How to implement the --verbose or -v option into a script?

Question

I know the --verbose or -v from several tools and I\'d like to implement this into some of my own scripts and tools.

I thought of placing:<

Answer 1

Building and simplifying @kindall's answer, here's what I typically use:

v_print = None
def main()
    parser = argparse.ArgumentParser()
    parser.add_argument('-v', '--verbosity', action="count", 
                        help="increase output verbosity (e.g., -vv is more than -v)")

    args = parser.parse_args()

    if args.verbosity:
        def _v_print(*verb_args):
            if verb_args[0] > (3 - args.verbosity):
                print verb_args[1]  
    else:
        _v_print = lambda *a: None  # do-nothing function

    global v_print
    v_print = _v_print

if __name__ == '__main__':
    main()

This then provides the following usage throughout your script:

v_print(1, "INFO message")
v_print(2, "WARN message")
v_print(3, "ERROR message")

And your script can be called like this:

% python verbose-tester.py -v
ERROR message

% python verbose=tester.py -vv
WARN message
ERROR message

% python verbose-tester.py -vvv
INFO message
WARN message
ERROR message

A couple notes:

1. Your first argument is your error level, and the second is your message. It has the magic number of 3 that sets the upper bound for your logging, but I accept that as a compromise for simplicity.
2. If you want v_print to work throughout your program, you have to do the junk with the global. It's no fun, but I challenge somebody to find a better way.

Answer 2

It might be cleaner if you have a function, say called vprint, that checks the verbose flag for you. Then you just call your own vprint function any place you want optional verbosity.

Answer 3

Use the logging module:

import logging as log
…
args = p.parse_args()
if args.verbose:
    log.basicConfig(format="%(levelname)s: %(message)s", level=log.DEBUG)
    log.info("Verbose output.")
else:
    log.basicConfig(format="%(levelname)s: %(message)s")

log.info("This should be verbose.")
log.warning("This is a warning.")
log.error("This is an error.")

All of these automatically go to stderr:

% python myprogram.py
WARNING: This is a warning.
ERROR: This is an error.

% python myprogram.py -v
INFO: Verbose output.
INFO: This should be verbose.
WARNING: This is a warning.
ERROR: This is an error.

For more info, see the Python Docs and the tutorials.