def create_friendship(self, access_token,
                          user_id=None, user_name=None):
        """doc: http://open.youku.com/docs/doc?id=28
        """
        url = 'https://openapi.youku.com/v2/users/friendship/create.json'
        data = {
            'client_id': self.client_id,
            'access_token': access_token,
            'user_id': user_id,
            'user_name': user_name
        }
        data = remove_none_value(data)
        r = requests.post(url, data=data)
        check_error(r)
        return r.json()