def get_pseudonym(nhs_number):
    global GENERATE_NEW

    pseudo = LOOKUP.get(nhs_number)
    if pseudo is not None:
        return pseudo

    if GENERATE_NEW is not True:
        print("I have encountered a new NHS number (%d) with no pseudonym.\n"
              "Should I generate new ones for any new NHS numbers I find "
              "from now on?" % nhs_number)
        response = raw_input("type y or n:")
        if response == 'y':
            GENERATE_NEW = True
        else:
            print("In that case, I will exit now.")
            exit()

    while True:
        digits = []
        s = ''
        tot = 0
        for i in range(9):
            if i == 0:
                digit = random.randint(1, 9)
            else:
                digit = random.randint(0, 9)
            digits.append(digit)
            s += str(digit)
            tot += digit * (10 - i)  # (10 - i) is the weighting factor

        checksum = 11 - (tot % 11)

        if checksum == 11:
            checksum = 0
        if checksum != 10:  # 10 is an invalid nhs number
            s += str(checksum)

            pseudo = int(s)
            LOOKUP[nhs_number] = pseudo

            return pseudo