[Zope] page templates and "python:"

Hi, what does exactly happen when I use "python:" into a zpt? Is it a problem if I have many "python:" in my page templates? What are the differences in performances of these two definitions? <metal:block tal:define="my_value my_object/getValue">...</metal:block> <metal:block tal:define="my_value python:my_object.getValue()">...</metal:block> thank you very much claudio :-)