[Python] Why isn't dict[str, str] assignable to Mapping[str | int, str]?

Given this code:

from collections.abc import Mapping

def my_fn(m: Mapping[str | int, str]):
print(m)

d = {"a": "b"}
my_fn(d)


both mypy 1.16.0 and pyright 1.1.400 report that it is invalid to assign d to the argument m. For example pyright outputs:

error: Argument of type "dict[str, str]" cannot be assigned to parameter "m" of type "Mapping[str | int, str]" in function "my_fn"
"dict[str, str]" is not assignable to "Mapping[str | int, str]"
Type parameter "_KT@Mapping" is invariant, but "str" is not the same as "str | int" (reportArgumentType)


Why is this the case? I understand why assigning dict[str, str] to a MutableMapping[str | int, str] would be bad (the callee could insert int keys into the dict), but Mapping is immutable.

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