slots=True do not play nice with __init_subclass__

[malinoff]

A class annotated with attr.s(slots=True) can't call super().__init__subclass__ within its own __init_subclass__ method - a TypeError: super(type, obj): obj must be an instance or subtype of type is raised.

Reproducible example (tested on 16.3.0, 17.2.0 and f7f53d4) is here: https://repl.it/Mmkb/0

[hynek]

Uh is that another thing with cells? Is it fixable? And more importantly, is this something we’re breaking with #260 for dict classes too? :|

[Tinche]

I'll take a look. This is a PEP 487 thing I see, Python 3.6+.

[Tinche]

It's fixable. I can push a commit to the replace-class branch a little later.

[Tinche]

So I'm sitting here trying to write a test for this and for some reason, the test I've written is passing even when I comment out the fix.

Turns out CPython will actually reuse closure cells sometimes. So if a class has a normal method that needs to be rewritten, the problem will not manifest. 😱

[hynek]

JFTR @malinoff, you prevented us from making a huge mistake and release it – in the last minute! So many thanks for that!

[malinoff]

@hynek my pleasure!