Idiomatic Python logging: format string + args list vs. inline string formatting - which is preferred?

Question

Is it advantageous to call logging functions with format string + args list vs. formatting inline?

I\'ve seen (and written) logging code that uses inline string form

Answer 1

IMHO, for messages that are very likely to be displayed, such as those given to error or warn it does not make much of a difference.

For messages that are less likely displayed, I would definitely go for the second version, mainly for performance reasons. I often give large objects as a parameter to info, which implement a costly __str__ method. Clearly, sending this pre-formatted to info would be a performance waste.

UPDATE

I just checked the source code of the logging module and, indeed, formatting is done after checking the log level. For example:

class Logger(Filterer):
    # snip
    def debug(self, msg, *args, **kwargs):
        # snip
        if self.isenabledfor(debug):
            self._log(debug, msg, args, **kwargs)

One can observe that msg and args are untouched between calling log and checking the log level.

UPDATE 2

Spired by Levon, let me add some tests for objects that have a costly __str__ method:

$ python -m timeit -n 1000000 -s "import logging" -s "logger = logging.getLogger('foo')" -s "logger.setLevel(logging.ERROR)" "logger.warn('%s', range(0,100))"
1000000 loops, best of 3: 1.52 usec per loop
$ python -m timeit -n 1000000 -s "import logging" -s "logger = logging.getLogger('foo')" -s "logger.setLevel(logging.ERROR)" "logger.warn('%s' % range(0,100))"
1000000 loops, best of 3: 10.4 usec per loop

In practice, this could give a fairly high performance boost.