[issue1692335] Fix exception pickling: Move initial args assignment to BaseException.__new__

Jason R. Coombs report at bugs.python.org
Wed Sep 22 15:39:21 CEST 2010

Jason R. Coombs <jaraco at jaraco.com> added the comment:

In msg108954, I believe belopolsky is mistaken in stating that "it would be easy to simply provide custom __getinitargs__ or __reduce__ to support it". It appears __getinitargs__ does not work on Python 2.5 or Python 2.7. Exceptions of the following class still raise a TypeError on unpickling:

class D(Exception):
    """Extension with values, init called with no args."""
    def __init__(self, foo):
        self.foo = foo

    def __getinitargs__(self):
        return self.foo,

Using __reduce__ does seem to work. I suspect this is because Exceptions are extension types.

I think the fundamental problem is that pickling exceptions does not follow the principle of least surprise. In particular:

 - Other built-in objects (dicts, lists, etc) don't require special handling (replace Exception with dict in the above example and it works).
 - It's not obvious how to write an Exception subclass that takes additional arguments and make it pickleable.
 - Neither the pickle documentation nor the Exception documentation describe how pickling is implemented in Exceptions.

Eric has provided some good use cases. Furthermore, it seems counter-intuitive to me to pass a subclass' custom arguments to the parent class. Indeed, even the official tutorial defines exception classes that are unpickleable (http://docs.python.org/tutorial/errors.html#tut-userexceptions).

If the use case is obvious enough that it shows up in the hello world tutorial, I don't think there should be any argument that it's not a common use case.

At the very least, there should be a section in the pickle documentation or Exception documentation describing how one should make a pickleable subclass. What would be better is for Exceptions to behave like most other classes when being pickled.

nosy: +jaraco

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