In Python, how to make data members visible to subclasses if not known when initializing an object?

The title is a bit long, but it should be pretty straightforward for someone well-aware of python.

I'm a python newbie. So, maybe i'm doing things in the 开发者_StackOverflow中文版wrong way.

Suppose I have a class TreeNode

class TreeNode(Node):
    def __init__(self, name, id):
        Node.__init__(self, name, id) 
        self.children = []

and a subclass with a weight:

class WeightedNode(TreeNode):
    def __init__(self,name, id):
        TreeNode.__init__(self, name, id)
        self.weight = 0

So far, i think I'm ok. Now, I want to add an object variable called father in TreeNode so that WeightedNode has also this member. The problem is that I don't know when initializing the object who is going to be the father. I set the father afterwards with this method in TreeNode :

def set_father(self, father_node):
    self.father = father_node 

The problem is then when i'm trying to access self.father in Weighted:

print 'Name %s Father %s '%(self.name, self.father.name)

I obtain:

AttributeError: WeightedNode instance has no attribute 'father'

I thought that I could make father visible by doing something in TreeNode.__init__ but i wasn't able to find what.

How can i do that ?

Thanks.

You could just initialize it with a default value:

self.father = None

That way the attribute will at least be recognized. And this is valid since at this point there really is no father.

In response to your statement on Justin's answer, try this:

print ' Name %s Father %s '%(str(self.name), str(self.father.name))

The str() command will get a string representation of an object even if it's None