Mypy complains with type-hints

First of all, this library is really, really handy. Logging is so much easier with it. Thanks! Regarding the issue:

I’m working on a project that enforces type-hints with Mypy, and I’m intercepting some logging activity from another process, thus using the InterceptHandler documented on the README file:

# main.py

from loguru import logger

class InterceptHandler(logging.Handler):
    def emit(self, record) -> None:
        # Get corresponding Loguru level if it exists
        try:
            level = logger.level(record.levelname).name
        except ValueError:
            level = record.levelno

        # Find caller from where originated the logged message
        frame, depth = logging.currentframe(), 2
        while frame.f_code.co_filename == logging.__file__:
            frame = frame.f_back
            depth += 1

        logger.opt(depth=depth, exception=record.exc_info).log(level, record.getMessage())


logging.basicConfig(handlers=[InterceptHandler()], level=0)

logger.remove()
logger.add(sys.stderr, format="<green>{time:HH:mm:ss.SSS}</green> | <level>{message}</level>")

logger.info("Hi!")

This is working perfectly, but Mypy is complaining about the missing type-hints:

main.py:4: error: Function is missing a type annotation
main.py:14: error: Incompatible types in assignment (expression has type "Optional[FrameType]", variable has type "FrameType")
Found 2 errors in 1 file (checked 1 source file)

The first one is noticing that the record argument is not typed and the second one is noticing the type of the variable is changing.

Is there a recommended way of solving them?

I tried to import Record from loguru to type-hint the record argument but got several errors after:

main.py:4: error: Argument 1 of "emit" is incompatible with supertype "Handler"; supertype defines the argument type as "LogRecord"
main.py:4: note: This violates the Liskov substitution principle
main.py:4: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#incompatible-overrides
main.py:7: error: "Record" has no attribute "levelname"
main.py:9: error: "Record" has no attribute "levelno"
main.py:17: error: "Record" has no attribute "exc_info"
main.py:17: error: "Record" has no attribute "getMessage"
Found 5 errors in 1 file (checked 1 source file)

[Delgan]

Hey @rafaellehmkuhl, glad you like Loguru, thanks!

About your issue: the InterceptHander inherits from logging.Handler. The emit() method is the one from the standard logging module, just as the record is. The record is not from a Loguru type, it’s a logging.LogRecord object. I guess Mypy should stop complaining if you use this type hint instead. 👍

[rafaellehmkuhl]

@patrickelectric came with this solution:

from logging import LogRecord
from types import FrameType
from typing import Any, List, Optional, Union

class InterceptHandler(logging.Handler):
    def emit(self, record: LogRecord) -> None:
        # Get corresponding Loguru level if it exists
        level: Union[int, str]
        try:
            level = logger.level(record.levelname).name
        except ValueError:
            level = record.levelno

        # Find caller from where originated the logged message
        frame: Optional[FrameType]
        frame, depth = logging.currentframe(), 2
        while frame and frame.f_code.co_filename == logging.__file__:
            frame = frame.f_back
            depth += 1

        logger.opt(depth=depth, exception=record.exc_info).log(level, record.getMessage())

I think its pretty much what is needed. Tested here and it works fine.