don't call format inside logger calls.

[crwilcox]

It is bad practice to call str.format() in a logger call because it's a waste of compute if the logger ever gets set so that debug calls are ignored. …

What you can do is in your logging.Formatter() call above is specify you want to use str.format()-style formatting. Then here you could do:
logger.debug("current max size {} is {}", path, max_size)

[crwilcox]

Also, logger.warning('error writing ' + str(e)) could use an exception-logging call which would log the traceback