Think carefully about why you want to do this; you probably don't. Consider not making B inherit from A.

The idea of subclassing is to specialise an object. In particular, children of a class should be valid instances of the parent class:

>>> class foo(dict): pass
>>> isinstance(foo(), dict)
... True
If you implement this behaviour (with e.g. x = property(lambda: AttributeError)), you are breaking the subclassing concept, and this is Bad.