Tornado: DummyFuture does not support blocking for results

Question

I am trying to get a very simple initial server which fetches a url (asynchronously) to work but it throws:

Exception: DummyFuture does not support blocking for results

There's this SO post but the answers do not include running a web server and trying to add the future to my loop as shown here throws:

RuntimeError: IOLoop is already running

This is the complete code:

import tornado.web
import tornado.gen
from tornado.httpclient import AsyncHTTPClient
from tornado.ioloop import IOLoop

URL = 'http://stackoverflow.com'


@tornado.gen.coroutine
def fetch_coroutine(url):
    http_client = AsyncHTTPClient()
    response = yield http_client.fetch(url)
    raise tornado.gen.Return(response.body)  # Python2


class MainHandler(tornado.web.RequestHandler):
    def get(self):
        self.write("Hello, world")


class FetchSyncHandler(tornado.web.RequestHandler):
    def get(self):
        data = fetch_coroutine(URL)
        print type(data)  # <class 'tornado.concurrent.Future'>
        self.write(data.result())


def make_app():
    return tornado.web.Application([
        (r"/", MainHandler),
        (r"/async", FetchSyncHandler),
    ])

if __name__ == "__main__":
    app = make_app()
    app.listen(9999)
    print 'starting loop'
    IOLoop.current().start()
    print 'loop stopped'

The loop is running, a future is returned. What is the issue?

Python 2.7.10
tornado==4.4.2

Answer 1

To get a result from a Future, yield it in a gen.coroutine function, or await it in an async def native coroutine. So replace your FetchSyncHandler with:

class FetchSyncHandler(tornado.web.RequestHandler):
    @gen.coroutine
    def get(self):
        data = yield fetch_coroutine(URL)
        self.write(data)

For more information, see the my Refactoring Tornado Coroutines or the Tornado coroutine guide.