When you set the delay parameter of the 
logging.handlers.RotatingFileHandler.__init__() function to True, two 
things go wrong:

1) A previously set formatter is reset (to None) upon lazy creating the 
StreamHandler. That means in practice that you always will have the 
default formatter which just formats the message.

2) Upon a roll-over, the delay parm does not seem to be used anymore as, 
even if there are no records to log, the new current logfile is always 
created.