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.

Fixes now in trunk and release26-maint.

Note that RotatingFileHandler will open the file when shouldRollover()
is called - which it is from emit() - and doRollover() will also open
the file even if a delay parameter is set.

TimedRotatingFileHandler will not open a file if delay is set until
rollover actually occurs, as shouldRollover goes by time only rather
than file size.

The delay parameter is intended only as a hint to the handlers to delay
opening until as long as possible, and they will try to honour this as
far as is practicable.