[Python] How can I subclass logging.Logger without breaking %(filename) in logging.Formatter's...

I am trying to write a custom logging.Logger subclass which is mostly working, but I run into issues when trying to use a logging.Formatter that includes the interpolated value %(filename) in the custom format, it prints the filename where my custom subclass is, rather than the filename of the code that called the logging function.

I've found a number of tutorials for subclassing Logger but none of them address the effects this has on the filename interpolation. Is there a straightforward solution to this without having to override large sections of logging.Logger?

Sample code defining my custom logger:

#-------------------------------------
# custom_logger.py
#-------------------------------------
import logging
import io
class CustomLogger(logging.Logger):

def __init__(self, *args, **kwargs):
super().__init__(*args, **kwargs)
self.setLevel(logging.DEBUG)

# create the record format
formatter = logging.Formatter(fmt = "%(filename)s - %(message)s")

# create the handler
self.stream = io.StringIO()
handler = logging.StreamHandler(self.stream)
handler.setFormatter(formatter)
self.addHandler(handler)

def debug(self, msg, *args, **kwargs):
super().debug(msg, *args, **kwargs)

# do some other stuff
...


#-------------------------------------
# test.py
#-------------------------------------
from custom_logger import CustomLogger
import logging

logging.setLoggerClass(CustomLogger)
myLog = logging.getLogger("myLog")
myLog.debug("hello world")
print(myLog.stream.getvalue())


Expected output: >>> test.py - hello world

Actual output: >>> custom_logger.py - hello world

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