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gh-132898: Add a note in `multiprocessing.Process` docs about creating a process in a REPL session by YvesDup · Pull Request #137118 · python/cpython · GitHub
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gh-132898: Add a note in multiprocessing.Process docs about creating a process in a REPL session #137118

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Merged
gpshead merged 8 commits into from
Jul 27, 2025
Merged

gh-132898: Add a note in multiprocessing.Process docs about creating a process in a REPL session #137118

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c64de22
initial commit
YvesDup Jul 23, 2025
dd2d43e
Merge branch 'main' into Update-doc-process-target-location
YvesDup Jul 26, 2025
607ae4a
Reword, expand, and clarify the limitation, highlighting the REPL case.
gpshead Jul 27, 2025
b9773c7
Move out of a .. note into the main text. mention in the high level …
gpshead Jul 27, 2025
4ad605a
move back to a note, it keeps it clear that this is "special"
gpshead Jul 27, 2025
2a115a3
editing for clarity
gpshead Jul 27, 2025
5f36fbf
formatting
gpshead Jul 27, 2025
4b848a9
add a pointer to the GH issue from the doc note
gpshead Jul 27, 2025
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editing for clarity
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@gpshead
gpshead committed Jul 27, 2025
commit 2a115a31475647b01b04d401bc2cb9d48883d9be
36 changes: 19 additions & 17 deletions Doc/library/multiprocessing.rst
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Expand Up @@ -545,37 +545,39 @@ 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 :meth:`Process.__init__` must be picklable.
This is particularly notable when trying to create a :class:`Process` or use
a :class:`concurrent.futures.ProcessPoolExecutor` from a REPL with a locally
defined *target* function.
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 raises an
:exc:`AttributeError` exception when starting the process as such as
*target* must have been defined within an importable module to under to be
unpickled.
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::
Example of this uncatchable error from the child::

>>> import multiprocessing as mp
>>> def knigit():
... print("knee!")
... print("Ni!")
...
>>> mp.Process(target=knigit).start()
>>> 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
<SpawnProcess name='SpawnProcess-1' pid=379473 parent=378707 stopped exitcode=1>

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
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`.

.. versionchanged:: 3.3
Expand Down Expand Up @@ -3107,7 +3109,7 @@ start method.

More picklability

Ensure that all arguments to :meth:`Process.__init__` are picklable.
Ensure that all arguments to :class:`Process` are picklable.
Also, if you subclass :class:`~multiprocessing.Process` then make sure that
instances will be picklable when the :meth:`Process.start
<multiprocessing.Process.start>` method is called.
Expand Down