Bad error message: mypy reports 'Name is not defined' when using an object member as super class of another class

[sandrinr]

Consider the following valid Python 3 code:

class A():
    pass

class B():
    def __init__(self):
        self.a = A

b = B()

class C(b.a):
    pass

Actual behavior:
Mypy will report Name 'b.a' is not defined on the line class C(b.a):.

Expected behavior:
No error reported by Mypy.

Python version: 3.8.2
Mypy version: 0.770 (master not checked)
Flags: no flags

[msullivan]

This is an incorrect error message, certainly. We're not going to support this code, though.

[vhawk19]

@msullivan Should the error message be changed?

[JelleZijlstra]

We'd be likely to accept a PR that improves the error message (something like "object member cannot be used as base class", I suppose).

[akash-suresh]

@JelleZijlstra This is my first time contributing. Can I pick this up?

[harshkumar02]

``I would like to contribute to this issue. Can I pull this up?

[PrasanthChettri]

Can I look into this, seems like I can handle this one

[PrasanthChettri]

@msullivan @JelleZijlstra I looked through the codebase where I can set up a check for this, I think I have to implement something around semanal/analyze base class or semanal/expr to analyzed type and have to check if the base class is a object member, I tried everything but I don't seem to know how to do this, could anybody guide me through this, I'm kind of a newbie.

[williamjmorenor]

Using per line comment as # type: ignore[name-defined] at less silence this error message

[SwagatSBhuyan]

If this issue is not solved, I would like to be assigned this @msullivan @JelleZijlstra

[kai3341]

Have similar issue:

from typing import Type

class A: ...
class SomeMixin: ...
class some_framework_app: ...

def subclass_a(something):
    class A1(A):
        attr = something

    return A1


class B(some_framework_app):
    C: Type[A]
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.C = subclass_a(self)

app = B()

class D(app.C, SomeMixin): ...  # <=== Name "app.C " is not defined