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Exceptions with required initialization arguments can't be unpickled:
>>> class E(Exception):
... def __init__(self, x):
... self.x = x
... 
>>> import pickle
>>> e = E(1)
>>> p = pickle.dumps(e, 1)
>>> pickle.loads(p)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/usr/local/python/2.5.1/lib/python2.5/pickle.py", line 1374, in loads
 return Unpickler(file).load()
 File "/usr/local/python/2.5.1/lib/python2.5/pickle.py", line 858, in load
 dispatch[key](self)
 File "/usr/local/python/2.5.1/lib/python2.5/pickle.py", line 1133, in load_reduce
 value = func(*args)
TypeError: __init__() takes exactly 2 arguments (1 given)
This is because __reduce__ defined in exceptions.c returns the type and the args variable, which an exception subclass might not populate.
Also, the reduce implementation doesn't properly serialize the message attribute.
I assume that the need for a custom reduce is due to the micro-optimization to store arge and message in C slots. Is this really necessary?