Question

I’m working on a project in Tornado that relies heavily on the asynchronous features of the library. By following the chat demo, I’ve managed to get long-polling working with my application, however I seem to have run into a problem with the way it all works.

Basically what I want to do is be able to call a function on the UpdateManager class and have it finish the asynchronous request for any callbacks in the waiting list. Here’s some code to explain what I mean:

update.py:

class UpdateManager(object):
  waiters = []
  attrs = []
  other_attrs = []

  def set_attr(self, attr):
    self.attrs.append(attr)

  def set_other_attr(self, attr):
    self.other_attrs.append(attr)

  def add_callback(self, cb):
    self.waiters.append(cb)

  def send(self):
    for cb in self.waiters:
      cb(self.attrs, self.other_attrs)

class LongPoll(tornado.web.RequestHandler, UpdateManager):
  @tornado.web.asynchronous 
  def get(self):
    self.add_callback(self.finish_request)

  def finish_request(self, attrs, other_attrs):
    # Render some JSON to give the client, etc...

class SetSomething(tornado.web.RequestHandler):
  def post(self):
    # Handle the stuff...
    self.add_attr(some_attr)

(There’s more code implementing the URL handlers/server and such, however I don’t believe that’s necessary for this question)

So what I want to do is make it so I can call UpdateManager.send from another place in my application and still have it send the data to the waiting clients. The problem is that when you try to do this:

from update import UpdateManager
UpdateManager.send()

it only gets the UpdateManager class, not the instance of it that is holding user callbacks. So my question is: is there any way to create a persistent object with Tornado that will allow me to share a single instance of UpdateManager throughout my application?

Answer 1

Don’t use instance methods – use class methods (after all, you’re already using class attributes, you just might not realize it). That way, you don’t have to instantiate the object, and can instead just call the methods of the class itself, which acts as a singleton:

class UpdateManager(object):
  waiters = []
  attrs = []
  other_attrs = []

  @classmethod
  def set_attr(cls, attr):
    cls.attrs.append(attr)

  @classmethod
  def set_other_attr(cls, attr):
    cls.other_attrs.append(attr)

  @classmethod
  def add_callback(cls, cb):
    cls.waiters.append(cb)

  @classmethod
  def send(cls):
    for cb in cls.waiters:
      cb(cls.attrs, cls.other_attrs)

This will make…

from update import UpdateManager
UpdateManager.send()

work as you desire it to.