How to log from your charm

See first: Juju | Log, Juju | How to manage logs

Ops configures Python logging and warnings to end up in the Juju log, and the Charmcraft profiles provide a logger object for your charm to use at the top of your src/charm.py file. This lets you use the Juju debug-log command to display logs from the charm. Note that it shows logs from the charm code, but not the workload.

Use the provided logger to log information relevant to the charm user. 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.info("Found a new thing: %r", current)
            self._stored.things.append(current)

Tip

In addition to logs from your charm, juju debug-log contains a wealth of information about everything happening in your model. To focus on what’s happening in your charm, you can adjust the logging configuration. For example, to limit logs to ones from your charm and the uniter operation (generally: which events are being emitted), and include DEBUG level logs, use:

juju debug-log --debug --include-module juju.worker.uniter.operation --include-module unit.<charm name>/<unit number>.juju-log

See more: logging

Best practice

Capture output to stdout and stderr in your charm and use the logging and warning functionality to send messages to the charm user, rather than rely on Juju capturing output.

In particular, you should avoid print() calls, and ensure that any subprocess calls capture output.

Best practice

Do not build log strings yourself: allow the logger to do this for you as required. 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}")

Best practice

Ensure that log messages are clear, meaningful, and provide enough information for the user to troubleshoot any issues. Avoid spurious logging. For instance, try not to log when event handlers are called, as the Juju controller does this automatically.

Best practice

Never log credentials or other sensitive information.