How to add a custom loglevel to Python's logging facility

I'd like to have loglevel TRACE (5) for my application, as I don't think that debug() is sufficient. Additionally log(5, msg) isn't what I want. How can I add a custom loglevel to a Python logger?

I've a mylogger.py with the following content:

import logging

@property
def log(obj):
    myLogger = logging.getLogger(obj.__class__.__name__)
    return myLogger

In my code I use it in the following way:

class ExampleClass(object):
    from mylogger import log

    def __init__(self):
        '''The constructor with the logger'''
        self.log.debug("Init runs")

Now I'd like to call self.log.trace("foo bar")

Edit (Dec 8th 2016): I changed the accepted answer to pfa's which is, IMHO, an excellent solution based on the very good proposal from Eric S.

Solution

To people reading in 2022 and beyond: you should probably check out the currently next-highest-rated answer here: https://stackoverflow.com/a/35804945/1691778

My original answer is below.

@Eric S.

Eric S.'s answer is excellent, but I learned by experimentation that this will always cause messages logged at the new debug level to be printed -- regardless of what the log level is set to. So if you make a new level number of 9, if you call setLevel(50), the lower level messages will erroneously be printed.

To prevent that from happening, you need another line inside the "debugv" function to check if the logging level in question is actually enabled.

Fixed example that checks if the logging level is enabled:

import logging
DEBUG_LEVELV_NUM = 9 
logging.addLevelName(DEBUG_LEVELV_NUM, "DEBUGV")
def debugv(self, message, *args, **kws):
    if self.isEnabledFor(DEBUG_LEVELV_NUM):
        # Yes, logger takes its '*args' as 'args'.
        self._log(DEBUG_LEVELV_NUM, message, args, **kws) 
logging.Logger.debugv = debugv

If you look at the code for class Logger in logging.__init__.py for Python 2.7, this is what all the standard log functions do (.critical, .debug, etc.).

I apparently can't post replies to others' answers for lack of reputation... hopefully Eric will update his post if he sees this. =)