django 中间件问题

`class SessionExpiredMiddleware(object):
    def process_request(self,request):
        if 'last_activity' in request.session:
            last_activity = request.session['last_activity']
            now = datetime.datetime.now()

            if (last_activity + datetime.timedelta(minutes=1)) < now:
                if 'user_name' in request.session:
                        del request.session['user_name']
                if 'last_activity' in request.session:
                        del request.session['last_activity']
                error_info = u'登录超时，请重新登录！'
                logging.info('Login session expired!')
                return render_to_response("framework/out_frame.html",{'error_info':error_info})

            if not request.is_ajax():
                # don't set this for ajax requests or else your
                # expired session checks will keep the session from
                # expiring :)
                request.session['last_activity'] = now

        return None`