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The case in msg76028 are probably not pointing to a bug. If subclass' __init__ passes its args to the base class __init__ (as it probably should), pickling works: 
class E(Exception):
 """Extension with values, init called with args."""
 def __init__(self, foo):
 self.foo = foo
 Exception.__init__(self, foo)
>>> from pickle import *
>>> loads(dumps(E(1)))
E(1,)
>>> _.foo
1
What would be a use case for not setting args? Even if there was a valid use case for it, it would be easy to simply provide custom __getinitargs__ or __reduce__ to support it.
Note that with msg76028 definitions, eval(repr(C('foo'))) also raises a TypeError, so properly supporting args hiding in exception subclasses would probably involve overriding __repr__ as well.