arg_positional_argument_optional.py

import argparse

def main():
    parser = argparse.ArgumentParser(
                    prog=__file__,
                    description='What the program does',
                    epilog='Text at the bottom of help')
    parser.add_argument('filename', nargs='?', choices=['install', 'update', 'remove'])
    args = parser.parse_args()
    print(args)

if __name__ == "__main__":
    main()