variable scope and referencing

Question

Here, I have copied a reference to the "root" to the "temp" variable before performing operation on root variable on a Binary Search Tree.

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right

class Solution:
    def insertIntoBST(self, root: Optional[TreeNode], val: int) -> Optional[TreeNode]:
        temp = root
        prev_node = None
        while True:
            if not root:
                if prev_node is None:
                    root = TreeNode(val)
                elif val > prev_node.val:
                    prev_node.right = TreeNode(val)
                else:
                    prev_node.left = TreeNode(val)
                break
            elif val > root.val:
                prev_node = root
                root = root.right
            elif val < root.val:
                prev_node = root
                root = root.left
        return temp

This approach works for almost all testcases but fails for when root = []

At this point I realized there is something wrong going on with variable scope, so I started debugging and changed my code to as shown below:

    def insertIntoBST(self, root: Optional[TreeNode], val: int) -> Optional[TreeNode]:
        temp = root
        prev_node = None
        while True:
            if not root:
                if prev_node is None:
                    temp = TreeNode(val)
                elif val > prev_node.val:
                    prev_node.right = TreeNode(val)
                else:
                    prev_node.left = TreeNode(val)
                break
            elif val > root.val:
                prev_node = root
                root = root.right
            elif val < root.val:
                prev_node = root
                root = root.left
        return temp

Now, second piece of code works perfectly, but even after debugging it line by line I could not understand what exactly went wrong with my first piece of code. I have a vague idea that it was related to the variable scope, but just can't put my finger on it. Any help is appreciated.

Answer 1

There are only really three lines executed in case root is []:

temp = root
    ...
    if not root:
        if prev_node is None:
            root = TreeNode(val)
    ...
return temp

So, yeah. temp is set to [], then root is set to a TreeNode (which doesn't affect temp), then temp (which is still []) is returned.