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Good Question! The answer is with __setstate__.

<snip>

It's allways there in every persistent object, it's defined by the Persistence class. If you have special behavior your want to add to that, you can override it.

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Are you aware of any working examples of this in action? Yes, the Catalog in 2.1 upgraded itself from the Catalog in 2.0: <snip>

Thanks for the info. With all the power and flexibility of Zope and ZClasses, it is becoming extremely clear to me that in order to create any 'real' products, writing code in Python is going to be a must. Oh well ....