https://github.com/python/cpython/commit/4a49a11c62ce1832912be3ac672f9539a9918496 commit: 4a49a11c62ce1832912be3ac672f9539a9918496 branch: 3.13 author: Miss Islington (bot) <[email protected]> committer: gpshead <[email protected]> date: 2025-12-15T22:18:57Z summary:
[3.13] gh-85222: Document the global start method side effect in multiprocessing (GH-136426) (#142771) gh-85222: Document the global start method side effect in multiprocessing (GH-136426) * Document the ctx parameter in some types in multiprocessing. * Reduce duplication while still linking to the central explanation from API points with the side effect. (cherry picked from commit 0978b9a7d5eedab19d7d39bd703ed5b55fd7aa5f) Co-authored-by: AN Long <[email protected]> Co-authored-by: Carol Willing <[email protected]> Co-authored-by: R Chintan Meher <[email protected]> files: M Doc/library/multiprocessing.rst diff --git a/Doc/library/multiprocessing.rst b/Doc/library/multiprocessing.rst index 898ab663a6ef02..e923a8a8df5739 100644 --- a/Doc/library/multiprocessing.rst +++ b/Doc/library/multiprocessing.rst @@ -507,6 +507,21 @@ Reference The :mod:`multiprocessing` package mostly replicates the API of the :mod:`threading` module. +.. _global-start-method: + +Global start method +^^^^^^^^^^^^^^^^^^^ + +Python supports several ways to create and initialize a process. +The global start method sets the default mechanism for creating a process. + +Several multiprocessing functions and methods that may also instantiate +certain objects will implicitly set the global start method to the system's default, +if it hasn’t been set already. The global start method can only be set once. +If you need to change the start method from the system default, you must +proactively set the global start method before calling functions or methods, +or creating these objects. + :class:`Process` and exceptions ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ @@ -877,6 +892,9 @@ For an example of the usage of queues for interprocess communication see locks/semaphores. When a process first puts an item on the queue a feeder thread is started which transfers objects from a buffer into the pipe. + Instantiating this class may set the global start method. See + :ref:`global-start-method` for more details. + The usual :exc:`queue.Empty` and :exc:`queue.Full` exceptions from the standard library's :mod:`queue` module are raised to signal timeouts. @@ -992,6 +1010,9 @@ For an example of the usage of queues for interprocess communication see It is a simplified :class:`Queue` type, very close to a locked :class:`Pipe`. + Instantiating this class may set the global start method. See + :ref:`global-start-method` for more details. + .. method:: close() Close the queue: release internal resources. @@ -1022,6 +1043,9 @@ For an example of the usage of queues for interprocess communication see :class:`JoinableQueue`, a :class:`Queue` subclass, is a queue which additionally has :meth:`task_done` and :meth:`join` methods. + Instantiating this class may set the global start method. See + :ref:`global-start-method` for more details. + .. method:: task_done() Indicate that a formerly enqueued task is complete. Used by queue @@ -1134,8 +1158,8 @@ Miscellaneous :mod:`multiprocessing` module. If *method* is ``None`` then the default context is returned. Note that if - the global start method has not been set, this will set it to the - default method. + the global start method has not been set, this will set it to the system default + See :ref:`global-start-method` for more details. Otherwise *method* should be ``'fork'``, ``'spawn'``, ``'forkserver'``. :exc:`ValueError` is raised if the specified start method is not available. See :ref:`multiprocessing-start-methods`. @@ -1146,10 +1170,9 @@ Miscellaneous Return the name of start method used for starting processes. - If the global start method has not been set and *allow_none* is - ``False``, then the start method is set to the default and the name - is returned. If the start method has not been set and *allow_none* is - ``True`` then ``None`` is returned. + If the global start method is not set and *allow_none* is ``False``, the global start + method is set to the default, and its name is returned. See + :ref:`global-start-method` for more details. The return value can be ``'fork'``, ``'spawn'``, ``'forkserver'`` or ``None``. See :ref:`multiprocessing-start-methods`. @@ -1376,6 +1399,9 @@ object -- see :ref:`multiprocessing-managers`. A barrier object: a clone of :class:`threading.Barrier`. + Instantiating this class may set the global start method. See + :ref:`global-start-method` for more details. + .. versionadded:: 3.3 .. class:: BoundedSemaphore([value]) @@ -1383,6 +1409,9 @@ object -- see :ref:`multiprocessing-managers`. A bounded semaphore object: a close analog of :class:`threading.BoundedSemaphore`. + Instantiating this class may set the global start method. See + :ref:`global-start-method` for more details. + A solitary difference from its close analog exists: its ``acquire`` method's first argument is named *block*, as is consistent with :meth:`Lock.acquire`. @@ -1397,6 +1426,9 @@ object -- see :ref:`multiprocessing-managers`. If *lock* is specified then it should be a :class:`Lock` or :class:`RLock` object from :mod:`multiprocessing`. + Instantiating this class may set the global start method. See + :ref:`global-start-method` for more details. + .. versionchanged:: 3.3 The :meth:`~threading.Condition.wait_for` method was added. @@ -1404,6 +1436,8 @@ object -- see :ref:`multiprocessing-managers`. A clone of :class:`threading.Event`. + Instantiating this class may set the global start method. See + :ref:`global-start-method` for more details. .. class:: Lock() @@ -1419,6 +1453,9 @@ object -- see :ref:`multiprocessing-managers`. instance of ``multiprocessing.synchronize.Lock`` initialized with a default context. + Instantiating this class may set the global start method. See + :ref:`global-start-method` for more details. + :class:`Lock` supports the :term:`context manager` protocol and thus may be used in :keyword:`with` statements. @@ -1469,6 +1506,9 @@ object -- see :ref:`multiprocessing-managers`. instance of ``multiprocessing.synchronize.RLock`` initialized with a default context. + Instantiating this class may set the global start method. See + :ref:`global-start-method` for more details. + :class:`RLock` supports the :term:`context manager` protocol and thus may be used in :keyword:`with` statements. @@ -1521,6 +1561,9 @@ object -- see :ref:`multiprocessing-managers`. A semaphore object: a close analog of :class:`threading.Semaphore`. + Instantiating this class may set the global start method. See + :ref:`global-start-method` for more details. + A solitary difference from its close analog exists: its ``acquire`` method's first argument is named *block*, as is consistent with :meth:`Lock.acquire`. @@ -1649,7 +1692,7 @@ processes. attributes which allow one to use it to store and retrieve strings -- see documentation for :mod:`ctypes`. -.. function:: Array(typecode_or_type, size_or_initializer, *, lock=True) +.. function:: Array(typecode_or_type, size_or_initializer, *, lock=True, ctx=None) The same as :func:`RawArray` except that depending on the value of *lock* a process-safe synchronization wrapper may be returned instead of a raw ctypes @@ -1663,9 +1706,13 @@ processes. automatically protected by a lock, so it will not necessarily be "process-safe". - Note that *lock* is a keyword-only argument. + *ctx* is a context object, or ``None`` (use the current context). If ``None``, + calling this may set the global start method. See + :ref:`global-start-method` for more details. -.. function:: Value(typecode_or_type, *args, lock=True) + Note that *lock* and *ctx* are keyword-only parameters. + +.. function:: Value(typecode_or_type, *args, lock=True, ctx=None) The same as :func:`RawValue` except that depending on the value of *lock* a process-safe synchronization wrapper may be returned instead of a raw ctypes @@ -1678,19 +1725,27 @@ processes. automatically protected by a lock, so it will not necessarily be "process-safe". - Note that *lock* is a keyword-only argument. + *ctx* is a context object, or ``None`` (use the current context). If ``None``, + calling this may set the global start method. See + :ref:`global-start-method` for more details. + + Note that *lock* and *ctx* are keyword-only parameters. .. function:: copy(obj) Return a ctypes object allocated from shared memory which is a copy of the ctypes object *obj*. -.. function:: synchronized(obj[, lock]) +.. function:: synchronized(obj, lock=None, ctx=None) Return a process-safe wrapper object for a ctypes object which uses *lock* to synchronize access. If *lock* is ``None`` (the default) then a :class:`multiprocessing.RLock` object is created automatically. + *ctx* is a context object, or ``None`` (use the current context). If ``None``, + calling this may set the global start method. See + :ref:`global-start-method` for more details. + A synchronized wrapper will have two methods in addition to those of the object it wraps: :meth:`get_obj` returns the wrapped object and :meth:`get_lock` returns the lock object used for synchronization. @@ -1808,8 +1863,9 @@ their parent process exits. The manager classes are defined in the *serializer* must be ``'pickle'`` (use :mod:`pickle` serialization) or ``'xmlrpclib'`` (use :mod:`xmlrpc.client` serialization). - *ctx* is a context object, or ``None`` (use the current context). See the - :func:`get_context` function. + *ctx* is a context object, or ``None`` (use the current context). If ``None``, + calling this may set the global start method. See + :ref:`global-start-method` for more details. *shutdown_timeout* is a timeout in seconds used to wait until the process used by the manager completes in the :meth:`shutdown` method. If the @@ -2293,7 +2349,9 @@ with the :class:`Pool` class. the worker processes. Usually a pool is created using the function :func:`multiprocessing.Pool` or the :meth:`Pool` method of a context object. In both cases *context* is set - appropriately. + appropriately. If ``None``, calling this function will have the side effect + of setting the current global start method if it has not been set already. + See the :func:`get_context` function. Note that the methods of the pool object should only be called by the process which created the pool. _______________________________________________ Python-checkins mailing list -- [email protected] To unsubscribe send an email to [email protected] https://mail.python.org/mailman3//lists/python-checkins.python.org Member address: [email protected]
