Bugs item #1742889, was opened at 2007-06-25 16:43
Message generated for change (Comment added) made by zseil
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Category: None
Group: Python 2.5
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Jim Fulton (dcjim)
Assigned to: Nobody/Anonymous (nobody)
Summary: Pickling of exceptions broken
Initial Comment:
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?
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>Comment By: Ziga Seilnacht (zseil)
Date: 2007-08-12 13:26
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Jim, could you please take a look at the pathes in #1692335?
See: http://www.python.org/sf/1692335
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Comment By: Georg Brandl (gbrandl)
Date: 2007-06-25 17:57
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No, I haven't anything written -- and this should be fixed anyway.
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Comment By: Jim Fulton (dcjim)
Date: 2007-06-25 17:57
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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.
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Comment By: Jim Fulton (dcjim)
Date: 2007-06-25 17:53
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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.
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Comment By: Georg Brandl (gbrandl)
Date: 2007-06-25 17:26
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AFAIR we were told that filling args and message is part of the exception
contract...
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