[Zope] Is there a function which evaluate a python expression situated in a string variable ?

Hi all, I have a string variable "expres" which contains a python expression. I would like to do <dtml-call "expres.manage_addFolder('new_folder')"> but Zope raises an error. expres is considered as a string and is not interpreted by Zope. So I wonder if there is a function which evaluate the python expression situated in my string variable... Thanks Frederic