Question

I’m relatively new to python (but not to programming), and I can’t explain the following behavior. It appears that a variable (the list “children” in my example) from one object (“child”) is getting overwritten by the value for that variable in a completely different object (“node”). To give some context, I’m trying to create a simple Node class to use in a tree structure. The node has children and a parent (all other nodes).

I can’t figure out why child.children gets the same value as node.children. Are they somehow referencing the same data? Why? Code and output follows:

class Node:
    children = []
    parent = 0
    visited = 0
    cost = 0
    position = (0, 0)
    leaf = 0

    def __init__(self, parent, pos):
        self.parent = parent
        self.position = pos

    def addChild(self, node):
        self.children += [node]

node = Node(0, (0,0))
child = Node(node, (3,2))

node.addChild(child)

print "node: ",
print node

print "node.childen: ",
print node.children

print "child: ",
print child

print "child.children",
print child.children

Output:

node:  <__main__.Node instance at 0x414b20>
node.childen:  [<__main__.Node instance at 0x414b48>]
child:  <__main__.Node instance at 0x414b48>
child.children [<__main__.Node instance at 0x414b48>]

As you can see, both node.children and child.children have the same value (a list containing child) even though I only updated node.children. Thanks for any help!

Answer 1

The children variable was declared as a class-level variable so it is shared amongst all instances of your Nodes. You need to declare it an instance variable by setting it in the initializer.

class Node:
    #children = [] # not here...
    parent = 0     # might want to rethink where you initialize these
    visited = 0
    cost = 0
    position = (0, 0)
    leaf = 0

    def __init__(self, parent, pos):
        self.parent = parent
        self.position = pos
        self.children = [] # ...but here

    def addChild(self, node):
        self.children += [node]