Why derivated exception can not be pickled ?

[Mathieu Courtois]

Here is my example :


import cPickle

ParentClass = object     # works
ParentClass = Exception  # does not

class MyError(ParentClass):
    def __init__(self, arg):
        self.arg = arg

    def __getstate__(self):
        print '#DBG pass in getstate'
        odict = self.__dict__.copy()
        return odict

    def __setstate__(self, state):
        print '#DBG pass in setstate'
        self.__dict__.update(state)

exc = MyError('IDMESS')

fo = open('pick.1', 'w')
cPickle.dump(exc, fo)
fo.close()

fo = open('pick.1', 'r')
obj = cPickle.load(fo)
fo.close()


1. With ParentClass=object, it works as expected.

2. With ParentClass=Exception, __getstate__/__setstate__ are not called.

Does anyone explain me why ?
Thanks.