No structural typing for mapping inside `KeysView`, `ValuesView` and `ItemsView`

[Gatsik]

Maybe it's intended to be like that, but if some class implements all the necessary methods for KeysView, ValuesView or ItemsView, mypy will generate errors, although python code will work:

from collections.abc import ItemsView, KeysView, ValuesView
from typing import Iterator


class Map[KT, VT]:
 def __init__(self, mapping: dict[KT, VT]) -> None:
 self._items: dict[KT, VT] = mapping

 def __getitem__(self, key: KT) -> VT:
 return self._items[key]

 def __len__(self) -> int:
 return len(self._items)

 def __iter__(self) -> Iterator[KT]:
 return iter(self._items)


a = Map[int, int]({3: 4})
keys = KeysView(a)
values = ValuesView(a)
items = ItemsView(a)

print(list(keys))
print(len(keys))

$ mypy t.py

t.py:20: error: Need type annotation for "keys" [var-annotated]
t.py:20: error: Argument 1 to "KeysView" has incompatible type "Map[int, int]"; expected "Mapping[Never, Any]" [arg-type]
t.py:21: error: Need type annotation for "values" [var-annotated]
t.py:21: error: Argument 1 to "ValuesView" has incompatible type "Map[int, int]"; expected "Mapping[Any, Never]" [arg-type]
t.py:22: error: Need type annotation for "items" [var-annotated]
t.py:22: error: Argument 1 to "ItemsView" has incompatible type "Map[int, int]"; expected "Mapping[Never, Never]" [arg-type]
Found 6 errors in 1 file (checked 1 source file)

$ python3 t.py

[3]
1

In my case I couldn't add multiple inheritance with Mapping because of metaclass conflict

[srittau]

Unfortunately, we use Mapping quite extensively in typeshed, where a proper (tighter) protocol would be more appropriate. Any PRs to improve the situation are welcome!