'prefix' is the start of the CC number as a string, any number of digits.
        'length' is the length of the CC number to generate. Typically 13 or 16

def _generate_number(self, prefix, length):
        """
        'prefix' is the start of the CC number as a string, any number of digits.
        'length' is the length of the CC number to generate. Typically 13 or 16
        """
        number = prefix
        # Generate random char digits
        number += '#' * (length - len(prefix) - 1)
        number = self.numerify(number)
        reverse = number[::-1]
        # Calculate sum
        tot = 0
        pos = 0
        while pos < length - 1:
            tot += Provider.luhn_lookup[reverse[pos]]
            if pos != (length - 2):
                tot += int(reverse[pos + 1])
            pos += 2
        # Calculate check digit
        check_digit = (10 - (tot % 10)) % 10
        number += str(check_digit)
        return number