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I'm sort of stumped as to how the subclass' assertions are applied in the absence of InitializeClass! This is not what I expected, I would have thought that differing assertions would only be applied if InitializeClass was called on the subclass. There's some magic going on here that I don't understand.

The magic is that Persistent has a __class_init__ that calls InitializeClass for you. (This attribute is actually set by App.PersistentExtra, called from Globals.)

Aha. This is what I was missing! I used to know that, too. ;-) Thanks for clearing it up! - C