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No structural typing for mapping inside KeysView, ValuesView and ItemsView #14555
Description
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]
1In my case I couldn't add multiple inheritance with Mapping because of metaclass conflict
Activity
srittau commentedon Aug 11, 2025
Unfortunately, we use
Mappingquite extensively in typeshed, where a proper (tighter) protocol would be more appropriate. Any PRs to improve the situation are welcome!