#! /usr/bin/env python
import argparse
import sys

class UniqueAppendAction(argparse.Action):
    def __call__(self, parser, namespace, values, option_string=None):
        unique_values = set(values)
        setattr(namespace, self.dest, unique_values)


def get_args(argv=None):
    parser = argparse.ArgumentParser()
    parser.add_argument("--myargs",
                        nargs='*',
                        default=['alpha', 'gamma'],
                        action=UniqueAppendAction,
                        choices=['alpha', 'beta', 'gamma'])

    return parser.parse_args(argv)


def main():
    args = get_args(sys.argv[1:])
    print(args.myargs)

if __name__ == '__main__':
    main()