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

Question

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")

Thanks in advance for your help.

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.

Answer 1

@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. =)

Answer 2

I took the "avoid seeing lambda" answer and had to modify where the log_at_my_log_level was being added. I too saw the problem that Paul did "I don't think this works. Don't you need logger as the first arg in log_at_my_log_level?" This worked for me

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

Answer 3

Combining all of the existing answers with a bunch of usage experience, I think that I have come up with a list of all the things that need to be done to ensure completely seamless usage of the new level. The steps below assume that you are adding a new level TRACE with value logging.DEBUG - 5 == 5:

1. logging.addLevelName(logging.DEBUG - 5, 'TRACE') needs to be invoked to get the new level registered internally so that it can be referenced by name.
2. The new level needs to be added as an attribute to logging itself for consistency: logging.TRACE = logging.DEBUG - 5.
3. A method called trace needs to be added to the logging module. It should behave just like debug, info, etc.
4. A method called trace needs to be added to the currently configured logger class. Since this is not 100% guaranteed to be logging.Logger, use logging.getLoggerClass() instead.

All the steps are illustrated in the method below:

def addLoggingLevel(levelName, levelNum, methodName=None):
    """
    Comprehensively adds a new logging level to the `logging` module and the
    currently configured logging class.

    `levelName` becomes an attribute of the `logging` module with the value
    `levelNum`. `methodName` becomes a convenience method for both `logging`
    itself and the class returned by `logging.getLoggerClass()` (usually just
    `logging.Logger`). If `methodName` is not specified, `levelName.lower()` is
    used.

    To avoid accidental clobberings of existing attributes, this method will
    raise an `AttributeError` if the level name is already an attribute of the
    `logging` module or if the method name is already present 

    Example
    -------
    >>> addLoggingLevel('TRACE', logging.DEBUG - 5)
    >>> logging.getLogger(__name__).setLevel("TRACE")
    >>> logging.getLogger(__name__).trace('that worked')
    >>> logging.trace('so did this')
    >>> logging.TRACE
    5

    """
    if not methodName:
        methodName = levelName.lower()

    if hasattr(logging, levelName):
       raise AttributeError('{} already defined in logging module'.format(levelName))
    if hasattr(logging, methodName):
       raise AttributeError('{} already defined in logging module'.format(methodName))
    if hasattr(logging.getLoggerClass(), methodName):
       raise AttributeError('{} already defined in logger class'.format(methodName))

    # This method was inspired by the answers to Stack Overflow post
    # http://stackoverflow.com/q/2183233/2988730, especially
    # http://stackoverflow.com/a/13638084/2988730
    def logForLevel(self, message, *args, **kwargs):
        if self.isEnabledFor(levelNum):
            self._log(levelNum, message, *args, **kwargs)
    def logToRoot(message, *args, **kwargs):
        logging.log(levelNum, message, *args, **kwargs)

    logging.addLevelName(levelNum, levelName)
    setattr(logging, levelName, levelNum)
    setattr(logging.getLoggerClass(), methodName, logForLevel)
    setattr(logging, methodName, logToRoot)

Answer 4

This question is rather old, but I just dealt with the same topic and found a way similiar to those already mentioned which appears a little cleaner to me. This was tested on 3.4, so I'm not sure whether the methods used exist in older versions:

from logging import getLoggerClass, addLevelName, setLoggerClass, NOTSET

VERBOSE = 5

class MyLogger(getLoggerClass()):
    def __init__(self, name, level=NOTSET):
        super().__init__(name, level)

        addLevelName(VERBOSE, "VERBOSE")

    def verbose(self, msg, *args, **kwargs):
        if self.isEnabledFor(VERBOSE):
            self._log(VERBOSE, msg, args, **kwargs)

setLoggerClass(MyLogger)

Answer 5

Who started the bad practice of using internal methods (self._log) and why is each answer based on that?! The pythonic solution would be to use self.log instead so you don't have to mess with any internal stuff:

import logging

SUBDEBUG = 5
logging.addLevelName(SUBDEBUG, 'SUBDEBUG')

def subdebug(self, message, *args, **kws):
    self.log(SUBDEBUG, message, *args, **kws) 
logging.Logger.subdebug = subdebug

logging.basicConfig()
l = logging.getLogger()
l.setLevel(SUBDEBUG)
l.subdebug('test')
l.setLevel(logging.DEBUG)
l.subdebug('test')

Answer 6

I find it easier to create a new attribute for the logger object that passes the log() function. I think the logger module provides the addLevelName() and the log() for this very reason. Thus no subclasses or new method needed.

import logging

@property
def log(obj):
    logging.addLevelName(5, 'TRACE')
    myLogger = logging.getLogger(obj.__class__.__name__)
    setattr(myLogger, 'trace', lambda *args: myLogger.log(5, *args))
    return myLogger

now

mylogger.trace('This is a trace message')

should work as expected.

Answer 7

I think you'll have to subclass the Logger class and add a method called trace which basically calls Logger.log with a level lower than DEBUG. I haven't tried this but this is what the docs indicate.

Answer 8

Tips for creating a custom logger:

1. Do not use _log, use log (you don't have to check isEnabledFor)
2. the logging module should be the one creating instance of the custom logger since it does some magic in getLogger, so you will need to set the class via setLoggerClass
3. You do not need to define __init__ for the logger, class if you are not storing anything

# Lower than debug which is 10
TRACE = 5
class MyLogger(logging.Logger):
    def trace(self, msg, *args, **kwargs):
        self.log(TRACE, msg, *args, **kwargs)

When calling this logger use setLoggerClass(MyLogger) to make this the default logger from getLogger

logging.setLoggerClass(MyLogger)
log = logging.getLogger(__name__)
# ...
log.trace("something specific")

You will need to setFormatter, setHandler, and setLevel(TRACE) on the handler and on the log itself to actually se this low level trace

Answer 9

This worked for me:

import logging
logging.basicConfig(
    format='  %(levelname)-8.8s %(funcName)s: %(message)s',
)
logging.NOTE = 32  # positive yet important
logging.addLevelName(logging.NOTE, 'NOTE')      # new level
logging.addLevelName(logging.CRITICAL, 'FATAL') # rename existing

log = logging.getLogger(__name__)
log.note = lambda msg, *args: log._log(logging.NOTE, msg, args)
log.note('school\'s out for summer! %s', 'dude')
log.fatal('file not found.')

The lambda/funcName issue is fixed with logger._log as @marqueed pointed out. I think using lambda looks a bit cleaner, but the drawback is that it can't take keyword arguments. I've never used that myself, so no biggie.

  NOTE     setup: school's out for summer! dude
  FATAL    setup: file not found.

Answer 10

In my experience, this is the full solution the the op's problem... to avoid seeing "lambda" as the function in which the message is emitted, go deeper:

MY_LEVEL_NUM = 25
logging.addLevelName(MY_LEVEL_NUM, "MY_LEVEL_NAME")
def log_at_my_log_level(self, message, *args, **kws):
    # Yes, logger takes its '*args' as 'args'.
    self._log(MY_LEVEL_NUM, message, args, **kws)
logger.log_at_my_log_level = log_at_my_log_level

I've never tried working with a standalone logger class, but I think the basic idea is the same (use _log).

Answer 11

Addition to Mad Physicists example to get file name and line number correct:

def logToRoot(message, *args, **kwargs):
    if logging.root.isEnabledFor(levelNum):
        logging.root._log(levelNum, message, args, **kwargs)

Answer 12

While we have already plenty of correct answers, the following is in my opinion more pythonic:

import logging

from functools import partial, partialmethod

logging.TRACE = 5
logging.addLevelName(logging.TRACE, 'TRACE')
logging.Logger.trace = partialmethod(logging.Logger.log, logging.TRACE)
logging.trace = partial(logging.log, logging.TRACE)

If you want to use mypy on your code, it is recommended to add # type: ignore to suppress warnings from adding attribute.

Answer 13

As alternative to adding an extra method to the Logger class I would recommend using the Logger.log(level, msg) method.

import logging

TRACE = 5
logging.addLevelName(TRACE, 'TRACE')
FORMAT = '%(levelname)s:%(name)s:%(lineno)d:%(message)s'


logging.basicConfig(format=FORMAT)
l = logging.getLogger()
l.setLevel(TRACE)
l.log(TRACE, 'trace message')
l.setLevel(logging.DEBUG)
l.log(TRACE, 'disabled trace message')

Answer 14

I'm confused; with python 3.5, at least, it just works:

import logging


TRACE = 5
"""more detail than debug"""

logging.basicConfig()
logging.addLevelName(TRACE,"TRACE")
logger = logging.getLogger('')
logger.debug("n")
logger.setLevel(logging.DEBUG)
logger.debug("y1")
logger.log(TRACE,"n")
logger.setLevel(TRACE)
logger.log(TRACE,"y2")

output:

DEBUG:root:y1

TRACE:root:y2

Answer 15

based on pinned answer, i wrote a little method which automaticaly create new logging levels

def set_custom_logging_levels(config={}):
    """
        Assign custom levels for logging
            config: is a dict, like
            {
                'EVENT_NAME': EVENT_LEVEL_NUM,
            }
        EVENT_LEVEL_NUM can't be like already has logging module
        logging.DEBUG       = 10
        logging.INFO        = 20
        logging.WARNING     = 30
        logging.ERROR       = 40
        logging.CRITICAL    = 50
    """
    assert isinstance(config, dict), "Configuration must be a dict"

    def get_level_func(level_name, level_num):
        def _blank(self, message, *args, **kws):
            if self.isEnabledFor(level_num):
                # Yes, logger takes its '*args' as 'args'.
                self._log(level_num, message, args, **kws) 
        _blank.__name__ = level_name.lower()
        return _blank

    for level_name, level_num in config.items():
        logging.addLevelName(level_num, level_name.upper())
        setattr(logging.Logger, level_name.lower(), get_level_func(level_name, level_num))

config may smth like that:

new_log_levels = {
    # level_num is in logging.INFO section, that's why it 21, 22, etc..
    "FOO":      21,
    "BAR":      22,
}

Answer 16

In case anyone wants an automated way to add a new logging level to the logging module (or a copy of it) dynamically, I have created this function, expanding @pfa's answer:

def add_level(log_name,custom_log_module=None,log_num=None,
                log_call=None,
                   lower_than=None, higher_than=None, same_as=None,
              verbose=True):
    '''
    Function to dynamically add a new log level to a given custom logging module.
    <custom_log_module>: the logging module. If not provided, then a copy of
        <logging> module is used
    <log_name>: the logging level name
    <log_num>: the logging level num. If not provided, then function checks
        <lower_than>,<higher_than> and <same_as>, at the order mentioned.
        One of those three parameters must hold a string of an already existent
        logging level name.
    In case a level is overwritten and <verbose> is True, then a message in WARNING
        level of the custom logging module is established.
    '''
    if custom_log_module is None:
        import imp
        custom_log_module = imp.load_module('custom_log_module',
                                            *imp.find_module('logging'))
    log_name = log_name.upper()
    def cust_log(par, message, *args, **kws):
        # Yes, logger takes its '*args' as 'args'.
        if par.isEnabledFor(log_num):
            par._log(log_num, message, args, **kws)
    available_level_nums = [key for key in custom_log_module._levelNames
                            if isinstance(key,int)]

    available_levels = {key:custom_log_module._levelNames[key]
                             for key in custom_log_module._levelNames
                            if isinstance(key,str)}
    if log_num is None:
        try:
            if lower_than is not None:
                log_num = available_levels[lower_than]-1
            elif higher_than is not None:
                log_num = available_levels[higher_than]+1
            elif same_as is not None:
                log_num = available_levels[higher_than]
            else:
                raise Exception('Infomation about the '+
                                'log_num should be provided')
        except KeyError:
            raise Exception('Non existent logging level name')
    if log_num in available_level_nums and verbose:
        custom_log_module.warn('Changing ' +
                                  custom_log_module._levelNames[log_num] +
                                  ' to '+log_name)
    custom_log_module.addLevelName(log_num, log_name)

    if log_call is None:
        log_call = log_name.lower()

    setattr(custom_log_module.Logger, log_call, cust_log)
    return custom_log_module