Nested Protocols are not checked by Mypy/Pylance

Question

I'm using Protocols to define some base classes
I want to just inherit the parent Protocol and have the children type checked automatedly.

Here is the example code:

class Child(Protocol):
   name: str


class Parent(Protocol):
   Child: Child


class FooBar(Parent):
   class Child:
       pass
       # No Error

Why is Mypy/Pylance not inferring that the Child Protocol requires a name

Answer 1

This is a known limitation in mypy, here's the corresponding issue. As pointed out in comments, Pyright detects this type violation correctly. If you define the type externally and assign, mypy also identifies the error:

from typing import Protocol


class Child(Protocol):
   name: str

class Parent(Protocol):
   Child: Child

class _Child:
    pass

class FooBar(Parent):
    Child = _Child  # E: Incompatible types in assignment (expression has type "type[_Child]", base class "Parent" defined the type as "Child")  [assignment]