Consider this program: class Descr(object): def __init__(self, name): self.name = name def __set__(self, instance, what): instance.__dict__[self.name] = what class X(object): attr = Descr("attr") x = X() print(x.attr) x.attr = 42 print(x.attr) It gives in output: <__main__.Descr object at 0x7fe1c9b28150> 42 The documentation [1] says that Descr is a data descriptor because it defines the __set__ method. It also states that data descriptors always override the value in the instance dictionary. So, the second line should also be the descriptor object according to the documentation. My question is: Is this a doc bug or a implementation bug? If the former, it will be the description of a data descriptor much less consistent, since it will require that a __get__ method be present, too. If the latter, the fix may break some programs relying on the ability to "cache" a value in the instance dictionary. [1] http://docs.python.org/reference/datamodel#invoking-descriptors -- Regards, Benjamin