Mypy complains when `[]` is used for `__slots__` definition

[sobolevn]

Bug Report

Slot definition in a form of __slots__ = [] raise an error:

ex.py:2: error: Need type annotation for "__slots__" (hint: "__slots__: List[<type>] = ...")

To Reproduce

Run mypy on this code:

class A:
    __slots__ = []

Expected Behavior I expect it to have the same effect as:

class A:
    __slots__ = ()

and to produce no errors. Moreover, mypy already knows that __slots__ should contain str items only as shown here:

class A:
    __slots__ = [1, 2]

Outputs:

ex.py:2: error: List item 0 has incompatible type "int"; expected "str"
ex.py:2: error: List item 1 has incompatible type "int"; expected "str"

Refs https://github.com/willmcgugan/rich/pull/1364

[sobolevn]

I would love to fix this. PR is incomming! 👍

[sobolevn]

Ok, we can generalize this problem to this case:

from typing import List

class Super:
    a: List[int]

class Child(Super):
    a = []

Right now it reports:

ex.py:7: error: Need type annotation for "a" (hint: "a: List[<type>] = ...")

The question is: should partial type be resolved in this general case? If yes, then I can modify the type checker to do it. If not, should __slots__ be the special case? If yes, what other props should be special cased?

[smheidrich]

#13494 which, AFAICT, should have fixed the generalized case mentioned by @sobolevn has been merged by now, but this is still broken for __slots__ specifically, I guess because __slots__ was removed from object again in https://github.com/python/typeshed/pull/6800?

[herebebeasties]

This is breaking default protobuf generated code type checking which is a shame. (See mentioned issues above.) Would be lovely to get this sorted out. 👍🏻