Setup and desired behavior:

1. Logging-configuration defined outside of settings.py
2. Want to still have logging with runserver

1.

# custom_logging.py
def set_logger(logger):
    # add formatters and handlers for the supplied logger
def init():
    # call set_logger on the desired loggers

2.

# apps.py
import sys
    [...]
    def ready():
        is_manage_py = any(arg.casefold().endswith("manage.py") for arg in sys.argv)
        is_runserver = any(arg.casefold() == "runserver" for arg in sys.argv)
        is_shell     = any(arg.casefold() == "shell" for arg in sys.argv)

        if is_manage_py and (is_runserver or is_shell):
            custom_logging.init()

All well and dandy … until you need your logs to rotate (logging.handlers.RotatingFileHandler for instance):

--- Logging error ---
Traceback (most recent call last):
  File "Liblogginghandlers.py", line 74, in emit
    self.doRollover()
  File "Liblogginghandlers.py", line 185, in doRollover
    self.rotate(self.baseFilename, dfn)
  File "Liblogginghandlers.py", line 116, in rotate
    os.rename(source, dest)
PermissionError: [WinError 32] The process cannot access the file because it is being used by another process: 'logsmain.log' -> 'logsmain.log.1'

Inspecting the file handle will reveal that Django is the only process holding a lock. So what gives? Restarting the IDE does nothing, restarting the PC does nothing, there are no duplicate processes.

Setting up an instance through gunicorn or similar doesn’t suffer this issue so the problem is likely not in the logging configuration itself, but rather with how/when/etc. the logging configuration gets called.

By way of print-debugging, I can see that the init() method gets run twice before the server has completely started but upon auto-reload it only gets run once.