[Zope] page templates and "python:"

[Claudio Battaglino]

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 :-)