[Zope] How to get $PRODUCT_HOME

Hello again, I just started with FS products and have trouble to figure some things out ...

From within zope I would like to call scripts that I provide in the product folder on the file system like eg.:

handle = os.popen('gnuplot %s/bin/loadgraphs.gplt' % product_folder, 'r') How can I get the path to the product folder? Thanks in advance! Greetings Roman