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Commit9da008d

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miss-islingtonYvesDupgpshead
authored
[3.14]gh-132898: Add a note inmultiprocessing.Process docs about creating a process in a REPL (GH-137118) (#137154)
Co-authored-by: Duprat <yduprat@gmail.com>Co-authored-by: Gregory P. Smith <greg@krypto.org>
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‎Doc/library/concurrent.futures.rst‎

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@@ -342,6 +342,11 @@ that :class:`ProcessPoolExecutor` will not work in the interactive interpreter.
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Calling:class:`Executor` or:class:`Future` methods from a callable submitted
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to a:class:`ProcessPoolExecutor` will result in deadlock.
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Note that the restrictions on functions and arguments needing to picklable as
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per:class:`multiprocessing.Process` apply when using:meth:`~Executor.submit`
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and:meth:`~Executor.map` on a:class:`ProcessPoolExecutor`. A function defined
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in a REPL or a lambda should not be expected to work.
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..class::ProcessPoolExecutor(max_workers=None, mp_context=None, initializer=None, initargs=(), max_tasks_per_child=None)
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An:class:`Executor` subclass that executes calls asynchronously using a pool

‎Doc/library/multiprocessing.rst‎

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@@ -97,6 +97,10 @@ To show the individual process IDs involved, here is an expanded example::
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For an explanation of why the ``if __name__ == '__main__'`` part is
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necessary, see:ref:`multiprocessing-programming`.
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The arguments to:class:`Process` usually need to be unpickleable from within
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the child process. If you tried typing the above example directly into a REPL it
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could lead to an:exc:`AttributeError` in the child process trying to locate the
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*f* function in the ``__main__`` module.
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.. _multiprocessing-start-methods:
@@ -233,9 +237,12 @@ processes for a different context. In particular, locks created using
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the *fork* context cannot be passed to processes started using the
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*spawn* or *forkserver* start methods.
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A library which wants to use a particular start method should probably
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use:func:`get_context` to avoid interfering with the choice of the
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library user.
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Libraries using:mod:`multiprocessing` or
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:class:`~concurrent.futures.ProcessPoolExecutor` should be designed to allow
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their users to provide their own multiprocessing context. Using a specific
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context of your own within a library can lead to incompatibilities with the
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rest of the library user's application. Always document if your library
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requires a specific start method.
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..warning::
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@@ -538,9 +545,42 @@ The :mod:`multiprocessing` package mostly replicates the API of the
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to pass to *target*.
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If a subclass overrides the constructor, it must make sure it invokes the
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base class constructor (:meth:`Process.__init__`) before doing anything else
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base class constructor (``super().__init__()``) before doing anything else
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to the process.
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..note::
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In general, all arguments to:class:`Process` must be picklable. This is
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frequently observed when trying to create a:class:`Process` or use a
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:class:`concurrent.futures.ProcessPoolExecutor` from a REPL with a
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locally defined *target* function.
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Passing a callable object defined in the current REPL session causes the
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child process to die via an uncaught:exc:`AttributeError` exception when
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starting as *target* must have been defined within an importable module
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in order to be loaded during unpickling.
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Example of this uncatchable error from the child::
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>>> import multiprocessing as mp
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>>> def knigit():
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... print("Ni!")
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...
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>>> process = mp.Process(target=knigit)
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>>> process.start()
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>>> Traceback (most recent call last):
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File ".../multiprocessing/spawn.py", line ..., in spawn_main
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File ".../multiprocessing/spawn.py", line ..., in _main
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AttributeError: module '__main__' has no attribute 'knigit'
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>>> process
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<SpawnProcess name='SpawnProcess-1' pid=379473 parent=378707 stopped exitcode=1>
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See:ref:`multiprocessing-programming-spawn`. While this restriction is
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not true if using the ``"fork"`` start method, as of Python ``3.14`` that
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is no longer the default on any platform. See
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:ref:`multiprocessing-start-methods`.
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See also:gh:`132898`.
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..versionchanged::3.3
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Added the *daemon* parameter.
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@@ -3058,10 +3098,10 @@ start method.
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More picklability
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Ensure that all arguments to:meth:`Process.__init__` are picklable.
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Also, if you subclass:class:`~multiprocessing.Process` thenmake sure that
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instances will be picklable when the:meth:`Process.start
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<multiprocessing.Process.start>` method is called.
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Ensure that all arguments to:class:`~multiprocessing.Process` are
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picklable.Also, if you subclass``Process.__init__``, you mustmake sure
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thatinstances will be picklable when the
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:meth:`Process.start<multiprocessing.Process.start>` method is called.
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Global variables
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