[Zope] <dtml-call expr="my_var='foo' "> fails

Hi! The following seems to be wrong: <dtml-call expr="my_var='foo' "> (I want to set the variable my_var of my python object) The DTML-File containing the dtml-call is a DTMLFile(...) of a python product <dtml-var expr="my_var"> is working How can I set a attribute of my object?