docs: def rand_str(max_len=6):
      """ 生成 ``max_len`` 长度的字符串
  
      - max_len < -len(NUM_LETTER_STR) will return blank str
  
      :param max_len:
      :type max_len:
      :return:
      :rtype:
      """
      v = [x for x in NUM_LETTER_STR]
      np.random.shuffle(v)
      return ''.join(v[:max_len])