EDIT: I added a pull request in python/typeshed to enable workaround 2.

• issue in typeshed: Logger.getChild bad signature typeshed#6606
• pr: Logger.getChild subclass compatible typehint typeshed#6609

So, let's say we want to use a custom logging.Logger sub-class with additional methods e.g. trace, verbose, passed, failed that provide specialized formatting and/or use custom log-levels.

• Python3 add logging level
• How to extend the logging.Logger Class?
• How to add a custom loglevel to Python's logging facility

However, the proposed solutions, either monkey-patching or using a logging.Logger subclass both do not work well with mypy, because the standard way to get a Logger is using logging.getLogger which is unaware of the subclass. See also this blog post by Sam Hooke.

The issue

Suppose we have the following setup code: In config/base module we define a new Logger subclass:

class MyLogger(logging.Logger):
 ...
logging.setLoggerClass(MyLogger)
logger = logging.getLogger(__name__)

Then in submodules:

import logging
logger = logging.getLogger(__name__)

We will get type errors (undefined attribute) each and every time we do logger.verbose / logger.trace etc. There was already a very brief discussion here python/typeshed#1801 without any good resolution.

Possible workarounds

So I was wondering, what could be an acceptable solution? Plastering # type: ignore every time we use logger.verbose does not seem reasonable to me. I think there are two possibilities, which however currently are not quite compatible with mypy.

Workaround proposal 1 - add explicit type hint for the subclass

import logging
from mymodule.logger import MyLogger
logger: MyLogger = logging.getLogger(__name__)

This does not quite work because getLogger has the following signature in typeshed/stdlib/logging`:

def getLogger(name: Optional[str] = ...) -> Logger: ...

Instead, we would need something like

T = TypeVar("T", bound=Logger) 
def getLogger(name: Optional[str] = ...) -> T: ...

In conjunction with a change of how TypeVar works: if no type hint is given assume the type is the bound type. This was discussed here python/mypy#4236 with no resolution.

Workaround proposal 2 - Import the Logger object and create a child.

from mymodule.logger import logger # returns actual object
logger = logger.getChild(__name__)

Here are 2 issues:

1. I am not sure what possible unintended consequences this could have compared to using getLogger.
2. We still need a signature change in typeshed/stdlib/logging: there getChild is defined as

def getChild(self, suffix: str) -> Logger: ...

However, we need something along the lines of

T = TypeVar("T", bound=Logger)
def getChild(self: T, suffix: str) -> T: ...

Cf. python/mypy#1212