def search_videos_by_tag(self, tag, category=None,
                             period='today',
                             orderby='relevance',
                             page=1, count=20):
        """doc: http://open.youku.com/docs/doc?id=80
        """
        url = 'https://openapi.youku.com/v2/searches/video/by_tag.json'
        params = {
            'client_id': self.client_id,
            'tag': tag,
            'period': period,
            'orderby': orderby,
            'page': page,
            'count': count
        }
        if category:
            params['category'] = category
        r = requests.get(url, params=params)
        check_error(r)
        return r.json()