如何访问n叉树中节点的父节点？

Question

I am trying to create a Node class that implements an n-ary tree but I also want to keep track of the parent of each node to trace back to the root.

class Node(object):
    def __init__(self, state, children=None, parent=None):
        self.state = state
        self.children = children or []
        self.parent = parent
    def add(self, obj):
        self.children.append(Node(obj))
        Node(obj).parent = self.state

This is what I have been working on. But when I check to see a node's parent, it prints None. In a binary tree, checking if the child was to the left or right makes it easy but for an n-ary tree, I don't really understand how to go about it with explicitly making it the parent.

I am pretty new to Python and coding so I would really appreciate it if anyone could help out! Thank you!

EDIT:

I ran this on the IDE:

>>> n = Node(4)
>>> l = [1,2,3]
>>> for i in l:
    n.add_child(i)


>>> n.children.state
Traceback (most recent call last):
  File "<pyshell#63>", line 1, in <module>
    n.children.state
AttributeError: 'list' object has no attribute 'state'
>>> for child in n.children:
    print(child.state)


1
2
3
>>> for child in n.children:
    print(child.parent)


None
None
None
>>> 

Answer 1

instead of self.parent = parent

you want to do

self.parent = None # for root node only

for child in children:
  child.parent = self

Answer 2

self.children.append(Node(obj))

Here you are creating a Node instance on the fly and appending it to the children list

Node(obj).parent = self.state

That is an extra instance of Node on the fly which you are assigning its parent to be self.state

I think you are trying to do something like this:

node = Node(obj)
node.parent = self
self.children.append(node)

With this you are creating an instance of Node and storing its reference in the variable node, then assigning its parent to be the current Node instance (with the self reference) and saving the reference to its children list.

Also, you could do:

self.children.append(Node(obj, None, self))

since you are assigning the parent in the constructor.