I was suprised by a change to the order of evaluation of members in the new object type. I haven't found an explanation for why the change was made. I was comfortable with the way it worked. Is there an advantage to the change?. In the classic python model the interpreter looked in the instance dictionary and if the name wasn't there it looked in the class dictionary. The following illustrates this evaluation order.
class C: def __init__(self): self.a = 4
c = C() c.a 4 C.a = 6 c.a 4 c.a = 8 c.a 8
With the new slots mechanism the order has been reversed. The class level dictionary is searched and then the slots are evaluated.
class B(object): __slots__ = ['a','b','c']
b = B() b.a = 4 b.a 4 B.a = 6 b.a 6 b.a = 8 Traceback (most recent call last): File "<pyshell#61>", line 1, in ? b.a = 8 AttributeError: 'B' object attribute 'a' is read-only