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

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.

Potentially this is fixed by #19233?

@A5rocks

@A5rocks nope, this is way more tricky: this only works because gmtime is a "special" C function. If you do the same with

import time

def my_gmtime(t: float | None = None, /) -> time.struct_time:
    return time.gmtime(t)

class Demo:
    will_bind = my_gmtime
    wont_bind = time.gmtime

Demo().wont_bind()
Demo().wont_bind(0)
Demo().will_bind()  # TypeError: 'Demo' object cannot be interpreted as an integer
Demo().will_bind(0)  # TypeError: my_gmtime() takes from 0 to 1 positional arguments but 2 were given

That happens when the RHS of the assignment is a builtin_function_or_method. We don't have typeshed information to distinguish those from plain python functions. Perhaps a short allowlist would do the trick? But gathering that take a lot of work... Can we script it?

Yeah we could have a list for typeshed and go with "if it's from a .pyi file then it's native" as a heuristic for everything else.

I'm not sure it's a good idea to allow this code if alternate implementations or compilations of Python would fail in this case, though. I'm in favor of saying this is intended...

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Mypy false positive for `converter` class attribute in `logging.Formatter` class. #19241

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Mypy false positive for converter class attribute in logging.Formatter class. #19241

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A5rocks commented on Jun 6, 2025

sterliakov commented on Jun 6, 2025

A5rocks commented on Jun 7, 2025

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Mypy false positive for `converter` class attribute in `logging.Formatter` class. #19241