How do I refactor a simple commandline script to be object oriented?

I don't have a lot of experience with object-oriented Python and want to refactor a simple command-line tool. My current script simply imports needed libraries, has a few function definitions, uses global variables (which I know is bad practice) and uses argparse all in one .py file, e.g.:

import argparse

dict = {}

#Some code to populate the dict, used both in checking the argument and later in the script

def check_value(value):
    if not dict.has_key(value):
        raise argparse.ArgumentTypeError("%s is invalid." % value)
    return value

parser = argparse.ArgumentParser(…)
parser.add_argument('…', type=check_value, help='…')

args = parser.parse_args()

# Some other code that uses the dict

As well, the way that I handle some of the argument parsing uses a function similar to "abc" which modifies a dictionary I need later in the script. How do I make this less ugly? Or is using global variables acceptable for a simple commandline script?