If several threads try to start a multiprocessing.Pool at the same time when no pool has been started before this often fails with an exception like this (the exact import varies):
Exception in thread Thread-2:
Traceback (most recent call last):
File "/tmp/py3.9.0rc1/lib/python3.9/threading.py", line 950, in _bootstrap_inner
self.run()
File "/tmp/py3.9.0rc1/lib/python3.9/threading.py", line 888, in run
self._target(*self._args, **self._kwargs)
File "/tmp/py3.9.0rc1/lib/python3.9/multiprocessing/context.py", line 118, in Pool
from .pool import Pool
ImportError: cannot import name 'Pool' from partially initialized module 'multiprocessing.pool' (most likely due to a circular import) (/tmp/py3.9.0rc1/lib/python3.9/multiprocessing/pool.py)
This happens even if Pool was imported before starting the threads and is new in 3.9. It's easy to work around by starting a pool in the main thread before starting the other threads.
I have attached a minimal example that triggers it. Tested on Debian stable and FreeBSD 11.3.