19-Create-a-Log-for-your-Python-application.md

Create a Log for your Python application

Creating Logs in Python

First import the logging module. Once we do that, you can type the following to log a message.

import logging
logging.warning('This is a warning')

WARNING:root:This is a warning

Logging Levels

There are different levels of severity in logging. The example above logged a warning message. The levels, in order of most severe to least, are critical, error, warning, info, and debug.

When importing logging, the default is warning. Any level below warning will NOT be returned in the terminal.

Practical Use of Log

When using logging in a project, you of couse start by importing logging. Then you have to configure it.

logging.basicConfig(filename='./demo.log', level=logging.DEBUG

This code is configured to write to a file name (aka demo.log) and sets the logging level to debug.

Here's how it looks using logs in a loop.

for i in range(0, 100):
  if i % 5 == 0:
    logging.debug('Found a number divisible by 5: {0}'.format(i))
  else:
    logging.info('At number {0}'.format(i))
logging.warning('Finished sequence')

Specify Log Level on the Command Line

To be able to specify the log level on the command line, we need import both the sys (allows us to read the command line arguments) and getopt(makes parsing command line arguments easier) modules.

Read the comments in the code for more info.

import sys
import getopt
import logging

log_level="INFO"

opts, args = getopt.getopt(sys.argv[1:], "l:", ["log="])
# created variables opts and args. Call on getopt to parse sys.argv aka system arguments
# slice the list to exclude first arg and look for 2 params, either l or log=

for opt, arg in opts:
    if opt in ("-l", "--log"):
        # if this evaluates to true, indicates log level parameter given to our application
        log_level = getattr(logging, arg.upper())
        # gets the numerica value of the level passed in

logging.basicConfig(filename='./demo.log', level=log_level, format='%(asctime)s %(levelname)s:%(message)s')
# notice that level is changed to machine the log_level variable created in the loop.