F821 false positive: `from .foo import Bar` doesn't mark `foo` as defined.

[alona-em]

Summary

spam/__init__.py

from .foo import Bar

print(foo.Bar)

spam/foo.py

class Bar:
    pass

ruff things this is an error.

% uvx ruff check ./spam/__init__.py --select=F821
F821 Undefined name `foo`
 --> spam/__init__.py:3:7
  |
1 | from .foo import Bar
2 |
3 | print(foo.Bar)
  |       ^^^
  |

Found 1 error.

But it works fine:

% python3 -c 'import spam'
<class 'spam.foo.Bar'>

This works due to weird python behavior (https://docs.python.org/3/reference/import.html#submodules).

(See also microsoft/pyright#782, where pyright used to have this same bug).

Version

ruff 0.15.0 (ce5f7b6 2026-02-03)

[ntBre]

Thanks for the report! This seems closely related to the F401 changes in #20200 and many of the linked issues, but it seems that this hasn't come up for F821, at least based on my searches.