node.py

class Node:
    def __init__(self, key, value, parent):
        self.key = key
        self.value = value
        self.parent = parent
        self.left = None
        self.right = None
        self.set_children()
        
    def add(self, key, value):
        self.path(key).add(key, value)

    def search(self, key):
        return self.path(key).value

    def path(self, key):
        if self.key > key:
            return self.left.path(key)
        if self.key < key:
            return self.right.path(key)
        return self

    def set_children(self):
        self.left = NullNode(self)
        self.right = NullNode(self)


class NullNode(Node):
    def __init__(self, parent):
        super().__init__(None, None, parent)

    def set_children(self):
        pass

    def add(self, key, value):
        self.__class__ = Node
        self.__init__(key, value, self.parent)

    def path(self, key):
        return self