extract_from_file.py

def file_to_list(fname):
    f = open(fname)
    lst = f.readlines()
    return [line.rstrip('\n') for line in lst]



def list_to_file(fname, lst):
    f = open(fname, "w")
    f.write("\n".join(lst))
    f.close()

def remove_from(list_from, list_source):
    """Remove every element in list_from 
       that exists in list_source"""

    result = list(list_from)

    for el in list_from:
        if el in list_source:
            print el
            result.remove(el)
    
    return result

subscribers = file_to_list('subscribers.txt')
users = file_to_list('users.txt')
unsubscribers = file_to_list('unsubscribers.txt')
users_non_subscribers = file_to_list('users_non_subscribers.txt')

# Users who are not subscribers
list_to_file('users_non_subscribers.txt', remove_from(users, subscribers))

# Subscribers who are not users
list_to_file('subscribers_non_users.txt', remove_from(subscribers, users))

# later
#list_to_file('test.txt', remove_from(users_non_subscribers, unsubscribers))