Python : Object attributes overwritten when creating a new object

Question

Here is a code I'm working on:

class Node:
    count = 0

    def __init__(self):
        Node.id = Node.count
        Node.count += 1

    @classmethod
    def get_count(cls):
        return Node.count


class Element:

    count = 0

    def __init__(self, type = "MTH20", group = 0):
        self.type = type
        self.group = group
        Element.count += 1

    def define_element(self):
        self.nodes = [-1]*3

        for l in range(len(self.nodes)):
            if self.nodes[l] == -1:
                self.nodes[l] = Node()
                print(self.nodes[l].id)

        for l in range(len(self.nodes)):

            print(self.nodes[l].id)  

ga = Element()

ga.define_element()

I would expect the output of this code to be:

0
1
2
0
1
2

However I get this:

0
1
2
2
2
2

My conclusion is that somehow the id attribute of a node object is overwritten when constructing a new node. However, I don't understand why it would do so, or how to avoid that.

Can anyone point out what I did wrong?

Answer 1

With Node.id you are accessing a class attribute. You want to store ids as instance attributes by doing self.id .

class Node:
    count = 0

    def __init__(self):
        self.id = Node.count # We want id to be an attribute of that specific node
        Node.count += 1

About your code

I want to point out that your Element.define_element method could be written in a more concise natural way.

class Element:

...

def define_element(self):
        self.nodes = [Node() for _ in range(3))]

        for node in self.nodes:
                print(node)