Subtype of mapping is not compatible type in `.update()` method

[Dreamsorcerer]

    async def psubscribe(self, *args: ChannelT, **kwargs: PubSubHandler):
        new_patterns: Dict[ChannelT, Optional[PubSubHandler]] = dict.fromkeys(args)
        new_patterns.update(kwargs)

This code (taken from aioredis) produces this error:
error: Argument 1 to "update" of "dict" has incompatible type "Dict[str, Callable[[Dict[str, str]], None]]"; expected "Mapping[Union[bytes, str, memoryview], Optional[Callable[[Dict[str, str]], None]]]" [arg-type]

But, the types are compatible as the keys and values are part of the expected union, I can't think of any operation that would make this unsafe. I understand it could be unsafe if the expectation was MutableMapping, as the function might assign any of the unioned types, but for Mapping this looks completely safe to me.

[JelleZijlstra]

For reference, dict.update is defined in typeshed as:

    @overload
    def update(self, __m: Mapping[_KT, _VT], **kwargs: _VT) -> None: ...
    @overload
    def update(self, __m: Iterable[Tuple[_KT, _VT]], **kwargs: _VT) -> None: ...
    @overload
    def update(self, **kwargs: _VT) -> None: ...

I need to think about this more but I guess it has to do with the invariant key type argument to Mapping.

[Dreamsorcerer]

For reference, errors come from this file: https://github.com/aio-libs/aioredis-py/blob/f5a8e80882c3dfa70cf9cc2e44d110faeb579d74/aioredis/client.py
Just search for the issue number (10970) to find them.

[mzapotoczny]

I stumbled across similar problem.

X = {(1, 2): 0, (1,): 1}
Y = {(1, 3): 2, (1, 4): 3}
X.update(Y)

causes

Argument 1 to "update" of "dict" has incompatible type "Dict[Tuple[int, int], int]"; expected "Mapping[Tuple[int, ...], int]"

while if we use other update definition:

X = {(1, 2): 0, (1,): 1}
Y = {(1, 3): 2, (1, 4): 3}
X.update(Y.items())

mypy passes.

[Dreamsorcerer]

Think that's a different issue. It doesn't recognise that Tuple[int, int] is compatible with Tuple[int, ...]. I don't think it has anything to do with Mapping.

You can just add an annotation to make it work:
Y: Dict[Tuple[int, ...], int]

Although interesting that it accepts ItemsView[Tuple[int, int], ...], but not Dict[Tuple[int, int], ...]... Maybe it is related...

[Dreamsorcerer]

Annotations seem to have changed to expect SupportsKeysAndGetItem, but otherwise this error still seems reproducible.