Logging verbosity control with a single argument

verbosity.py

"""
A demonstration of how to control stdout verbosity with argparse and logging.

Examples of running this file and its expected output:

$ python verbosity.py
CRITICAL :: A critical message.

$ python verbosity.py -v
CRITICAL :: A critical message.
ERROR :: An error message.

$ python verbosity.py -vv
CRITICAL :: A critical message.
ERROR :: An error message.
WARNING :: A warning message.

$ python verbosity.py -vvv
CRITICAL :: A critical message.
ERROR :: An error message.
WARNING :: A warning message.
INFO :: An info message.

$ python verbosity.py -vvvv
CRITICAL :: A critical message.
ERROR :: An error message.
WARNING :: A warning message.
INFO :: An info message.
DEBUG :: A debug message.
"""

import argparse
import logging

stream_formatter = logging.Formatter(fmt='%(level) :: %(message)s')
stream_handler = logging.StreamHandler()
stream_handler.setFormatter(stream_formatter)

logger = logging.getLogger('verbosity')
logger.addHandler(stream_handler)

parser = argparse.ArgumentParser(prog='verbosity')
parser.add_argument('-v', dest='log_level', action='count', help='Increase verbosity of stdout messages')

if __name__ == '__main__':
    args = parser.parse_args()
    
    # a custom log level with a value higher than critical
    # could be created for a "SILENT" level, but for the 
    # sake of demonstration, default logging levels are used
    effective_log_level = logging.CRITICAL
    level = args.log_level or 0
    
    # the following numbers may change depending on how log levels are set up
    modifier = level * 10  
    if modifier > 40:
        modifier = 40
    effective_level = effective_level - modifier
    
    logger.setLevel(effective_level)
    
    logger.critical('A critical message.')
    logger.error('An error message.')
    logger.warn('A warning message.')
    logger.info('An info message.')
    logger.debug('A debug message.')