Python logging

We can track events in a software application, this is known as logging.  Let's start with a simple example, we will log a warning message.

As opposed to just printing the errors, logging can be configured to disable output or save to a file. This is a big advantage to simple printing the errors.

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Logging example

import logging
# print a log message to the console.
logging.warning('This is a warning!')

This will output:

WARNING:root:This is a warning!

We can easily output to a file:

import logging
logging.basicConfig(filename='program.log',level=logging.DEBUG)
logging.warning('An example message.')
logging.warning('Another message')

The importance of a log message depends on the severity.

Level of severity

The logger module has several levels of severity. We set the level of severity using this line of code:

logging.basicConfig(level=logging.DEBUG)

These are the levels of severity:

The default logging level is warning, which implies that other messages are ignored. If you want to print debug or info log messages you have to change the logging level like so:

import logging
logging.basicConfig(level=logging.DEBUG)
logging.debug('Debug message')

Time in log

You can enable time for logging using this line of code:

logging.basicConfig(format='%(asctime)s %(message)s')

An example below:

import logging
logging.basicConfig(format='%(asctime)s %(message)s', level=logging.DEBUG)
logging.info('Logging app started')
logging.warning('An example logging message.')
logging.warning('Another log message')

Output:

2015-06-25 23:24:01,153 Logging app started
2015-06-25 23:24:01,153 An example message.
2015-06-25 23:24:01,153 Another message

Type	Description
DEBUG	Information only for problem diagnostics
INFO	The program is running as expected
WARNING	Indicate something went wrong
ERROR	The software will no longer be able to function
CRITICAL	Very serious error