[Zope] ZPT to Python(script) to a different page

[AdvertisingDept]

Newbie requests Style help please...

I have a ZopePageTemplate containing a form.
The action of the form points back to itself.
This puts the user in a loop.

At the top of the form, a tal:define statement
calls a python script to perform validation
and decision making logic.

If the validation fails, as it will on the
first ZPT rendering, the script
returns the validation errors and
the user is still on the originating ZPT 
containing the form.

If the validation fails, due to bad
input, the script returns the validation
errors and the user is still on the 
originating ZPT.

If the validation passes,
I would like the user's browser
to display different ZPTs based on 
the script's decision logic.

What are the various means for the 
python script to cause the user's browser
to display a new ZPT/zope object/html page?

Or should I pass back to the ZPT
and write tal:condition tags?

Then what are the various means for the 
ZPT to cause the user's browser to
display a new ZPT/zope object/html page?

Can I simulate a user clicking on a link?

thanks in advance