def get_incrementer(num, num_digits):
    """Returns string containing num prefixed by zeros such that the final length is equal to num_digits
    :param num: an integer number
    :param num_digits: the desired length of the returned number
    :return: string number
    """
    if type(num) != int:
        print('Number is not an Int.')
        return None
    if num >= 10 ** num_digits:
        print('NUMBER IS LARGER THAN THE MAX POSSIBLE BASED ON num_digits')
        return None
    elif num > 9999999:
        number_length = 8
    elif num > 999999:
        number_length = 7
    elif num > 99999:
        number_length = 6
    elif num > 9999:
        number_length = 5
    elif num > 999:
        number_length = 4
    elif num > 99:
        number_length = 3
    elif num > 9:
        number_length = 2
    else:
        number_length = 1
    char = ''
    for i in range(num_digits - number_length):
        char = char + '0'
    return char + str(num)
