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. :-)
>>
>> 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?
>
> 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())
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__']
... return x
...
>>> c = mc('c', (object,), { '__slots__' : ['foo'], '__getstate__' : lambda self: tuple() } )
>>> 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 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?
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.
--
Dave Abrahams
Boost Consulting
www.boost-consulting.com
RetroSearch is an open source project built by @garambo | Open a GitHub Issue
Search and Browse the WWW like it's 1997 | Search results from DuckDuckGo
HTML:
3.2
| Encoding:
UTF-8
| Version:
0.7.4