Mypy mistakes instance attrs for class attrs?

[poneill]

Bug Report

Consider the following class definition:

from typing import Any


class Foo:
    @classmethod
    def bar(cls) -> Any:
        return cls.baz  # mypy correctly catches attr-defined error

Mypy (1.4.1) correctly notices that baz is not defined. However, if we amend the class definition slightly by adding baz as an instance attr like so:

class Foo2:
    def __init__(self, baz: int) -> None:
        self.baz = baz

    @classmethod
    def bar(cls) -> int:
        return cls.baz  # but mypy doesn't catch the attr-defined error here

now mypy doesn't spot the error anymore: it seems to be confused about whether baz is an instance or a class attr.

To Reproduce

Please see playground gist here.

Expected Behavior

I would expect mypy to raise an [attr-defined] error for Foo2 just as for Foo1.

Actual Behavior

main.py:7: error: "type[Foo]" has no attribute "baz"  [attr-defined]
Found 1 error in 1 file (checked 1 source file)

[But mypy finds nothing for Foo2.]

Your Environment

• Mypy version used: 1.4.1
• Mypy command-line flags: --strict
• Mypy configuration options from mypy.ini (and other config files): NA
• Python version used: 3.11

Thanks!

[erictraut]

I think this is a duplicate of #240.

[AlexWaygood]

I think this is a duplicate of #240.

Indeed

[ikonst]

Maybe we should split reading and writing? #240 is about writing to the class instance, which @JukkaL proceeded to comment might be a tough call to make.

[erictraut]

FWIW, here is a description of how I addressed this problem in pyright.

[poneill]

Maybe we should split reading and writing? #240 is about writing to the class instance, which @JukkaL proceeded to comment might be a tough call to make.

I'm far from an expert on this topic, but this issue (is about reading instance attributes as class attributes) does seem distinct from #240 (about writing them). (This issue is, however, a duplicate of #240 (comment), #15307, #1097 and others).

The important difference between #240 and this issue, AIUI, is that the snippet in #240 doesn't raise an AttributeError at runtime, so mypy is simply following python's counterintuitive behavior in this case. The snippet in this issue, however, will raise AttributeError:

foo2 = Foo2(1)
foo2.baz  # AttributeError: type object 'Foo2' has no attribute 'baz'

It sounds like there are deep-seated reasons why this error is hard to catch statically, but python does indeed consider it an error.