The logging module has already 5 log levels: CRITICAL, ERROR, WARNING, INFO, DEBUG.
https://docs.python.org/dev/library/logging.html#logging-levels
(I don't count NOTSET, I don't consider it to be "usable", at least not in an application or a library.)

For a large application, sometimes we need to a 6th level since DEBUG might be too verbose for some customers (random value: 1 MiB of log per minute). The OpenStack project has 2 additional logging levels: TRACE and AUDIT. See the Oslo Log project used by all OpenStack projects:

https://github.com/openstack/oslo.log/blob/master/oslo_log/handlers.py#L39

The typical usage for TRACE is to log a Python traceback when an exception is logged:

logger.debug("Oops: something bad happened! %s", exc)
for line in my_exception_formatter(exc):
   logger.trace(line)

In software development, "trace" term is commonly associated to "debug traces", as in "How to get debug traces?". Or more commonly in Python: "traceback" or "stack trace".

An additional level helps to filter logs, but also to colorize logs differently.

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I don't propose to add a new AUDIT level since it's not well defined and it's less popular in OpenStack. For example, the Oslo Log module adds a new logger.trace() method, but no logger.audit().