I was trying to use multiprocessing (via a concurrent.futures.ProcessPoolExecutor) and encountered an error when pickling a custom Exception.  On closer examination I was able to create a simple test case that only involves pickle:


import pickle
class StrRegexError(Exception):
    def __init__(self, *, pattern):
        self.pattern = pattern

data = pickle.dumps(StrRegexError(pattern='test'))
instance = pickle.loads(data)


[pts/11@peru /srv/ansible]$ python3.8 ~/p.py
Traceback (most recent call last):
  File "/home/badger/p.py", line 7, in <module>
    instance = pickle.loads(data)
TypeError: __init__() missing 1 required keyword-only argument: 'pattern'

pickle can handle mandatory keyword args in other classes derived from object; it's only classes derived from Exception that have issues.