TypedDict marked as @final not assignable to Mapping

[DetachHead]

from typing import Mapping, TypedDict, final

@final
class Foo(TypedDict):
    foo: int
    
foo: Foo

bar: Mapping[str, int] = foo # error: Incompatible types in assignment (expression has type "Foo", variable has type "Mapping[str, int]")  [assignment]

playground

[erictraut]

Mypy is correct in generating an error here. A TypedDict may have subtypes that include additional keys that are not specified in the parent class, so it's unsafe to assume that foo is compatible with Mapping[str, int].

If a TypedDict were marked as @final, it would be safe to assume that it is compatible with Mapping[str, <union or join of all value types>], but it's rare to see @final TypedDicts, and no Python type checkers currently implement this special-case logic. That would be an enhancement request rather than a bug report.

[DetachHead]

oops i don't know how i missed that

[AlexWaygood]

Closing, since the basic feature request here is "support TypedDicts decorated with @final", which is a duplicate of #7981.