[Zope] Problem: Accessing ZClass propertysheet properties

[Roger Lam]

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.