[Zope] Problem: Accessing ZClass propertysheet properties

I followed an sample from "Chapter 12: Extending Zope" of Zope Book however the following pythonscripts didn't work. ================================================== ## Script (Python) "isHungry" ## """ Returns true if the animal hasn't eaten in over 8 hours """ from DateTime import DateTime if (DateTime().timeTime() - self.last_meal_time.timeTime() > 60 * 60 * 8): return 1 else: return 0 ================================================== The problem: "self" not found! I've tried "container", "context" and other possible mixes and still don't have any clue to get it work. Please help. Thank you. Roger.