def update(self, distributor_id, grade_id, session, trade_type=None):
        '''taobao.fenxiao.cooperation.update 更新合作关系等级
        
        供应商更新合作的分销商等级'''
        request = TOPRequest('taobao.fenxiao.cooperation.update')
        request['distributor_id'] = distributor_id
        request['grade_id'] = grade_id
        if trade_type!=None: request['trade_type'] = trade_type
        self.create(self.execute(request, session), fields=['is_success'])
        return self.is_success