Building Chain of Nodes?

Question

I have a Node class. Then I'm building a chain of Nodes. They are connected with one another via self.prev_node and self.next_nodes, also the Chain : self.last holds a link to the last element of the chain. I'm using a method like this to add a new node :

class Node(object):

def __init__(self, next_nodes=[], prev_node=None):
    self.next_nodes = next_nodes
    self.prev_node = prev_node


class Chain:

def __init__(self):
   self.start = Node()
   self.last = self.start 

def add_node(self, node):
   node.prev_node = self.last
   self.last.next_nodes.append(node)
   self.last = node

The problem is that if I add several nodes :

c.start.next_nodes
[Node:2, Node:3, Node:4]

c.start.next_nodes[0].next_nodes
[Node:2, Node:3, Node:4]

instead it should be :

c.start.next_nodes
[Node:2]


c.start.next_nodes[0].next_nodes
[Node:3]

For some reason the next_nodes list seems to be duplicate/the-same.

What I'm doing wrong ?

Answer 1

You're passing mutable "[]" in init as a parameter.

So each node you make is referring to the same memory location thus ending up with all same "next_nodes".