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logging ⚬⃕ᵖʸ｜Documentation｜1st｜20251021003703-00-⌔

logging — Logging facility for Python — Python 3 documentation

logging — Logging facility for Python

Source code: Lib/logging/init.py

This module defines functions and classes which implement a flexible event logging system for applications and libraries.

The key benefit of having the logging API provided by a standard library module is that all Python modules can participate in logging, so your application log can include your own messages integrated with messages from third-party modules.

Here’s a simple example of idiomatic usage:

# myapp.py
import logging
import mylib
logger = logging.getLogger(__name__)
 
def main():
   logging.basicConfig(filename='myapp.log', level=logging.INFO)
   logger.info('Started')
   mylib.do_something()
   logger.info('Finished')
 
if __name__ == '__main__':
   main()

# mylib.py
import logging
logger = logging.getLogger(__name__)
 
def do_something():
   logger.info('Doing something')

If you run myapp.py, you should see this in myapp.log:

INFO:__main__:Started
INFO:mylib:Doing something
INFO:__main__:Finished

The key feature of this idiomatic usage is that the majority of code is simply creating a module level logger with getLogger(__name__), and using that logger to do any needed logging. This is concise, while allowing downstream code fine-grained control if needed. Logged messages to the module-level logger get forwarded to handlers of loggers in higher-level modules, all the way up to the highest-level logger known as the root logger; this approach is known as hierarchical logging.

For logging to be useful, it needs to be configured: setting the levels and destinations for each logger, potentially changing how specific modules log, often based on command-line arguments or application configuration. In most cases, like the one above, only the root logger needs to be so configured, since all the lower level loggers at module level eventually forward their messages to its handlers. basicConfig() provides a quick way to configure the root logger that handles many use cases.

The module provides a lot of functionality and flexibility. If you are unfamiliar with logging, the best way to get to grips with it is to view the tutorials (see the links above and on the right).

The basic classes defined by the module, together with their attributes and methods, are listed in the sections below.

• Loggers expose the interface that application code directly uses.
• Handlers send the log records (created by loggers) to the appropriate destination.
• Filters provide a finer grained facility for determining which log records to output.
• Formatters specify the layout of log records in the final output.

Printed 2026-06-28.

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