It seems that critical, warning, and error do all work, though.
It does change which messages are printed to stdout/err (I don't know which one exactly is used)
I think the problem is that the the log level of the log handler handling the file logging is manually set to WARNING, which overrides the log level set for the logger.
do not set the file log handlers's log level.
The handler now inherits the log level from the logger, which can be changed by qtile.
Should fix #179