Python Logging Demo

logger_config.json

{
  "version": 1,
  "loggers": {
    "": {
      "level"   : "DEBUG",
      "handlers": ["console_handler"]
    }
  },
  "handlers": {
    "console_handler": {
      "level"    : "DEBUG",
      "formatter": "logfmt",
      "class"    : "logging.StreamHandler"
    }
  },
  "formatters": {
    "logfmt": {
      "format" : "dt=%(asctime)s lvl=%(levelname)s msg=\"%(message)s\"",
      "datefmt": "%Y-%m-%d_%H:%M:%S"
    }
  }
}

logging_demo.py

#!/usr/bin/python3

'''
Demonstrate how logging in python works
https://docs.python.org/3.8/library/logging.html
'''

import logging
import logging.config
import json     # for loading config from file

if __name__ == '__main__':

  CONFIG_FROM_FILE = True

  if CONFIG_FROM_FILE:
    # load the config from a file
    # fileConfig() is getting deprecated, so use the newer dictConfig()
    # https://docs.python.org/3/library/logging.config.html#logging.config.dictConfig
    with open('logger_config.json', 'r') as f:
      cd = json.load(f)

    logging.config.dictConfig(cd)

    # create the logger
    logger = logging.getLogger('my-logger-name')

  else: # config not from file - do it from here

    logger = logging.getLogger('my-logger-name')

    # set output to terminal only
    # each output stream gets its own handler
    handler = logging.StreamHandler()

    # set the logging level
    # logging levels: https://docs.python.org/3.8/library/logging.html#logging-levels
    # CRITICAL, ERROR, WARNING, INFO, DEBUG, NOTSET
    handler.setLevel(logging.DEBUG)

    # set the log format
    formatter = logging.Formatter('%(asctime)s - %(name)s - %(levelname)s - %(message)s')
    handler.setFormatter(formatter)
    logger.addHandler(handler)


  # output messages at different levels
  logger.debug("this is a DEBUG message")
  logger.info("this is an info message")
  logger.warning("this is a warning message")
  logger.error("this is an error message")
  logger.critical("this is a CRITICAL message")

  logger.log(logging.INFO, "this is another way to make an INFO message")

  # a special case of ERROR - adds exception info to the logging message
  try:
    raise Exception("someone set us up the bomb")
  except Exception as e:
    logger.exception("this is an EXCEPTION message")