Phil Harris wrote:

I have a ZPT page which can optionally get a parameter, dn, which gets used in a call to a Script (Python).

If I was doing this with DTML, I would use something like,

<dtml-unless dn> <dtml-call "REQUEST.set('dn','o=cm')"> </dtml-unless>

Quick note - you're more likely to get responses on the ZPT mailing list. On to the question: tal:define="dn_def string:o=cm; dn request/dn | dn_def" ...will set 'dn' equal to the request form value if it exists, otherwise to the string "o=cm". You can extend this to deal with keyword arguments to the template, like so: tal:define="dn_def string:o=cm; dn options/dn | request/dn | dn_def" ...and of course, if the default value is accessible as a path, there's no need for the two-step: tal:define="dn request/dn | here/dn_property" Finally, if you want to use a Python expression, you can do: tal:define="dn python:path('request/dn') or 'o=cm'" Cheers, Evan @ digicool