Logger

• There are six levels for logging in Python & each level is associated with an integer that indicates the log severity:
  • NOTSET=0
  • DEBUG=10
  • INFO=20
  • WARN=30
  • ERROR=40
  • CRITICAL=50

  

Logger hierarchy

Root Logger

• Changing the basicConfig of the root logger will also affect the log events of the other (lower) loggers in the hierarchy.

import logging
# config for root logger
logging.basicConfig(level=logging.INFO, format='%(name)s - %(levelname)s - %(message)s')
logging.info("This is info from root logger")

Custom Logger with Handler

• Sample logger class: MyLogger
• Logger is probably the one that will be used directly the most often in the code and which is also the most complicated. A new logger can be obtained by logger = logging.getLogger(__name__)
• The log handler is the component that effectively writes/displays a log: Display it in the console (via StreamHandler), in a file (via FileHandler), or even by sending you an email via SMTPHandler, etc.
• A logger has three main fields:
  • propagate Decides whether a log should be propagated to the logger’s parent. By default, its value is True.
  • level Like the log handler level, the logger level is used to filter out “less important” logs. Except, unlike the log handler, the level is only checked at the “child” logger; once the log is propagated to its parents, the level will not be checked. This is rather an un-intuitive behavior.
  • handlers The list of handlers that a log will be sent to when it arrives to a logger. This allows a flexible log handling—for example, you can have a file log handler that logs all DEBUG logs and an email log handler that will only be used for CRITICAL logs. In this regard, the logger-handler relationship is similar to a publisher-consumer one: A log will be broadcast to all handlers once it passes the logger level check.

Best practice

• Configure the root logger but never use it in your code—e.g., never call a function like logging.info(), which actually calls the root logger behind the scenes.
• To use the logging, make sure to create a new logger by using logging.getLogger(logger name)
  • In your application with multiple modules is to create an internal logger using the __name__ global variable (logger = logging.getLogger(__name__)).
  • This will create a logger with the name of your module and ensures no name collisions.

Logging Tips

• Trace back error with logger.exception(f"Cannot divide x={x} with y={y}")