[Python] python subclass access to class variable of parent

I was surprised to to learn that a class variable of a subclass can't access a class variable of the parent without specifically indicating the class name of the parent:

>>> class A(object):
... x = 0
...
>>> class B(A):
... y = x+1
...
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 2, in B
NameError: name 'x' is not defined
>>> class B(A):
... y = A.x + 1
...
>>> B.x
0
>>> B.y
1


Why is it that in defining B.y I have to refer to A.x and not just x? This is counter to my intuition from instance variables, and since I can refer to B.x after B is defined.

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