Logging function

When to use logging

Task you want to perform The best tool for task
Display console output for ordinary usage of a command line script or program print()
Report events that occur during normal operation of a program (e.g. for status monitoring or fault investigation) logging.info() (or logging.debug() for very detailed output for diagnostic purposes)
Issue a warning regarding a particular runtime event warnings.warn() in library code if the issue is avoidable and the client application should be modified to eliminate the warning; logging.warning() if there is nothing the client application can do about the situation, but the event should still be noted
Report an error regarding a particular runtime event Raise an exception
Report suppression of an error without raising an exception (e.g. error handler in a long-running server process) logging.error(), logging.exception() or logging.critical() as appropriate for the specific error and application domain

Logging level

Level When it’s used
DEBUG Detailed information, typically of interest only when diagnosing problems.
INFO Confirmation that things are working as expected.
WARNING(default) An indication that something unexpected happened, or indicative of some problem in the near future (e.g. ‘disk space low’). The software is still working as expected.
ERROR Due to a more serious problem, the software has not been able to perform some function.
CRITICAL A serious error, indicating that the program itself may be unable to continue running.

The default level is WARNING, which means that only events of this level and above will be tracked, unless the logging package is configured to do otherwise.

DEBUG > INFO > WARNING > ERROR > CRITICAL
This means that, if the level is WARNING, then WARNING, ERROR, CRITICAL information will be tracked; If the level is INFO, then INFO, WARNING, ERROR and CRITICAL infomation will be tracked.

Events that are tracked can be handled in different ways. The simplest way of handling tracked events is to print them to the console. Another common way is to write them to a disk file.

A simple example

import logging
logging.warning('Watch out!')  # will print a message to the console
logging.info('I told you so')  # will not print anything

output:

WARNING:root:Watch out!

Logging to a file

import logging
logging.basicConfig(filename='example.log',level=logging.DEBUG)
logging.debug('This message should go to the log file')
logging.info('So should this')
logging.warning('And this, too')

output:

DEBUG:root:This message should go to the log file
INFO:root:So should this
WARNING:root:And this, too

If you run the above script several times, the messages from successive runs are appended to the file example.log. If you want each run to start afresh, not remembering the messages from earlier runs, you can specify the filemode argument, by changing the call in the above example to:

logging.basicConfig(filename='example.log', filemode='w', level=logging.DEBUG)

The output will be the same as before, but the log file is no longer appended to, so the messages from earlier runs are lost.

Logging from multiple modules

# myapp.py
import logging
import mylib

def main():
    logging.basicConfig(filename='myapp.log', level=logging.INFO)
    logging.info('Started')
    mylib.do_something()
    logging.info('Finished')

if __name__ == '__main__':
    main()
# mylib.py
import logging

def do_something():
    logging.info('Doing something')

output of file:

INFO:root:Started
INFO:root:Doing something
INFO:root:Finished

Logging variable data

import logging
logging.warning('%s before you %s', 'Look', 'leap!')

output:

WARNING:root:Look before you leap!

Changing the format of displayed messages

import logging
logging.basicConfig(format='%(levelname)s:%(message)s', level=logging.DEBUG)
logging.debug('This message should appear on the console')
logging.info('So should this')
logging.warning('And this, too')

output:

DEBUG:This message should appear on the console
INFO:So should this
WARNING:And this, too

Displaying the date/time in messages

ISO8601(default) format

import logging
logging.basicConfig(format='%(asctime)s %(message)s')
logging.warning('is when this event was logged.')

output:

2018-03-20 14:07:19,536 is when this event was logged.

particular format

import logging
logging.basicConfig(format='%(asctime)s %(message)s', datefmt='%m/%d/%Y %I:%M:%S %p')
logging.warning('is when this event was logged.')

output

03/20/2018 02:10:09 PM is when this event was logged.