diff --git a/Doc/library/concurrent.futures.rst b/Doc/library/concurrent.futures.rst index 4e7db8fc854521..73134f6a3987bf 100644 --- a/Doc/library/concurrent.futures.rst +++ b/Doc/library/concurrent.futures.rst @@ -243,6 +243,11 @@ that :class:`ProcessPoolExecutor` will not work in the interactive interpreter. Calling :class:`Executor` or :class:`Future` methods from a callable submitted to a :class:`ProcessPoolExecutor` will result in deadlock. +Note that the restrictions on functions and arguments needing to picklable as +per :class:`multiprocessing.Process` apply when using :meth:`~Executor.submit` +and :meth:`~Executor.map` on a :class:`ProcessPoolExecutor`. A function defined +in a REPL or a lambda should not be expected to work. + .. class:: ProcessPoolExecutor(max_workers=None, mp_context=None, initializer=None, initargs=(), max_tasks_per_child=None) An :class:`Executor` subclass that executes calls asynchronously using a pool diff --git a/Doc/library/multiprocessing.rst b/Doc/library/multiprocessing.rst index 92d6bf9a07a30e..9b173540219c77 100644 --- a/Doc/library/multiprocessing.rst +++ b/Doc/library/multiprocessing.rst @@ -97,6 +97,10 @@ To show the individual process IDs involved, here is an expanded example:: For an explanation of why the ``if __name__ == '__main__'`` part is necessary, see :ref:`multiprocessing-programming`. +The arguments to :class:`Process` usually need to be unpickleable from within +the child process. If you tried typing the above example directly into a REPL it +could lead to an :exc:`AttributeError` in the child process trying to locate the +*f* function in the ``__main__`` module. .. _multiprocessing-start-methods: @@ -218,9 +222,12 @@ processes for a different context. In particular, locks created using the *fork* context cannot be passed to processes started using the *spawn* or *forkserver* start methods. -A library which wants to use a particular start method should probably -use :func:`get_context` to avoid interfering with the choice of the -library user. +Libraries using :mod:`multiprocessing` or +:class:`~concurrent.futures.ProcessPoolExecutor` should be designed to allow +their users to provide their own multiprocessing context. Using a specific +context of your own within a library can lead to incompatibilities with the +rest of the library user's application. Always document if your library +requires a specific start method. .. warning:: @@ -518,9 +525,42 @@ The :mod:`multiprocessing` package mostly replicates the API of the to pass to *target*. If a subclass overrides the constructor, it must make sure it invokes the - base class constructor (:meth:`Process.__init__`) before doing anything else + base class constructor (``super().__init__()``) before doing anything else to the process. + .. note:: + + In general, all arguments to :class:`Process` must be picklable. This is + frequently observed when trying to create a :class:`Process` or use a + :class:`concurrent.futures.ProcessPoolExecutor` from a REPL with a + locally defined *target* function. + + Passing a callable object defined in the current REPL session causes the + child process to die via an uncaught :exc:`AttributeError` exception when + starting as *target* must have been defined within an importable module + in order to be loaded during unpickling. + + Example of this uncatchable error from the child:: + + >>> import multiprocessing as mp + >>> def knigit(): + ... print("Ni!") + ... + >>> process = mp.Process(target=knigit) + >>> process.start() + >>> Traceback (most recent call last): + File ".../multiprocessing/spawn.py", line ..., in spawn_main + File ".../multiprocessing/spawn.py", line ..., in _main + AttributeError: module '__main__' has no attribute 'knigit' + >>> process + + + See :ref:`multiprocessing-programming-spawn`. While this restriction is + not true if using the ``"fork"`` start method, as of Python ``3.14`` that + is no longer the default on any platform. See + :ref:`multiprocessing-start-methods`. + See also :gh:`132898`. + .. versionchanged:: 3.3 Added the *daemon* parameter. @@ -2993,10 +3033,10 @@ start method. More picklability - Ensure that all arguments to :meth:`Process.__init__` are picklable. - Also, if you subclass :class:`~multiprocessing.Process` then make sure that - instances will be picklable when the :meth:`Process.start - ` method is called. + Ensure that all arguments to :class:`~multiprocessing.Process` are + picklable. Also, if you subclass ``Process.__init__``, you must make sure + that instances will be picklable when the + :meth:`Process.start ` method is called. Global variables pFad - Phonifier reborn

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