Python

UNSET
DEBUG, logging.debug()
INFO, logging.info()
WARNING, default level, only events of this level and above will be tracked
- warnings.warn(), in library code if the issue is avoidable and the client application should be modified to eliminate the warning
- logging.warning(), if there is nothing the client application can do about the situation, but the event should still be noted
ERROR
- Report suppression of an error without raising an exception
- logging.error(), logging.exception()
CRITICAL, logging.critical()

#WARNING is defalut level
import logging

logging.warning('Watch out!')  # will print a message to the console
logging.info('I told you so')  # will not print anything

import logging
logging.basicConfig(level=logging.DEBUG) # adjust level

logging.warning('Watch out!') # will print
logging.info('I told you so') # will print

import logging

logging.basicConfig(filename='log.txt', level=logging.DEBUG)

logging.warning('Watch out!')
logging.info('I told you so')

# l2.py
import logging

def do_something():
    logging.info('Logging in lib ...');

# l1.py
import logging
import l2

def main():
    logging.basicConfig(filename='log.txt', level=logging.INFO)

    logging.warning('Watch out!')
    logging.info('I told you so')

    l2.do_something()

if __name__ == '__main__':
    main()

import logging

logging.basicConfig(format='[%(levelname)s] (%(threadName)-10s) %(name)s %(asctime)s : %(message)s', filename='log.txt', level=logging.INFO)

logging.warning('Watch out!')
logging.info('I told you so')

[WARNING] (MainThread) root 2023-07-06 06:21:26,437 : Watch out!
[INFO] (MainThread) root 2023-07-06 06:21:26,437 : I told you so

Logger, Expose the interface that application code directly uses

Handler, Send the log records (created by loggers) to the appropriate destination

Filter, Determining which log records to output

Formatter, Specify the layout of log records in the final output

LogRecord, instances are created automatically by the Logger every time something is logged

#WARNING is defalut level
import logging

class UserRunningProgramFilter(logging.Filter):
    def filter(self, record):
        print(record.module, record.name, record.pathname, record.getMessage())
        if record.name != 'root': # returns True to log it or False to discard it
            return True

def get_logger():
    # create a logger
    logger = logging.getLogger()
    logger.setLevel(logging.DEBUG) # set up logging level

    # create handler
    handler = logging.FileHandler('log.txt', mode='a', encoding=None, delay=False)

    # create formater
    formatter = logging.Formatter('%(asctime)s - %(name)s - %(levelname)s - %(message)s')

    handler.addFilter(UserRunningProgramFilter())
    handler.setFormatter(formatter) # set up format for handler
    logger.addHandler(handler) # add handler for logger

    return logger

logger = get_logger()

def main():

    logger.warning('Watch out!')
    logger.info('I told you so')

    logger.info('End ...')

if __name__ == '__main__':
    main()

Create rotating log files

Output logging into a new log file each time logging some messages

import logging
import logging.handlers

logger = logging.getLogger()
logger.setLevel(logging.DEBUG)

handler = logging.handlers.RotatingFileHandler(
    'log.txt',
    maxBytes=2,
    backupCount=3, # create 4 log rotating log files
)
logger.addHandler(handler)

# Log some messages
for i in range(10):
    logger.debug('i = %d' % i)

Logger instances are configured in a tree structure, based on their names

root has no name

If a logger does not have any handlers, the message is handed to its parent for processing

In the following sample code, loggers in the module l2 and the package use the handlers defined in parent module l1

l1.py

#WARNING is defalut level
import logging
import l2
from package.l3 import do_something

def get_logger():
    # create a logger
    logger = logging.getLogger()
    logger.setLevel(logging.DEBUG) # set up logging level

    # create handler
    handler = logging.FileHandler('log.txt', mode='a', encoding=None, delay=False)

    # create formater
    formatter = logging.Formatter('%(asctime)s - %(name)s - %(levelname)s - %(message)s')
    handler.setFormatter(formatter) # set up format for handler
    logger.addHandler(handler) # add handler for logger

    return logger

logger = get_logger()

def main():

    logger.warning('Watch out!')
    logger.info('I told you so')

    l2.do_something()
    do_something()

    logger.info('End ...')

if __name__ == '__main__':
    main()

l2.py

import logging

logger = logging.getLogger('l2')

def do_something():
    logger.info('Do Something ...')

__init__.py

# empty

l3.py

import logging

logger = logging.getLogger('package.l3')

def do_something():
    logger.info('Do Something ...')

log.txt

2023-07-06 06:36:24,954 - root - WARNING - Watch out!
2023-07-06 06:36:24,954 - root - INFO - I told you so
2023-07-06 06:36:24,954 - l2 - INFO - Do Something ...
2023-07-06 06:36:24,955 - package.l3 - INFO - Do Something ...
2023-07-06 06:36:24,955 - root - INFO - End ...

Integrates warning into logging using the WARNING level

import logging
import warnings

logging.basicConfig(
    filename='log.txt',
    level=logging.INFO,
)

# output warning on screen
warnings.warn('This warning is not sent to the logs')

# send warning information to logging
logging.captureWarnings(True)

# output warning to logging file
warnings.warn('This warning is sent to the logs')
warnings.warn('End ...')