import inspect


class A():
    def __init__(self):
        self.stack = inspect.stack()
        self.children = []

    def add_child(self, cls):
        child = cls()
        self.children.append(child)
        return child

class B(A):
    pass

class C(A):
    pass

node_classes = [A, B, C]

def load_nodes(N):
    root = A()
    for i in range(N):
        child_i = root.add_child(node_classes[i%len(node_classes)])
        for j in range(N):
            child_j = child_i.add_child(node_classes[(i*j)%len(node_classes)])

load_nodes(20)