Support custom __getstate__, __setstate__ for slotted classes (or improve docs)?

[andhus]

The assertion fails in the following example, but is ok with slots=False:

import attr, pickle

@attr.s(slots=True)
class MyClass(object):
    a = attr.ib()
    not_picklable = attr.ib()

    def __getstate__(self):
        return self.a, "replacement"

    def __setstate__(self, state):
        self.a, self.not_picklable = state

mc = MyClass("a", "b")
mc_new = pickle.loads(pickle.dumps(mc))
assert mc_new.not_picklable == "replacement"

This is clearly because attrs auto creates these methods on the new slots-class: https:/python-attrs/attrs/blob/master/src/attr/_make.py#L601

Is there another prefered way to solve this, or would it be possible to support these methods also for slots-classes? If not, it would be good to clarify this in the documentation for slotted-classes, it says:

You can support protocol 0 and 1 by implementing __getstate__ and __setstate__ methods yourself. Those methods are created for frozen slotted classes because they won’t pickle otherwise. Think twice before using pickle though.

This is confusing to me, since implementing these methods have no effect for slotted classes?

Somewhat related:
#139
#475

[hynek]

You have opened this issue in the best possible moment because I'm prototyping a new API right now and this might go in. No promises about timeline and whether it'll be python2-compatible for now tho.

[andhus]

Glad to hear! In the meantime, I made this suggestion: #513. I think it makes sense. Maybe I'm missing something, but isn't the docs (referenced above) misleading/inconsistent with current behaviour?

[andhus]

Ok, read the docs more carefully and now understand that "You can support protocol 0 and 1 by implementing __getstate__ and __setstate__ methods yourself" refers to __slots__ classes in general but not to attr.s(slots=True) classes. Still don't see any downside from making it possible to define these methods in attr.s(slots=True) classes.

[hynek]

Ok, read the docs more carefully and now understand

With that you're way ahead of me because I don't understand that paragraph at all anymore. 😅

It would be great if we could rewrite it once this has been resolved.