Assigning empty dict to Mapping derived class

[AntoineD]

This code is OK for mypy

from collections.abc import Mapping
MyMapping=Mapping[str, str]
# class MyMapping(Mapping[str, str]): ...
x: MyMapping = {}

but this one is not

from collections.abc import Mapping
# MyMapping=Mapping[str, str]
class MyMapping(Mapping[str, str]): ...
x: MyMapping = {}

mypy --strict foo.py
foo.py:4: error: Incompatible types in assignment (expression has type "dict[Never, Never]", variable has type "MyMapping")  [assignment]

Could someone explain why and how to deal with it?

[erictraut]

Mypy is correct here. The expression {} constructs an instance of dict. The dict class is a subclass of (i.e. derives from) the Mapping class. It's not a subclass of MyMapping.

If you want to create an instance of MyMapping, you'd need to call its constructor rather than using the built-in syntax {}.

[AntoineD]

Thank you, I feel dumb, that Never thing puzzled me.