Logging in Python - Cheat Sheet

I’ve written a piece about logging Log4j2 for Kotlin

and the motivation to use logging in Python is the same:

Logging is a common good practice in software engineering. It enables you to monitor applications in production to gather information about crashes and other malfunctions for further analysis. It is the “little brother” of debugging and often a precursor for setting up test cases which can lead to reproducing the bugs on the developers machine.

Table of Contents

Setup

The good thing about Python: batteries included!

import logging

and Bob’s your uncle!

The Most Basic Logging

import logging

logger = logging.getLogger()

if __name__ == '__main__':
    # setup
    sh = logging.StreamHandler()
    sh.setLevel(logging.INFO)
    logger.addHandler(sh)
    logger.setLevel(logging.INFO)
    # logging
    logger.info("hello from main")

Let’s call it the Hello World of logging:

get a logger
setup a stream handler
set the log level
start logging away

The output is

hello from main

Wait, what? Could have done it with print as well.

Fair enough add some

Log output Formatting

import logging

logger = logging.getLogger()

if __name__ == '__main__':
    # setup
    sh = logging.StreamHandler()
    sh.setLevel(logging.INFO)
    FORMAT_STRING = '%(asctime)s - %(name)s - %(levelname)s - %(message)s'
    formatter = logging.Formatter(FORMAT_STRING)
    sh.setFormatter(formatter)
    logger.addHandler(sh)
    logger.setLevel(logging.INFO)
    # logging
    logger.info("hello from main")

adding a formatter to the stream handler we now get

2022-01-26 13:29:49,166 - root - INFO - hello from main

which definitely looks more like real logging

Other Handlers

Just printing whoops logging to the command line is not very interesting. When your software runs on a server you want to have a log file as well. Here you are:

import logging

logger = logging.getLogger()

if __name__ == '__main__':
    # setup
    FORMAT_STRING = '%(asctime)s - %(name)s - %(levelname)s - %(message)s'
    formatter = logging.Formatter(FORMAT_STRING)
    fh = logging.FileHandler('logging_tutorial.log', encoding="UTF-8")
    fh.setLevel(logging.DEBUG)
    fh.setFormatter(formatter)
    logger.addHandler(fh)
    logger.setLevel(logging.INFO)
    # logging
    logger.info("hello from main")

This produces a nice log file called logging_tutorial.log

You can decide if you want to have the behavior of appending (which is default) or you can erase the log with every rerun by providing mode=’w’ to the FileHandler constructor.

Logging modules and classes

Modules

import logging

logger = logging.getLogger(__name__)


def do_some_logging():
    logger.info("Hello from mymodule")

Classes

import logging

logger = logging.getLogger(__name__)


class MyClass:
    def do_some_logging(self):
        logger.error("do_some_logging: right here - right now")

When logging modules or classes you can use the __name__ to let the logger spit out the module name into the log.

Caveat: Do not use __name__ in your main module then!

import logging

logger = logging.getLogger() # don't use __name__ here!


def setup_logging():
    logger.setLevel(logging.INFO)
    ch = logging.StreamHandler()
    ch.setLevel(logging.INFO)
    formatter = logging.Formatter('%(asctime)s - %(name)s - %(levelname)s - %(message)s')
    ch.setFormatter(formatter)
    logger.addHandler(ch)


if __name__ == '__main__':
    setup_logging()
    logger.info("hello from main")

    from my_module import do_some_logging
    do_some_logging()

    from MyClass import MyClass
    myClass = MyClass()
    myClass.do_some_logging()

Now we get:

2022-01-26 14:16:54,300 - root - INFO - hello from main
2022-01-26 14:16:54,300 - my_module - INFO - Hello from mymodule
2022-01-26 14:16:54,300 - MyClass - ERROR - do_some_logging: right here - right now

	root-Logger & __name__	named-Logger
in main	logger = logging.getLogger()	logger = logging.getLogger("my_application")
in module	module_logger = logging.getLogger(__name__)	module_logger = logging.getLogger("my_application.my_module")
in class	class MyClass: def __init__(self): self.logger = logging.getLogger(__name__)	class MyClass: def __init__(self): self.logger = logging.getLogger("my_application.MyClass")

root-Logger & __name__

named-Logger

in main

logger = logging.getLogger()

logger = logging.getLogger("my_application")

in module

module_logger = logging.getLogger(__name__)

module_logger = logging.getLogger("my_application.my_module")

in class

class MyClass:
    def __init__(self):
        self.logger = logging.getLogger(__name__)

class MyClass:
    def __init__(self):
        self.logger = logging.getLogger("my_application.MyClass")

Log Levels

CRITICAL = 50
FATAL = CRITICAL
ERROR = 40
WARNING = 30
WARN = WARNING
INFO = 20
DEBUG = 10
NOTSET = 0

Logging in Python – Cheat Sheet