<p>Welcome to the wonderful world of Nevow, 
       <span nevow:data="group_name" nevow:render="string" />!</p>

class Page(rend.Page):
    docFactory = loaders.htmlfile('hellohtml.html')
    def data_group_name (self, context, data):
	def succeed (s):
	    return s.upper ()
	d = defer.Deferred ().addCallback (succeed)
	reactor.callLater (10, d.callback, 'chipy')
	return d

Note this doesn't handle failure of the callback. Deferred has additional methods

addErrback (error)

addCallbacks