Binary Tree Locking

This problem was asked by Google.

Implement locking in a binary tree. A binary tree node can be locked or unlocked only if all of its descendants or ancestors are not locked.

Design a binary tree node class with the following methods:

is_locked, which returns whether the node is locked lock, which attempts to lock the node. If it cannot be locked, then it should return false. Otherwise, it should lock it and return true. unlock, which unlocks the node. If it cannot be unlocked, then it should return false. Otherwise, it should unlock it and return true. You may augment the node to add parent pointers or any other property you would like. You may assume the class is used in a single-threaded program, so there is no need for actual locks or mutexes. Each method should run in O(h), where h is the height of the tree.

My Solution(Python):

from tree import Node
import random

class TreeNode(Node):
    def __init__(self, val=None):
        super().__init__(data=val)
        self._lock_state = False

    def insertNode(self, val):
        if self.left is None and self.right is not None:
            self.left = TreeNode(val)
            self.left.parent = self
        elif self.right is None and self.left is not None:
            self.right = TreeNode(val)
            self.right.parent = self
        elif self.left is None and self.right is None:
            choice = random.choice(['LEFT', 'RIGHT'])
            if choice=='LEFT':
                self.left = TreeNode(val)
                self.left.parent = self
            else:
                self.right = TreeNode(val)
                self.right.parent = self
        elif self.left is not None and self.right is not None:
            choice = random.choice(['LEFT', 'RIGHT'])
            if choice=='LEFT':
                self.left.insertNode(val)
            else:
                self.right.insertNode(val)

    def lock(self):
        if self.parent._lock_state or self.left._lock_state or self.right._lock_state:
            return False, 'Not possible'
        self._lock_state = True
        return True

    def unlock(self):
        if self.parent._lock_state or self.left._lock_state or self.right._lock_state:
            return False, 'Not possible'
        self._lock_state = False
        return True

    def is_locked(self):
        return self._lock_state

if __name__=='__main__':
    tree = TreeNode('*')
    for _ in range(20):
        val = random.randint(1, 100)
        tree.insertNode(val)
    tree.prettyPrint()
    print('\n\n\n')
    print(tree.left.lock()) #should lock and returnn true
    print(tree.is_locked()) #should be false
    print(tree.left.is_locked()) #should be true
    print(tree.left.left.is_locked()) #should be false
    print(tree.left.left.unlock()) #should not be possible
    print(tree.left.unlock()) #should be possible
    print(tree.left.left.is_locked()) #should be false