python / python/mypy

Mypy false positive for `converter` class attribute in `logging.Formatter` class.

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Description

Bug Report

Mypy false positive: Mypy thinks that setting the converter class attribute for a subclass of logging.Formatter to something like time.gmtime is wrong.

To Reproduce

class UTCFormatter(logging.Formatter):
    """An exact match for the `logging.Formatter` class, except that it logs the time in UTC."""
    converter = time.gmtime

Expected Behavior
No error, since time.gmtime is annotated as def gmtime(seconds: float | None = None, /) -> struct_time: ... and the converter attribute is annotated as converter: Callable[[float | None], struct_time].

Actual Behavior

logger.py:45: error: Incompatible types in assignment (expression has type "Callable[[], struct_time]", base class "Formatter" defined the type as "Callable[[float | None], struct_time]") [assignment]

Your Environment

  • Mypy version used: 1.16.0 (compiled: yes)
  • Mypy command-line flags: None
  • Mypy configuration options from mypy.ini (and other config files): None
  • Python version used: 3.13.4 (on Linux)

By the way, this issue didn't exist in my Python3.12 environment with an older version of mypy installed, but I don't have access to that environment right now so I can't provide which mypy version it had.

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First steps

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Research direction

No repository file or test is named. Reproduce the UTCFormatter example with mypy 1.16.0 on Python 3.13.4, then locate handling of subclass assignments to callable class attributes and add a regression test; done means no assignment error for time.gmtime while incompatible overrides remain diagnosed.

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
48/100

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