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:

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])