Introduction to logging in Python

tabularasa8931@gmail.com (Han) — Sat, 17 May 2025 00:00:00 +0000

A gentle, practical introduction to `logging` in Python

Why bother with a dedicated logging library?

Prints don’t scale. print() is fine during quick experiments, but real programs need a record that can be filtered, rotated, or shipped elsewhere.
Separation of concerns. You decide what to log in your code; logging decides where and how to write it (console, file, etc.).
Built-in, no extra dependency. The standard library’s logging module is powerful enough for most applications.

Core concepts

Concept	Role in the ecosystem	Typical examples
Logger	The entry point your code calls (`logger.info(...)`). You can have many, one per module.	`"__main__"`, `"my_package.worker"`
Handler	Decides where the record goes.	`StreamHandler` (stdout), `FileHandler`, `TimedRotatingFileHandler`, `SMTPHandler`
Formatter	Decides how the record looks.	`'%(asctime)s - %(levelname)s - %(name)s - %(message)s'`

A minimal logger

 
import logging

logging.basicConfig(
 level=logging.INFO,
 format="%(levelname)s | %(message)s"
)

logging.info("Hello, world!")

basicConfig is a one-liner good for small scripts.
In bigger projects, mixing multiple modules / log files, you’ll want finer control.

Rotating files at midnight

Rotating a log file means creating a new log file after a certain time or size limit is reached. In this case, a new file is created every night at midnight. Only the most recent two log files are kept—yesterday’s and today’s—while older ones are deleted automatically.