logging.getLogger() infers wrong type when used in combination with logging.setLoggerClass()

[zmwangx]

Consider test.py:

import logging

class Logger(logging.Logger):
    pass

logging.setLoggerClass(Logger)
logger = logging.getLogger(__name__)  # type: Logger

$ mypy test.py
test.py:7: error: Incompatible types in assignment (expression has type "logging.Logger", variable has type "test.Logger")

The problem here is that logging.getLogger() actually returns a logger of the class set previously by logging.setLoggerClass(), but the stub is inflexible and just assumes logging.Logger.

I'm not sure how to fix this, or whether it's fixable with mypy's current feature set. I'd be happy to take this on if someone could point me in the right direction.

In the meantime, is there a better workaround than just # type: ignore the line?

[zmwangx]

Uh, I realized my workaround is wrong; using a # type: ignore only suppresses the error on that line, and mypy, not getting a type hint as in # type: Logger, moves on with the assumption that logger is logging.Logger, and declares any custom method as nonexistent:

import logging

class Logger(logging.Logger):
    def foo(self):
	pass

logging.setLoggerClass(Logger)
logger = logging.getLogger(__name__)  # type: ignore
logger.foo()

$ mypy test.py
test.py:9: error: "Logger" has no attribute "foo"

So I learned a new workaround — casting:

import logging
from typing import cast

class Logger(logging.Logger):
    def foo(self):
        pass

logging.setLoggerClass(Logger)
logger = cast(Logger, logging.getLogger(__name__))
logger.foo()

$ mypy test.py  # this time happy

This feels rather intrusive, but I can settle with this if there's no satisfactory fix.

[JelleZijlstra]

This doesn't feel like something the type system can support in a straightforward manner. I think a cast is your best bet.

[JelleZijlstra]

I don't think we should fix this in typeshed. The only fix I can see is to make getLogger return Any, but I feel like that sacrifices too much type safety in the common case for a rare use case.

Feel free to reopen if you think there is a tractable fix.