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

    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.

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.

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.

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.

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...

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

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Subtype of mapping is not compatible type in `.update()` method #10970

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Subtype of mapping is not compatible type in .update() method #10970

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JelleZijlstra commented on Aug 11, 2021

Dreamsorcerer commented on Aug 21, 2021

mzapotoczny commented on Nov 9, 2021

Dreamsorcerer commented on Nov 9, 2021

Dreamsorcerer commented on Jul 14, 2025

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Subtype of mapping is not compatible type in `.update()` method #10970