Abstract __getitem__ in Sequence doesn't type check

[MichalKononenko]

I have the following code snippet

from typing import Sequence

class AbstractSequence(Sequence[float]):
    @overload
    def __getitem__(self, index: int) -> float:
        pass

    @overload
    def __getitem__( 
            self, index: slice
    ) -> 'AbstractSequence':
        pass

    @abc.abstractmethod
    def __getitem__( 
            self, index: Union[int, slice]
        ) -> Union[float, 'AbstractSequence']:
        raise NotImplementedError()

Mypy claims that the signature of the abstract getitem is incompatible with that of the supertype. Can anyone give me a hand with this?

[posita]

I don't think this is specific to typeshed. Even without Sequence, it still fails (which seems super weird, given python/mypy#10895):

# test_case.py

from abc import abstractmethod
from typing import List, Union, overload

class GetItemBase:
    @overload
    def __getitem__(self, key: int) -> int:
        ...
    @overload
    def __getitem__(self, key: slice) -> List[int]:
        ...
    @abstractmethod
    def __getitem__(self, key: Union[int, slice]) -> Union[int, List[int]]:
        ...

class GetItemTest(GetItemBase):
    def __getitem__(self, key: Union[int, slice]) -> Union[int, List[int]]:  # <- typing error despite being identical
        return list(range(10))[key]

Runtime:

% python --version
Python 3.9.6
% pip list
Package           Version
----------------- --------
mypy              0.910
mypy-extensions   0.4.3
pip               21.2.1
setuptools        57.4.0
toml              0.10.2
typing-extensions 3.10.0.0
% mypy --config=/dev/null test_case.py
/dev/null: No [mypy] section in config file
test_case.py:18: error: Signature of "__getitem__" incompatible with supertype "GetItemBase"
Found 1 error in 1 file (checked 1 source file)
% mypy --version
mypy 0.910

[Akuli]

GetItemBase says that instance[some_integer] must be an integer and instance[some_slice] must be a list. The inheriting class doesn't provide any guarantees like this; as far as mypy knows, instance[some_integer] with that __getitem__ method could be an integer or a list.

To fix this, you can # type: ignore the erroring line or copy/paste the overloads to the subclass:

class GetItemBase:
    @overload
    def __getitem__(self, key: int) -> int:
        ...
    @overload
    def __getitem__(self, key: slice) -> List[int]:
        ...
    @abstractmethod
    def __getitem__(self, key: Union[int, slice]) -> Union[int, List[int]]:
        ...

class GetItemTest(GetItemBase):
    @overload
    def __getitem__(self, key: int) -> int:
        ...
    @overload
    def __getitem__(self, key: slice) -> List[int]:
        ...
    def __getitem__(self, key: Union[int, slice]) -> Union[int, List[int]]:  # <- typing error despite being identical
        return list(range(10))[key]