import sys

from twisted.internet.defer import Deferred
from twisted.web.template import Element, XMLString, flatten, renderer

sample = XMLString(
    """
    <div xmlns:t="http://twistedmatrix.com/ns/twisted.web.template/0.1">
    Before waiting ...
    <span t:render="wait"></span>
    ... after waiting.
    </div>
    """
)


class WaitForIt(Element):
    def __init__(self):
        Element.__init__(self, loader=sample)
        self.deferred = Deferred()

    @renderer
    def wait(self, request, tag):
        return self.deferred.addCallback(lambda aValue: tag("A value: " + repr(aValue)))


def done(ignore):
    print("[[[Deferred fired.]]]")


print("[[[Rendering the template.]]]")
it = WaitForIt()
flatten(None, it, sys.stdout.write).addCallback(done)
print("[[[In progress... now firing the Deferred.]]]")
it.deferred.callback("<value>")
print("[[[All done.]]]")