The format method of logging.Formatter is buggy in that
it doesn't call the method formatException if the cache
record.exc_text is set. If you have two Formatters that
should format the same log record differently (i.e. each
has its own overriding formatException method), the
formatException method of the second formatter will
never be called because the cache has been set by the
first formatter. The proper way of using the cache is
IMHO to check the cache only in the method
formatException of logging.Formatter.