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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?

AFAIR we were told that filling args and message is part of the exception contract...

I'm not aware of any such contract. Can you point to anything in writing?
See for example:
file:///home/jim/Documentation/Python-Docs-2.4.1/tut/node10.html#SECTION0010500000000000000000
which teaches people to create custom exceptions that:
- don't set args ro message and
- won't be unpicklable in Python 2.5.
Also, as I mentioned, the reduce implementation doesn't preserve the message, so even if that was the contract, the contract is broken.

I'll note that I think the right thing to do is to:
- Take args and message out of the C struct.
- inherit the default reduce behavior from object.

No, I haven't anything written -- and this should be fixed anyway.

Jim, could you please take a look at the pathes in #1692335?
See: http://www.python.org/sf/1692335 

Closing in favor of #1692335.