pickle demo

pickledemo

>>> class Error(Exception):
...     def __init__(self, foo, bar):
...         return super().__init__(foo, bar)
...
>>> Error('blah', 'blah')
Error('blah', 'blah')
>>> pickle.loads(pickle.dumps(Error('blah', 'blah')))
Error('blah', 'blah')

Fails:

>>> class Error(Exception):
...     def __init__(self, foo, bar):
...         super().__init__((foo, bar))
...
>>> import pickle; pickle.loads(pickle.dumps(Error('blah', 'blah'), protocol=pickle.HIGHEST_PROTOCOL))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Error.__init__() missing 1 required positional argument: 'bar'


Sottile says due to Exception's reduce method