> Guido van Rossum <guido@python.org> writes:
> >> > I think you could subclass the metaclass, override __new__, and delete
> >> > the bogus __getstate__ from the type's __dict__. Then you'll get the
> >> > default pickling behavior which ignores slots; that should work just
> >> > fine in your case. :-)
[David]
> >> Ooh, that's sneaky! But I can't quite see how it works. The error
> >> message I quoted at the top about __getstate__ happens when you try to
> >> pickle an instance of the class. If I delete __getstate__ during
> >> __new__, it won't be there for pickle to find when I try to do the
> >> pickling. What will keep it from inducing the same error?
[Guido]
> > Just try it. There are many ways to customize pickling, and if
> > __getstate__ doesn't exist, pickling is done differently.
>
> Since this doesn't work:
>
> >>> d = type('d', (object,), { '__slots__' : ['foo'] } )
> >>> pickle.dumps(d())
Um, you're changing the rules in the middle of the game. You said you
had an *empty* __slots__. My recommendation only applied to that
case. I also thought you were doing this from C, not from Python, but
I may be mistaken.
> I'm still baffled as to why this works:
>
> >>> class mc(type):
> ... def __new__(self, *args):
> ... x = type.__new__(self, *args)
> ... del args[2]['__getstate__']
Hm. I don't think that x.__dict__ is args[2]; it's a copy, and
deleting __getstate__ from the arguments doesn't make any difference
to this example.
> ... return x
> ...
> >>> c = mc('c', (object,), { '__slots__' : ['foo'], '__getstate__' : lambda self: tuple() } )
Why are you passing a __getstate__ in? The point was getting rid of
the __getstate__ that type.__new__ inserts.
> >>> pickle.dumps(c())
> 'ccopy_reg\n_reconstructor\np0\n(c__main__\nc\np1\nc__builtin__\nobject\np2\nNtp3\nRp4\n.'
>
> especially since:
>
> >>> dir(d) == dir(c)
> 1
I think you have been testing something very different from what you
think you did here. dir(d) == dir(c) because they both have a
__getstate__; but d.__getstate__ is a built-in that raises an
exception, while c.__getstate__ is the lambda you passed in.
And have you tried unpickling yet? I expect it to fail.
> I don't see the logic in the source for object.__reduce__(), so where
> is it? OK, I see it in typeobject.c. But now:
>
> >>> c.__getstate__
> <unbound method c.<lambda>>
>
> OK, this seems to indicate that my attempt to remove __getstate__ from
> the class __dict__ was a failure. That explains why pickling c works,
> but not why you suggested that I remove __getstate__ inside of
> __new__. Did you mean for me to do something different?
Yes. I was assuming you'd do this at the C level. To do what I
suggested in Python, I think you'd have to write this:
class M(type):
def __new__(cls, name, bases, dict):
C = type.__new__(cls, name, bases, dict)
del C.__getstate__
return C
> I note that c's __slots__ aren't pickled at all, which I guess was the
> point of the __getstate__ requirement:
>
> >>> x = c()
> >>> x.foo = 1
> >>> pickle.dumps(x) == pickle.dumps(c())
> 1
>
> Fortunately, in our case the __slots__ are empty so it doesn't matter.
Right.
--Guido van Rossum (home page: http://www.python.org/~guido/)
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