Hello, Here is this problem again that has reappeared !!!! I already posted about this problem some months ago, but did not manage to make it work, and I had to rewrite all the class from scratch. (of course, I did a lot of cut and past, but you have to do a lot of manual work recreating the Properties, and you loose all the instance you had created). Here is my problem: I have 3 Products PA PB PC and I have the 3 classes A, B and C defined in each of the different Products. PA has class A PB has class B PC has class C. Besides, the inheritence tree is the following: A <-- B <-- C. (A inherits from B, etc.) The problem is that after some time (after many export and re-import of Products), the inheritance does not work anymore between the package. For instance if I define a new method in class A, it is invisible from class B. Or if I redefine a method in a Class A, the class B still see the old method of class B. Note that at the beginning, when you create those three packages and classes, everything seems to work fine. (I have done the test). Did someone already encountered a similar problems ? Do someone has an idea what could be wrong, and has some idea about how to fix the problem ? (my guess is that something wrong happens after too many export and imports which confuses python). Thierry Nabeth Research Fellow, INSEAD CALT (The Centre for Advanced Learning Technologies) http://www.insead.edu/CALT/ Thierry.Nabeth@insead.fr