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gh-85299: Add note warning about entry point guard for asyncio example #93457

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merged 5 commits into from
Oct 16, 2022
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gh-85299: Add note warning about entry point guard for asyncio example #93457

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97d022e
Add note warning about entry point guard for asyncio example
slateny Jun 3, 2022
16cb255
Add entry guard, clarify note on platform
slateny Oct 15, 2022
0129bdf
Add ref target in multiprocessing
slateny Oct 16, 2022
fd68a5e
Use rewording for multiprocessing
slateny Oct 16, 2022
8ba4b0c
Add missing grave sign
slateny Oct 16, 2022
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8 changes: 7 additions & 1 deletion Doc/library/asyncio-eventloop.rst
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Expand Up @@ -1206,7 +1206,13 @@ Executing code in thread or process pools
pool, cpu_bound)
print('custom process pool', result)

asyncio.run(main())
if __name__ == '__main__':
asyncio.run(main())

Note that on Windows and macOS, the entry point guard may be necessary as the
example would otherwise cause a :exc:`RuntimeError`. For more details, see the
`Safe importing of main module
<https://docs.python.org/3/library/multiprocessing.html#the-spawn-and-forkserver-start-methods>`_.
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@gvanrossum gvanrossum Oct 15, 2022 •
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I propose to simplify the note to the following:

Note that the entry point guard (if __name__ == '__main__') is required for option 3, due to the peculiarities of multiprocessing, which is used by ProcessPoolExecutor. See (link).

There should be three cross-references:

  • multiprocessing (the module)
  • ProcessPoolExecutor (the class)
  • Safe importing of main module

For the latter I recommend to add a label to the target section in multiprocessing.rst rather than linking to the website. (@CAM-Gerlach can provide details on how to do this.)

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My understanding though is that option 3 fails when spawn/forkserver is the process start method, and only Windows + mac default to spawn, while unix defaults to fork, so saying 'is required' may be a bit too strong. If 'may be required' instead, then the reader might wonder why 'may', in which case I think a link to https://docs.python.org/3/library/multiprocessing.html#contexts-and-start-methods somewhere would help explain things.

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That's too subtle for the readers. We're not stating the law, we're just explaining the example.

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See my comment for a suggestion implementing the above and the details Guido mentioned.

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This method returns a :class:`asyncio.Future` object.

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