Need help with simple OOP Python question

[Kristofer Tengström]

Thanks everyone, moving the declaration to the class's __init__ method
did the trick. Now there's just one little problem left. I'm trying to
create a list that holds the parents for each instance in the
hierarchy. This is what my code looks like now:

-----------------------------------------

class A:
    def __init__(self, parents=None):
        self.sub = dict()
        if parents:
            self.parents = parents
        else:
            self.parents = []
    def sub_add(self, cls):
        hierarchy = self.parents
        hierarchy.append(self)
        obj = cls(hierarchy)
        self.sub[obj.id] = obj

class B(A):
    id = 'inst'

base = A()
base.sub_add(B)
base.sub['inst'].sub_add(B)

print
print vars(base)
print
print vars(base.sub['inst'])
print
print vars(base.sub['inst'].sub['inst'])

---------------------------------------------

The output from this program is the following:

{'parents': [<__main__.A instance at 0x02179468>, <__main__.B instance
at 0x021794B8>], 'sub': {'inst': <__main__.B instance at 0x021794B8>}}

{'parents': [<__main__.A instance at 0x02179468>, <__main__.B instance
at 0x021794B8>], 'sub': {'inst': <__main__.B instance at 0x021794E0>}}

{'parents': [<__main__.A instance at 0x02179468>, <__main__.B instance
at 0x021794B8>], 'sub': {}}

As you can see, the problem looks similar to the one before: All the
instances have an identical parent list. However, I don't understand
why as self.parents is declared in the __init__ method. Any ideas?
What I want is for the first instance to have an empty list, the
second to have one element in the list and the third to have two
parent elements.