The following program says hello to the user. It takes one positional argument, the name of the user, and can also be told the greeting.

import argparse

parser = argparse.ArgumentParser()

parser.add_argument('name', 
    help='name of user'
)

parser.add_argument('-g', '--greeting', 
    default='Hello',
    help='optional alternate greeting'
)

args = parser.parse_args()

print("{greeting}, {name}!".format(
       greeting=args.greeting,
       name=args.name)
)

$ python hello.py --help
usage: hello.py [-h] [-g GREETING] name

positional arguments:
  name                  name of user

optional arguments:
  -h, --help            show this help message and exit
  -g GREETING, --greeting GREETING
                        optional alternate greeting

$ python hello.py world
Hello, world!
$ python hello.py John -g Howdy
Howdy, John!

For more details please read the argparse documentation.