27. Logging

The logging module is the standard-library way to record what a program is doing. It is better than scattering print() calls through larger programs because it has levels, formatting, and better control.

27.1. Basic logging

Set up a basic logger and emit messages at several levels.

import logging

logging.basicConfig(level=logging.INFO)

logging.debug('debug details')
logging.info('application started')
logging.warning('low disk space')
logging.error('something went wrong')

27.2. Logging to a file

You can write log output to a file instead of the console.

import logging

logging.basicConfig(
    filename='app.log',
    level=logging.INFO,
    format='%(levelname)s:%(message)s',
)

logging.info('file logger ready')
logging.warning('watch this condition')

27.3. Why logging matters

Logging is especially helpful when a script grows into a real application. It tells you what happened, when it happened, and where something failed.

27.4. Exercise

Create a script that processes a list of usernames. Log an INFO message for each valid username and a WARNING message for any empty username.

27.4.1. Solution

import logging

logging.basicConfig(level=logging.INFO)

for username in ['ava', '', 'mia']:
    if not username:
        logging.warning('empty username encountered')
    else:
        logging.info('processing %s', username)