__slots__ requires explicit type definition and must be Any (and nothing else) when used with Protocols #11013
Description
# test_case1.py
from __future__ import annotations
from typing import Protocol, runtime_checkable
@runtime_checkable
class Foo(Protocol):
__slots__ = ()
def foo(self):
...
class Bar:
__slots__ = ("_t",)
def __init__(self):
self._t = True
def foo(self):
pass
print(f"isinstance(Bar(), Foo) == {isinstance(Bar(), Foo)}") # <- prints … == True
foo: Foo = Bar() # <- errors here% python test_case1.py
isinstance(Bar(), Foo) == True
% mypy --version
mypy 0.910
% mypy --config-file=/dev/null test_case1.py
/dev/null: No [mypy] section in config file
test_case1.py:19: error: Incompatible types in assignment (expression has type "Bar", variable has type "Foo")
test_case1.py:19: note: Following member(s) of "Bar" have conflicts:
test_case1.py:19: note: __slots__: expected "Tuple[]", got "Tuple[str]"
The "fix" is to provide Tuple[str, ...] as the type for __slots__:
# test_case2.py
from __future__ import annotations
from typing import Protocol, Tuple, runtime_checkable
@runtime_checkable
class Foo(Protocol):
__slots__: Tuple[str, ...] = ()
def foo(self):
...
class Bar:
__slots__ : Tuple[str, ...] = ("_t",)
def __init__(self):
self._t = True
def foo(self):
pass
print(f"isinstance(Bar(), Foo) == {isinstance(Bar(), Foo)}") # still True
foo: Foo = Bar() # <- works now?% mypy --config-file=/dev/null test_case2.py
/dev/null: No [mypy] section in config file
Success: no issues found in 1 source file
Shouldn't mypy already know the type of __slots__?