Hi, In a product I am writing, I am trying to add a ComputedAttribute to an instance of a Folder descendant class. This is the code I use: def addComputedAttribute(self, attributeName, sourceCode): methodName = '_ca_get%s' % attributeName self.addMethod(methodName, sourceCode) setattr(self, attributeName, ContextComputedAttribute(self.__class__.__dict__[methodName])) where at first I defined ContextComputed Attribute like this: import ComputedAttribute _ComputedAttribute = ComputedAttribute.ComputedAttribute del ComputedAttribute class ContextComputedAttribute(_ComputedAttribute): pass This put the Folder in a bad state : when I access it later, the Zope instance crashes. So I supposed I needed to use a persistent Attribute and I tried with ContextComputedAttribute class defined like this: class ContextComputedAttribute(Persistent): def __init__(self, test): pass In this case, the Folder stays in a good state but it does not fullfill the requirements as ContextComputedAttribute does not descend from the real Zope ComputedAttribute class. My problem is the following : I cannot define a class like this: import ComputedAttribute _ComputedAttribute = ComputedAttribute.ComputedAttribute del ComputedAttribute class ContextComputedAttribute(_ComputedAttribute, Persistent): pass In this case, I get the following error TypeError: too many datafull base classes which has to do with too many ExtensionClass derivation... Can someone tell me how to define a class with both ComputedAttribute and Persistent behaviours ? Thanks a lot. -- Godefroid Chapelle BubbleNet sprl rue Victor Horta, 30 1348 Louvain-la-Neuve Belgium Tel + 32 (10) 457490 Mob + 32 (477) 363942 TVA 467 093 008 RC Niv 49849