[issue45914] Very first multiprocessing example not working on Windows 11

[Chang Zhou]

New submission from Chang Zhou <quattrozhou at gmail.com>:

Very first multiprocessing example not working on Windows 11

https://docs.python.org/3/library/multiprocessing.html

from multiprocessing import Pool

def f(x):
    return x*x

if __name__ == '__main__':
    with Pool(5) as p:
        print(p.map(f, [1, 2, 3]))

Tried Python 3.7, 3.8, 3.9, 3.10
Also tried Ubuntu which works fine.
But on Windows (clean installed python):


>>> from multiprocessing import Pool
>>> def f(x):
...     return x*x
...
>>> with Pool(5) as p:
...   print(p.map(f, [1, 2, 3]))
...
Process SpawnPoolWorker-14:
Process SpawnPoolWorker-13:
Traceback (most recent call last):
Process SpawnPoolWorker-15:
  File "C:\Program Files\Python39\lib\multiprocessing\process.py", line 315, in _bootstrap
    self.run()
Traceback (most recent call last):
  File "C:\Program Files\Python39\lib\multiprocessing\process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
  File "C:\Program Files\Python39\lib\multiprocessing\process.py", line 315, in _bootstrap
    self.run()
  File "C:\Program Files\Python39\lib\multiprocessing\pool.py", line 114, in worker
    task = get()
  File "C:\Program Files\Python39\lib\multiprocessing\process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
  File "C:\Program Files\Python39\lib\multiprocessing\queues.py", line 368, in get
    return _ForkingPickler.loads(res)
  File "C:\Program Files\Python39\lib\multiprocessing\pool.py", line 114, in worker
    task = get()
AttributeError: Can't get attribute 'f' on <module '__main__' (built-in)>
  File "C:\Program Files\Python39\lib\multiprocessing\queues.py", line 368, in get
    return _ForkingPickler.loads(res)
AttributeError: Can't get attribute 'f' on <module '__main__' (built-in)>
Traceback (most recent call last):
  File "C:\Program Files\Python39\lib\multiprocessing\process.py", line 315, in _bootstrap
    self.run()
  File "C:\Program Files\Python39\lib\multiprocessing\process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
  File "C:\Program Files\Python39\lib\multiprocessing\pool.py", line 114, in worker
    task = get()
  File "C:\Program Files\Python39\lib\multiprocessing\queues.py", line 368, in get
    return _ForkingPickler.loads(res)
AttributeError: Can't get attribute 'f' on <module '__main__' (built-in)>

----------
components: Windows
messages: 407186
nosy: paul.moore, quattrozhou, steve.dower, tim.golden, zach.ware
priority: normal
severity: normal
status: open
title: Very first multiprocessing example not working on Windows 11
type: crash
versions: Python 3.10, Python 3.7, Python 3.8, Python 3.9

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue45914>
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