calling a python script from ZPT

hi
i have a python script called getnames
i call it like this
<p tal:repeat="results python:context.getnames(table=tablename)">

now this script is in ploneroot/mainpage

if i call this from the mainpage folder or any sub folder there is no 
problem. But i want to call it from somewhere else in the site

i tried:
python:${plone_url}.mainpage.getnames()
and
python:${plone_url}/mainpage/getnames()

but both dont compile - any clues?
-- 
regards
kg

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