gistfile1.py

import io
import sys

def read_file(path, data):
  with io.open(path) as file:
    for domain in file.readlines():
      data.add(domain.strip().lower())

def write_file(data, path='whitelist.txt'):
  with io.open(path, 'w') as file:
    file.write('\n'.join(sorted(data)))

if __name__ == '__main__':
  domains = set()
  for path in sys.argv[1:]:
    read_file(path, domains)
  write_file(domains)