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?
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.