Hi! I am facing the following common problem: Let's say I have a filesystem-based Python Zope Product that uses propertysheets. I have defined a couple of properties and already have a number of instances of the class. Now I add a new property to the code. While this would not break anything if I worked with ZClasses, the filesystem-based propertysheet implementation will raise an error if I visit my existing instances' Properties tag because the property is not there. To avoid that, I have used the trick to add a default value for the new property in the class definition: class News(...): meta_type = 'News' ... myNewProperty='Green' ... def ... etc. This works perfectly, but I have been told that I should not do this because of the way ExtensionClasses work. Does anybody know more about that? Cheers Joachim