title: Tornado coroutine
body: <p>I am trying to learn tornado coroutines, but I have error using below code.</p>
  
  <pre><code>Traceback (most recent call last):
    File "D:\projekty\tornado\env\lib\site-packages\tornado\web.py", line 1334, in _execute
      result = yield result
    File "D:\projekty\tornado\env\lib\site-packages\tornado\gen.py", line 628, in run
      value = future.result()
    File "D:\projekty\tornado\env\lib\site-packages\tornado\concurrent.py", line 109, in result
      raise_exc_info(self._exc_info)
    File "D:\projekty\tornado\env\lib\site-packages\tornado\gen.py", line 631, in run
      yielded = self.gen.throw(*sys.exc_info())
    File "index.py", line 20, in get
      x = yield 'test'
    File "D:\projekty\tornado\env\lib\site-packages\tornado\gen.py", line 628, in run
      value = future.result()
    File "D:\projekty\tornado\env\lib\site-packages\tornado\concurrent.py", line 111, in result
      raise self._exception
  BadYieldError: yielded unknown object 'test'
  </code></pre>
  
  <p>Code: </p>
  
  <pre><code>from tornado.ioloop import IOLoop
  from tornado.web import RequestHandler, Application, url
  from tornado import gen
  
  class HelloHandler(RequestHandler):
      @gen.coroutine
      def get(self):
          x = yield 'test'
          self.render('hello.html')
  
  
  def make_app():
      return Application(
          [url(r"/", HelloHandler)], 
          debug = True
      )
  
  def main():
      app = make_app()
      app.listen(8888)
      IOLoop.instance().start()
  
  main()
  </code></pre>