python - tonado raise gen.Return 错误

Question

最近在使用 Tornado，用到 gen.coroutine 和 yield 配合，但是出了些问题，一直不明白！

代码：

class BaseHandler(tornado.web.RequestHandler):
    @gen.coroutine
    def args_kwargs(self,pro):
        try:
            kwargs = self.get_argument("data",None)
            if kwargs:
                code="-10000"
                raise gen.Return(code)
        except:
            print traceback.format_exc()

class EventAPIHandler(BaseHandler):
    @gen.coroutine
    def post(self):
        try:
            code = yield self.args_kwargs("event")
            if code:
                self.write(re_code[code])
                self.finish()
        except Exception,e:
            print traceback.format_exc()

错误为：

Traceback (most recent call last):
  File "server.py", line 124, in args_kwargs
    raise gen.Return(code)
Return

不能返回数据，请问有大神知道原因吗？请指教，非常感谢！

Answer 1

Original, remove the try except, try except will catch the raise, change it to the following, it should be OK

class BaseHandler(tornado.web.RequestHandler):
    @gen.coroutine
    def args_kwargs(self,pro):
        kwargs = self.get_argument("data", None)
        if kwargs:
            code = "-10000"
            raise gen.Return(code)
        else:
            raise gen.Return("xxx")
        
class EventAPIHandler(BaseHandler):
    @gen.coroutine
    def post(self):
        try:
            code = yield self.args_kwargs("event")
            if code:
                self.write(re_code[code])
                self.finish()
        except Exception,e:
            err = traceback.format_exc()
            self.write(err)
            self.finish() 
           

Answer 2

In Python2.x, the generator cannot be directly return 值，所以Tornado把值包在一个特殊的异常里返回出来，这个异常就是gen.Return，所以你的try..except会抓到这个异常并报错，所以改下BaseHandler.args_kwargs的代码，可参照：
BTW.貌似args_kwargs里没有yield，貌似只是拿个参数而已，没必要用gen.coroutine right?

class BaseHandler(tornado.web.RequestHandler):
    @gen.coroutine
    def args_kwargs(self,pro):
        try:
            kwargs = self.get_argument("data",None)
        except Exception as e:
            print e
        else:
            code = "-10000" if kwargs else "0000000"
            raise gen.Return(code)


class EventAPIHandler(BaseHandler):
    @gen.coroutine
    def post(self):
        try:
            code = yield self.args_kwargs("event")
            if code:
                self.write(re_code[code])
                self.finish()    # gen.coroutine会自动finish()，如果只是想结束连接，可把这一行替换为 return
        except Exception,e:
            print traceback.format_exc()