def strftime(date_time=None, time_format=None):
        """
        将 datetime 对象转换为 str

        :param:
            * date_time: (obj) datetime 对象
            * time_format: (sting) 日期格式字符串
        :return:
            * date_time_str: (string) 日期字符串
        """
        if not date_time:
            datetime_now = datetime.now()
        else:
            datetime_now = date_time
        if not time_format:
            time_format = '%Y/%m/%d %H:%M:%S'
        return datetime.strftime(datetime_now, time_format)