Alexander Belopolsky <belopol...@users.sourceforge.net> added the comment:
On Wed, Sep 22, 2010 at 9:39 AM, Jason R. Coombs <rep...@bugs.python.org> wrote: > .. It appears __getinitargs__ does not work on Python 2.5 or Python 2.7. Yes, __getinitargs__ is only used for old style classes. I was wrong on that point. > 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 > Exception.__init__(self) > > def __getinitargs__(self): > return self.foo, > The problem with your D class is that it does not provide correct args attribute which is expected from an exception class: () > Using __reduce__ does seem to work. I suspect this is because Exceptions are > extension types. > There are two ways to fix this class. Both fix the args issue as well: 1. Pass foo to Exception.__init__ like this: Exception.__init__(self, foo) in D.__init__. 2. Explicitly initialize self.args: self.args = foo, > 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). Other built-in objects don't provide an API for retrieving their init arguments. > - It's not obvious how to write an Exception subclass that takes additional > arguments and make it pickleable. AFAICT, this is python subclassing 101: if base class __init__ uses arguments, they should be passed to it by subclass' __init__. > - Neither the pickle documentation nor the Exception documentation describe > how pickling is implemented in Exceptions. > Exception documentation may be improved by adding a section on subclassing. Note that the args argument was not even mentioned in 2.7 documentation, so some discussion of its role may be helpful somewhere. > 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). > Well, the tutorial examples should probably be changed. In these examples base class __init__ is not called at all which is probably not a good style. > 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. > I am not arguing against simplifying Exception subclassing. I just don't see an easy solution that would not promote subclasses with unusable args attribute. I also disagree with this issue classified as a bug. It may be a valid feature request, but not a bug. In any case, no proponent of this feature has come up with a patch for 3.2 so far and in my view, this would be a prerequisite for moving this forward. > At the very least, there should be a section in the pickle documentation or > Exception > documentation describing how one should make a pickleable subclass. .. I agree, but again someone has to step in to write such section. Improving documentation may also be the only solution for the 2.x series. ---------- _______________________________________ Python tracker <rep...@bugs.python.org> <http://bugs.python.org/issue1692335> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
