import logging
from logging import Formatter
from logging.handlers import RotatingFileHandler

handler = RotatingFileHandler('site.log', maxBytes=1000000, backupCount=1)
handler.setLevel(logging.DEBUG)
handler.setFormatter(Formatter('%(asctime)s %(levelname)s: %(message)s '
                              '[in %(pathname)s:%(lineno)d]'))
app.logger.addHandler(handler)