Gavin Wiggins

Command-line Arguments with Python

The argparse module provides an interface to write command-line programs in Python. It is part of the Python standard library so there's no need for external dependencies. A basic example is given below that demonstrates positional and optional arguments. The second example shows the use of subparsers to create subcommands.

Basic example

This example uses argparse to create positional argument n and an optional argument -d.

# main.py

import argparse

# Create command-line parser and arguments
parser = argparse.ArgumentParser()
parser.add_argument('n', type=int, help='number of items to print')
parser.add_argument('-d', '--done', action='store_true', help='print when done')
args = parser.parse_args()

# Get an integer value
n = args.n
for n in range(n):
    print('Item ', n)

# Get a boolean value
if args.done:
    print('Program is done.')

The command-line program shown above is in a file named main.py. Output from running the program in the terminal are shown below.

$ python main.py 8
Item  0
Item  1
Item  2
Item  3
Item  4
Item  5
Item  6
Item  7

$ python main.py 8 --done
Item  0
Item  1
Item  2
Item  3
Item  4
Item  5
Item  6
Item  7
Program is done.

$ python main.py -h
usage: main.py [-h] [-d] n

positional arguments:
  n           number of items to print

optional arguments:
  -h, --help  show this help message and exit
  -d, --done  print when done

Subcommands example

This example uses subparsers to create two subcommands start and stop. Output from running the program with the subcommands is also shown below.

# main.py

import argparse


def start(args):
    """Invoked by the `start` subcommand."""
    print("start it up")


def stop(args):
    """Invoked by the `stop` subcommand."""
    print("shut it down")
    if args.baz:
        print("with", args.baz)


def main():
    """Run command line parsers."""
    parser = argparse.ArgumentParser(description="Example with subparsers.")
    parser.add_argument("--num", type=int, help="number of items to print")

    subparsers = parser.add_subparsers(title="subcommands", help="valid subcommands")

    # Create subparser for `start` subcommand
    parser_start = subparsers.add_parser("start", help="start the operation")
    parser_start.set_defaults(func=start)

    # Create subparser for `stop` subcommand
    parser_b = subparsers.add_parser("stop", help="shutdown the operation")
    parser_b.set_defaults(func=stop)
    parser_b.add_argument("--baz", choices="XYZ", help="choose a letter")

    # Get command line arguments
    args = parser.parse_args()

    # Print out numbers if `num` option is given
    if args.num:
        n = args.num
        for i in range(n):
            print("item number", i)

    # Run the function associated with a subcommand
    if hasattr(args, "func"):
        args.func(args)


if __name__ == "__main__":
    main()

$ python main.py --num 5
item number 0
item number 1
item number 2
item number 3
item number 4

$ python main.py start
start it up

$ python main.py stop
shut it down

$ python main.py stop --baz X
shut it down
with X

$ python main.py --help
usage: main.py [-h] [--num NUM] {start,stop} ...

Example with subparsers.

options:
  -h, --help    show this help message and exit
  --num NUM     number of items to print

subcommands:
  {start,stop}  valid subcommands
    start       start the operation
    stop        shutdown the operation