code: def get_rand(num_digit_min, num_digit_max):
          
          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
docs: Generates a character string of digits. Number of digits are
          between self.num_digit_min and self.num_digit_max
          Returns
          -------
          str