logging.config.fileConfig gotcha

If you use logging.config.fileConfig (e.g. because you use paster serve something.ini to deploy your WSGI apps) you should know about this.

By default fileConfig disables all pre-existing loggers if they (or their parent loggers) are not explicitly mentioned in your .ini file.

This can result in unintuitive behaviour:

(if you don't see the embedded example, you can find it at https://gist.github.com/1642893).

If you have Python 2.6 or later (and you should), you can turn this off by passing disable_existing_loggers=False to fileConfig(). But what if it's not you calling fileConfig() but your framework (e.g. the above-mentioned paster serve)?

Now usually paster serve tries to configure logging before importing any of your application modules, so there should be no pre-existing loggers to disable. Sometimes, however, this doesn't work for one reason or another, and you end up with your production server suppressing warnings and errors that should not be suppressed. I haven't actually figured out yet who's responsible for those early imports in the application I'm working on (until today I assumed, incorrectly, that paster imports the module containing your WSGI app before it calls fileConfig).

If you're not sure if this bug can bite you or not, check that you don't have any disabled loggers by doing something like

import logging
assert not any(getattr(logger, 'disabled', False)
               for logger in logging.getLogger().manager.loggerDict.values())

while your application is running.