[Zope] newbie q -- 2.3.0a1, file object & precondition

[John Gerard Malecki]

hi,

i'm very new to zope and am trying to get familiar with the variety of
mechanisms i can use.  zope is indeed a very nice package!

i am starting with version 2.3.0a1 (because it matches the book).
i've not yet had success getting preconditions to work with file
objects.  i hope i'm not doing anything too bone-headed.

here is what i have: in a folder i have file objects T and U.  T has
the precondition 'preco(1)' and U has the precondition 'preco(0)'.  in
the same folder (or above, it doesn't matter) i have the python script
preco with the contents

    if r == 0:
	raise "preco"
    else:
	return r

when i the manager or type in the URL directly i can view/download
both T and U.  i was hoping only T would be "visible".  

i'm not sure how to debug something like this.  if you can tell me how
i can debug this on my own that would be best.

-cheers