python / python/mypy

Name binding can be wrong when unrelated forward references are involved

Open
#16,570 0 comments 0 reactions 0 assignees View on GitHub

Nobody has claimed this yet.

bug
Dominant language
Python
Stars
20.6k
Forks
3.3k
PR merge metrics
PR metrics pending

Description

Bug Report

There is a very weird issue with classes in functions that are bound to names outside of the function, but only if forward references are involved. It is not a reveal_type issue and can occur with normal inheritance like class Foo(A) as well, which is how it was discovered.

To Reproduce

class A: pass
class B: pass

def foo() -> None:
    class Unrelated(list['ForwardReference']): ...
    class ForwardReference(): pass
    class A():
        def x(self) -> None: ...

def bar() -> None:
    reveal_type(A)  # Should be E: Revealed type is "def () -> errorx2.A"
    reveal_type(B)
    A().x()  # Should be an AttributeError

Will result in:

main.py:11: note: Revealed type is "def () -> __main__.A@7"
main.py:12: note: Revealed type is "def () -> __main__.B"
Success: no issues found in 1 source file

See also https://mypy-play.net/?mypy=latest&python=3.11&gist=7238759e8e27220c3b67ac9d1114d45b

Expected Behavior

The reveal type should be def () -> __main__.A instead of def () -> __main__.A@7.

Your Environment

  • Mypy version used: Current latest (see Playground)
  • Mypy command-line flags: None
  • 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

Start by running the provided Python reproducer with mypy and compare the revealed type for A with the expected output. Trace name binding for local classes, unrelated forward references, and inheritance; done means the revealed type refers to the module-level A and the example reports the expected invalid attribute access.

Written by the indexing model from the issue text.

Assessment

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

Get new issues in your inbox

A short digest of beginner-friendly GitHub issues.