code: def _gen_sample(self):
          
          num_str = self.get_rand(self.num_digit_min, self.num_digit_max)
          return self.captcha.image(num_str), num_str
docs: Generate a random captcha image sample
          Returns
          -------
          (numpy.ndarray, str)
              Tuple of image (numpy ndarray) and character string of digits used to generate the image