Ett litet exempel

Leta efter duplicerade användarnamn i /etc/passwd:
    seen = {}
    num_dup = 0
    f = open("/etc/passwd")
    for line in f.readlines():
        (user, rest) = line.split(":",1)
        if seen.has_key(user):
            print "User",user,"has been seen before!"
            num_dup += 1
        else:
            seen[user]= 1 # Any value would do
    print "Number of duplications:", num_dup