I really think test in isolation seems great but it depends on bit of codes.

Some bits of code are integration of your code in the framework. I don't think it make sense to test them in isolation, but rather do integration test. That is the case for object creation if you did not modify eg. __init__ on an Archetype content type.

Other bits are logics and you shall unit test them in isolation.

My feeling is that separing logic piece from integration code is the right way to be able to do good test. It's not always easy :-)