My implementation of a random generator of valid credit card numbers

cc_number_generator.py

from numpy import random


def luhn_checksum(n):
    """Calculates Luhn's Checksum of a number. n must be a string.

    Check Wikipedia to understand the algorithm."""

    even = 0
    odd = 1

    # If number has an even length, you double the digits in odd indexes.
    # Otherwise, you double the digits in even indexes
    if len(n) % 2 == 0:
        to_double = odd
    else:
        to_double = even

    sum_digits = []
    for i in range(len(n)):
        if (i + 1) % 2 == to_double:
            sum_digit = (int(n[i]) * 2) % 9
            sum_digits.append(sum_digit)
        else:
            sum_digits.append(int(n[i]))

    return sum(sum_digits) % 10


def is_luhn_valid(n):
    return luhn_checksum(n) == 0


def generate(prefix, credit_card_number_length):
    """Generate a valid credit card number with length
    'credit_card_number_length' and prefix 'prefix'.

    'prefix' must be a string."""

    len_check_digit = 1
    id_length = credit_card_number_length - len(prefix) - len_check_digit

    array_random_int_digits = random.choice(range(0, 10), id_length)
    random_id = ''.join(map(lambda x: str(x), array_random_int_digits))

    check_digit = '0'
    cc_number = prefix + random_id + check_digit
    cc_checksum = luhn_checksum(cc_number)

    if cc_checksum != 0:
        check_digit = str(10 - cc_checksum)
        cc_number = cc_number[:-1] + check_digit

    return cc_number


# Sanity check tests
prefix = "372542"
for i in range(1, 1000):
    n = generate(prefix, len(prefix) + i)
    assert is_luhn_valid(n)