure.
.. c:function:: int Py_IsInitialized()
Return true (nonzero) when the Python interpreter has been initialized, false
(zero) if not. After :c:func:Py_FinalizeEx is called, this returns false until
:c:func:Py_Initialize is called again.
.. c:function:: int Py_IsFinalizing()
Return true (non-zero) if the main Python interpreter is
:term:shutting down <interpreter shutdown>. Return false (zero) otherwise.
.. versionadded:: 3.13
.. c:function:: int Py_FinalizeEx()
Undo all initializations made by :c:func:Py_Initialize and subsequent use of
Python/C API functions, and destroy all sub-interpreters (see
:c:func:Py_NewInterpreter below) that were created and not yet destroyed since
the last call to :c:func:Py_Initialize. This is a no-op when called for a second
time (without calling :c:func:Py_Initialize again first).
Since this is the reverse of :c:func: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 while :c:func:Py_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
:c:func: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 (:meth:~object.__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 calls :c:func:Py_Initialize and :c:func:Py_FinalizeEx
more than once. :c:func: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 as :py:mod:atexit
handlers, object finalizers, or any code that may be run while flushing the
stdout and stderr files.
.. audit-event:: cpython._PySys_ClearAuditHooks "" c.Py_FinalizeEx
.. versionadded:: 3.6
.. c:function:: void Py_Finalize()
This is a backwards-compatible version of :c:func:Py_FinalizeEx that
disregards the return value.
.. c:function:: int Py_BytesMain(int argc, char **argv)
Similar to :c:func:Py_Main but argv is an array of bytes strings,
allowing the calling application to delegate the text decoding step to
the CPython runtime.
.. versionadded:: 3.8
.. c:function:: int Py_Main(int argc, wchar_t **argv)
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
:ref:using-on-cmdline.
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 :c:func:main function, except that the argv entries are first
converted to wchar_t using :c:func:Py_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 as :c:func:Py_RunMain.
In terms of the CPython runtime configuration APIs documented in the
:ref:runtime configuration <init-config> 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 :c:func:Py_Initialize, :c:func:Py_InitializeEx or
:c:func:Py_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).
.. c:function:: int Py_RunMain(void)
Executes the main module in a fully configured CPython runtime.
Executes the command (:c:member:PyConfig.run_command), the script
(:c:member:PyConfig.run_filename) or the module
(:c:member: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 :c:member:PyConfig.inspect is not set (the default), the return value
will be 0 if the interpreter exits normally (that is, without raising
an exception), the exit status of an unhandled :exc:SystemExit, or 1
for any other unhandled exception.
If :c:member:PyConfig.inspect is set (such as when the :option:-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 a :exc:SystemExit, as specified above.
This function always finalizes the Python interpreter before it returns.
See :ref:Python Configuration <init-python-config> for an example of a
customized Python that always runs in isolated mode using
:c:func:Py_RunMain.
.. c:function:: int PyUnstable_AtExit(PyInterpreterState *interp, void (*func)(void *), void *data)
Register an :mod:atexit callback for the target interpreter interp.
This is similar to :c:func:Py_AtExit, but takes an explicit interpreter and
data pointer for the callback.
There must be an :term:attached thread state for interp.
.. versionadded:: 3.13
.. _cautions-regarding-runtime-finalization:
In the late stage of :term:interpreter shutdown, after attempting to wait for
non-daemon threads to exit (though this can be interrupted by
:class:KeyboardInterrupt) and running the :mod:atexit functions, the runtime
is marked as finalizing: :c:func:Py_IsFinalizing and
:func:sys.is_finalizing return true. At this point, only the finalization
thread that initiated finalization (typically the main thread) is allowed to
acquire the :term:GIL.
If any thread, other than the finalization thread, attempts to attach a :term: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 :term:thread state C APIs
have never had any error reporting or handling expectations at :term: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.
.. c:function:: void Py_SetProgramName(const wchar_t *name)
.. index:: single: Py_Initialize() single: main()
This API is kept for backward compatibility: setting
:c:member:PyConfig.program_name should be used instead, see :ref:Python Initialization Configuration <init-config>.
This function should be called before :c:func:Py_Initialize is called for
the first time, if it is called at all. It tells the interpreter the value
of the argv[0] argument to the :c:func:main function of the program
(converted to wide characters).
This is used by 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 :c:func:Py_DecodeLocale to decode a bytes string to get a
:c:expr:wchar_t* string.
.. deprecated-removed:: 3.11 3.15
.. c:function:: const char* Py_GetVersion()
Return the version of this Python interpreter. This is a string that looks something like ::
"3.0a5+ (py3k:63103M, May 12 2008, 00:53:55) \n[GCC 4.2.3]"
.. index:: single: version (in module sys)
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 :data:sys.version.
See also the :c:var:Py_Version constant.
.. c:function:: const char* Py_GetPlatform()
.. index:: single: platform (in module sys)
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 as sys.platform.
.. c:function:: const char* Py_GetCopyright()
Return the official copyright string for the current Python version, for example
'Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam'
.. index:: single: copyright (in module sys)
The returned string points into static storage; the caller should not modify its
value. The value is available to Python code as sys.copyright.
.. c:function:: const char* Py_GetCompiler()
Return an indication of the compiler used to build the current Python version, in square brackets, for example::
"[GCC 2.7.2.2]"
.. index:: single: version (in module sys)
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.
.. c:function:: const char* Py_GetBuildInfo()
Return information about the sequence number and build date and time of the current Python interpreter instance, for example ::
"#67, Aug 1 1997, 22:34:28"
.. index:: single: version (in module sys)
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.
.. c:function:: void PySys_SetArgvEx(int argc, wchar_t **argv, int updatepath)
.. index:: single: main() single: Py_FatalError() single: argv (in module sys)
This API is kept for backward compatibility: setting
:c:member:PyConfig.argv, :c:member:PyConfig.parse_argv and
:c:member:PyConfig.safe_path should be used instead, see :ref:Python Initialization Configuration <init-config>.
Set :data:sys.argv based on argc and argv. These parameters are
similar to those passed to the program's :c:func:main function with the
difference that the first entry should refer to the script file to be
executed rather than the executable hosting the Python interpreter. If there
isn't a script that will be run, the first entry in argv can be an empty
string. If this function fails to initialize :data:sys.argv, a fatal
condition is signalled using :c:func:Py_FatalError.
If updatepath is zero, this is all the function does. If updatepath
is non-zero, the function also modifies :data:sys.path according to the
following algorithm:
argv[0], the absolute
path of the directory where the script is located is prepended to
:data:sys.path.0 or argv[0] doesn't point
to an existing file name), an empty string is prepended to
:data:sys.path, which is the same as prepending the current working
directory (".").Use :c:func:Py_DecodeLocale to decode a bytes string to get a
:c:expr:wchar_t* string.
See also :c:member:PyConfig.orig_argv and :c:member:PyConfig.argv
members of the :ref:Python Initialization Configuration <init-config>.
.. note::
It is recommended that applications embedding the Python interpreter
for purposes other than executing a single script pass 0 as updatepath,
and update :data:sys.path themselves if desired.
See :cve:2008-5983.
On versions before 3.1.3, you can achieve the same effect by manually
popping the first :data:`sys.path` element after having called
:c:func:`PySys_SetArgv`, for example using::
PyRun_SimpleString("import sys; sys.path.pop(0)\n");
.. versionadded:: 3.1.3
.. deprecated-removed:: 3.11 3.15
.. c:function:: void PySys_SetArgv(int argc, wchar_t **argv)
This API is kept for backward compatibility: setting
:c:member:PyConfig.argv and :c:member:PyConfig.parse_argv should be used
instead, see :ref:Python Initialization Configuration <init-config>.
This function works like :c:func:PySys_SetArgvEx with updatepath set
to 1 unless the :program:python interpreter was started with the
:option:-I.
Use :c:func:Py_DecodeLocale to decode a bytes string to get a
:c:expr:wchar_t* string.
See also :c:member:PyConfig.orig_argv and :c:member:PyConfig.argv
members of the :ref:Python Initialization Configuration <init-config>.
.. versionchanged:: 3.4 The updatepath value depends on :option:-I.
.. deprecated-removed:: 3.11 3.15
.. c:function:: void Py_SetPythonHome(const wchar_t *home)
This API is kept for backward compatibility: setting
:c:member:PyConfig.home should be used instead, see :ref:Python Initialization Configuration <init-config>.
Set the default "home" directory, that is, the location of the standard
Python libraries. See :envvar: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 :c:func:Py_DecodeLocale to decode a bytes string to get a
:c:expr:wchar_t* string.
.. deprecated-removed:: 3.11 3.15