Why does adding a child to one node cause it to be added to all nodes?

Question

I am trying to make a class to be used for some sort of tree graph. The class has the following attributes: value, label, children and parent. In Python I have the following lines of code:

import numpy as np

class testclass:
    def __init__(self, v, l, c=[], P = np.nan):
        self.value = v
        #self.parent = p
        self.children = c
        self.parent = P
        self.label = l
        
    
    def addChild(self, node):
        self.children.append(node)

The method addChild is supposed to simply add one instance to the list known as children of the current instance. However, when I run the following:

> Node1 = testclass(1,'x')

> Node2 = testclass(2, 'y')

> Node1.children.append(Node2)

for some reason this adds Node2 as a child to Node1 and itself. Node2's children shouldn't have changed but calling Node2.children yields a non-empty list! What am I doing wrong?

Edit: I saw that the question here seems to showcase a similar problem. But I still cannot get my head around a solution for my case. I have tried calling the append method directly in the console: > Node1.children.append(Node2) and have also tried defining it by

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