Generates a character string of digits. Number of digits are
        between self.num_digit_min and self.num_digit_max
        Returns
        -------
        str

def get_rand(num_digit_min, num_digit_max):
        """Generates a character string of digits. Number of digits are
        between self.num_digit_min and self.num_digit_max
        Returns
        -------
        str
        """
        buf = ""
        max_len = random.randint(num_digit_min, num_digit_max)
        for i in range(max_len):
            buf += str(random.randint(0, 9))
        return buf