How to log a message in a charm

See first: Juju | Log, Juju | How to manage logs > Manage the logging configuration

To log a message in a charm, import Python’s logging module, then use the getLogger() function with the desired level. For example:

import logging
# ...
logger = logging.getLogger(__name__)

class HelloOperatorCharm(ops.CharmBase):
    # ...

    def _on_config_changed(self, _):
        current = self.config["thing"]
        if current not in self._stored.things:
            # Note the use of the logger here
            logger.debug("found a new thing: %r", current)
            self._stored.things.append(current)

See more:

Tips for good practice:

  • Note that some logging is performed automatically by the Juju controller, for example when an event handler is called. Try not to replicate this behaviour in your own code.

  • Keep developer specific logging to a minimum, and use logger.debug() for such output. If you are debugging a problem, ensure you comment out or remove large information dumps (such as config files, etc.) from the logging once you are finished.

  • When passing messages to the logger, do not build the strings yourself. Allow the logger to do this for you as required by the specified log level. That is:

# Do this!
logger.info("Got some information %s", info)
# Don't do this
logger.info("Got some information {}".format(info))
# Or this ...
logger.info(f"Got some more information {more_info}")

Last updated 14 days ago. Help improve this document in the forum.