"error: Name is not defined" and confusing behavior on instance-level classes

[slavfox]

Running mypy on the following example:

from typing import Type

class Foo:
    def __init__(self):
        class Bar:
            pass

        self.Bar: Type[Bar] = Bar

foo = Foo()
bar: foo.Bar = foo.Bar()

reveal_type(foo.Bar)
reveal_type(bar)

results in the following slightly confusing and seemingly contradictory output:

example.py:11: error: Name 'foo.Bar' is not defined  # on bar: foo.Bar
example.py:13: note: Revealed type is 'Type[example.Bar@5]'  # on reveal_type(foo.Bar)
example.py:14: note: Revealed type is 'Any'  # on reveal_type(bar)

Ideally, I would have expected reveal_type(bar) to reveal example.Bar@5. I'm not sure if there is a way to correctly type this code without resorting to # type: ignore right now.

Tested on mypy 0.761, Python 3.8.1.

[ilevkivskyi]

Although the error message is not perfect it is technically correct. Type named foo.Bar can't be statically resolved.

On one hand, this is a really bad pattern, since a new nested class will be created on each instantiation: Foo().Bar and Foo().Bar are two different classes. On the other hand this (surprisingly to me) not the first time this issue appears. So it would make sense to improve the error message.

[slavfox]

Yeah, I'm aware this is impossible to type, and for a good reason:

class Foo:
    def __init__(self, baz):
        if baz:
            class Bar(int): ...
        else:
            class Bar(dict): ...
        self.Bar = Bar

This is strictly about the seemingly contradictory 'foo.Bar' is not defined when the type is, in fact, known to mypy, just incompatible with an attempt at static typing.

Apologies if I wasn't clear in the original report 😅