Command line arguments

For standalone programs, we often want to have our program take command line arguments that affect the runtime behavior of our program. There are a variety of mechanisms to do this in python, but the best option is the argparse module.

Here’s an example of using argparse to take a variety of options:

#!/usr/bin/env python3

# to get usage: use -h
import argparse


def setup_args():

    # simple example of argparse

    parser = argparse.ArgumentParser()
    parser.add_argument("-a", help="the -a option", action="store_true")
    parser.add_argument("-b", help="-b takes a number", type=int, default=0)
    parser.add_argument("-c", help="-c takes a string", type=str, default=None)
    parser.add_argument("--darg", help="the --darg option", action="store_true")
    parser.add_argument("--earg", help="--earg takes a string", type=str, metavar="test",
                        default="example string")

    # extra arguments (positional)
    parser.add_argument("extras", metavar="extra", type=str, nargs="*",
                        help="optional positional arguments")

    return parser.parse_args()


if __name__ == "__main__":

    args = setup_args()


    if args.a:
        print("-a set")
    print(f"-b = {args.b}")
    print(f"-c = {args.c}")
    if args.darg:
        print("--dargs set")
    print(f"--earg value = {args.earg}")

    print(" ")
    print("extra positional arguments: ")
    if len(args.extras) > 0:
        for e in args.extras:
            print(e)

A nice feature of argparse is that it automatically generates help for us. If we place the above code in argparse_example.py then we can do:

python argparse_example.py --help