python / python/mypy

False positive comparison-overlap for nested function and nested class

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bug
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Python
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Description

Bug Report

Nested functions inside methods incorrectly resolve attributes of their first argument relative to the surrounding class.

To Reproduce

class foo:
    a: str = "foo"
    
class bar:
    class a:
        pass
    
    def test(self) -> None:
        def inner1(f: foo, s: str) -> bool:
            return f.a == s
        
        def inner2(s: str, f: foo) -> bool:
            return f.a == s

https://mypy-play.net/?mypy=latest&python=3.11&flags=strict&gist=4c27f653074cd1c3a48bb16f49ca2168

Expected Behavior

As far as I can tell this should be valid code, even though this rarely happens in practice due to CamelCase vs snake_case naming conventions.

Actual Behavior

inner1 fails to type-check, because the a attribute is incorrectly assumed to be the a class inside bar

main.py:10: error: Non-overlapping equality check (left operand type: "Type[a]", right operand type: "str")  [comparison-overlap]

Interestingly, inner2 is fine.

Your Environment

  • Mypy version used: 1.2.0
  • Mypy command-line flags: --strict
  • Mypy configuration options from mypy.ini (and other config files): none
  • Python version used: 3.11

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Reproduce the example from the issue in mypy with Python 3.11 and --strict, then trace how the nested functions resolve the attribute access in the comparison. The fix is done when inner1 no longer reports a comparison-overlap error while inner2 remains valid and the regression is covered by a test.

Written by the indexing model from the issue text.

Assessment

Tech stack
python
Domain
devtools
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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