Tornado nested callback example

from tornado import gen
import tornado.web

class MainHandler(tornado.web.RequestHandler):
    @tornado.web.asynchronous
    @gen.engine
    def get(self):
        cool_result = yield gen.Task(self.nested, 2)
        self.write(cool_result)

    @gen.engine
    def nested(self, result_outer, callback = {}):
        result = yield gen.Task(self.asum, [1,result_outer])
        callback(result)

    def asum(self, l, callback={}):
        r = sum(l)
        return callback(r)