[Twisted-web] Dynamic size of formless.Lists

[twisted-web@twistedmatrix.com]

Hi,
I'm starting to write my own nevow site and truly love it

my question is regarding formless.List.
I took one of dp's examples from http://soundfarmer.com/content/code/formpost3.tac
Which shows how to create a form with a list of  form objects (formless.List )
My question:
Suppose I would like to define dynamicly the size of the list.
Instead of :
addresses = [Address(), Address(), Address()] 
Something like :
addresses = getDeferredFromSomewhere()
addresses.addCallback(….)
Is there any simple way of doing it?
Thanks in advance 
	-- Aragon