Question

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.

Answer 1

In Python, the body of a class is executed in its own namespace before the class is created (after which, the members of that namespace become the members of the class). So when the interpreter reaches y = x+1, class B does not exist yet at that point and, therefore, has no parent.

For more details, see http://docs.python.org/reference/compound_stmts.html#class-definitions