mypy infers the wrong type for classmethods

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#3,482 3 comments 1 reaction 0 assignees View on GitHub

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
25/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
python
Domain
devtools

Research direction

Start with the simplified classmethod example and compare mypy's inferred type for hi with the classmethod annotation in typeshed's stdlib/3/builtins.pyi. Trace the callable inference path to determine whether the real classmethod type can be preserved, and use a focused regression test for the __func__ access as the completion check.

Written by the indexing model from the issue text.

Description

bug topic-descriptors topic-runtime-semantics

We have some code that simplifies to this:

class A:
    @classmethod
    def hi(cls):
        pass

    bye = classmethod(hi.__func__)

(The real code is wrapping an async classmethod with a sync version; there's another async-to-sync wrapper in there.)

Mypy doesn't like this code because it doesn't know that classmethods have a __func__ attribute. The typeshed annotation for classmethod includes __func__, but mypy doesn't infer the real classmethod type for hi (which is the actual type at runtime), it just infers a general callable type.

This may not be easily fixed until we have protocol support and can more easily have various callable subtypes?

I'm also OK if this is closed as "just use cast() if you're accessing private attributes of a classmethod"; it's certainly not high priority.

Dominant language
Python
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