twisted 4 non block web server

from twisted.internet import  reactor
from twisted.internet.task import deferLater
from twisted.web.resource import Resource
from twisted.web.server import Site, NOT_DONE_YET

import time

class BusyPage(Resource):
    isLeaf = True

    def _delayedRender(self, request):
        request.write("Finally done, at %s" % (time.ctime(),))
        request.finish()

    def render_GET(self, request):
        d = deferLater(reactor, 5, lambda : request)
        d.addCallback(self._delayedRender)
        return NOT_DONE_YET

factory = Site(BusyPage())
reactor.listenTCP(8000, factory)
reactor.run()