Skip to content

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

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
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

Changes from 1 commit
Commits
Show all changes
8 commits
Select commit Hold shift + click to select a range
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
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
Prev Previous commit
Next Next commit
editing for clarity
  • Loading branch information
@gpshead
gpshead committed Jul 27, 2025
commit 2a115a31475647b01b04d401bc2cb9d48883d9be
36 changes: 19 additions & 17 deletions Doc/library/multiprocessing.rst
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -545,37 +545,39 @@
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 @@

More picklability

Ensure that all arguments to :meth:`Process.__init__` are picklable.
Ensure that all arguments to :class:`Process` are picklable.

Check warning on line 3112 in Doc/library/multiprocessing.rst

View workflow job for this annotation

GitHub Actions / Docs / Docs 

py:class reference target not found: Process [ref.class]
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
Loading