Parse hostnames from URLs

parse_hostnames.py

#!/usr/bin/python
from urlparse import urlparse
from argparse import ArgumentParser
import sys

def main():
    parser = ArgumentParser(description='Provide file with URLs and file to save extracted hostnames')
    parser.add_argument('-i', dest='input_filename', help='Enter name of input file, containing URLs', metavar='FILE')
    parser.add_argument('-o', dest='output_filename', help='Enter name of output file to save extracted hostnames', metavar='FILE')
    if len(sys.argv) == 1:
        parser.print_help()
        sys.exit(1)
    args = parser.parse_args()

    hostnames = []
    with open(args.input_filename, 'r') as input_file, open(args.output_filename, 'wb') as output_file:
        for line in input_file:
            hostname = urlparse(line.strip('\n')).netloc
            if hostname not in hostnames:
                hostnames.append(hostname)
        for hostname in hostnames:
            output_file.write(hostname + '\n')


if __name__ == '__main__':
    main()