BST Delete node method removes entire subtree. Python

Question

I'm struggling getting my Delete function to work for my BST class, I mostly understand the idea behind but can't quite get the implementation right.

Here in DataStructures.py I have a class for the BST, it pulls from another class DataStructures which just takes in a list of arguments for values.

# Class for Binary Search Tree (BST)
class BST(DataStructures):
    
    def __init__(self, *args):
        self.root = None
        for arg in args:
            self.insert(arg)

    # Function to insert into tree
    def insert(self, data):
        new_node = self.BSTNode(data)
        if self.root is None:
            self.root = new_node
        else:
            current_node = self.root
            while current_node != None:
                if current_node.data < new_node.data:
                    if current_node.right is None:
                        current_node.right = new_node
                        return
                    else:
                        current_node = current_node.right
                elif current_node.data > new_node.data:
                    if current_node.left is None:
                        current_node.left = new_node
                        return
                    else:
                        current_node = current_node.left

    # Function to search BST
    def search(self, value):
        node = self.root
        count = 1
        if node is None:
            print("Tree is Empty")
        elif value == node.data:
            print(f"{value} in line {count} of tree.")
        else:
            while node != None and node.data != value:
                count +=1
                if node.data < value:
                    node = node.right
                elif node.data > value:
                    node = node.left
            if node is None:
                print(f"{value} Not in Tree")
            else:
                print(f"{value} in line {count} of tree.")

    # Function to get min value in BST
    def getMin(self, node = "root"):
        if node == "root":
            node = self.root
        min = node.data
        while node.left != None:
            min = node.left.data
            node = node.left
        return min

    # Function to get max value in BST
    def getMax(self, node = "root"):
        if node == "root":
            node = self.root
        max = node.data
        while node.right != None:
            max = node.right.data
            node = node.right
        return max

    # Function to delete a node in BST
    def delNode(self, value, node = "root"):
        if node == "root":
            node = self.root
        if node is None:
            return node
        if value < node.data:
            node.left = self.delNode(value, node.left)
        elif value > node.data:
            node.right = self.delNode(value, node.right)
        else:
            if node.left is None:
                return node.right
            elif node.right is None:
                return node.left
            node.data = self.getMin(node.right)
            node.right = self.delNode(node.data, node.right)
            return node

    # Function to traverse in-order BST
    def inOrder(self, node = "root"):
        if node == "root":
            node = self.root
        if node:                    # Recursively print left-right most nodes on tree
            self.inOrder(node.left)
            print(node.data, end=" ")
            self.inOrder(node.right)

    # Function to get BST height
    def getHeight(self, node = "root"):
        if node == "root":
            node = self.root
        if node is None:
            return 0
        leftHeight = self.getHeight(node.left)      # Recursively get height of left/right nodes then return the larger
        rightHeight = self.getHeight(node.right)    # of them, adding 1 to their increment each time, 
        return max(leftHeight, rightHeight) + 1

    # Function to see if BST is balanced
    def getBalance(self, node = "root"):
        if node == "root":
            node = self.root
        if node is None:
            return True
        left_height = self.getHeight(node.left)     # Get height from either side of our root node
        right_height = self.getHeight(node.right)
        if((left_height - right_height) > 1 or (left_height - right_height) < -1):
            return False
        left_balance = self.getBalance(node.left)   # Check if each subtree is balanced, return True if so
        right_balance = self.getBalance(node.right)
        if left_balance == True and right_balance == True:
            return True

    # String Method for BST see repr function in BSTNode Class
    def __str__(self) -> str:
        if self.root is None:
           return "Tree is Empty"
        else:
            return repr(self)

    def __repr__(self):
        return repr(self.root)
            
    # Class for Nodes of the BST
    class BSTNode():
        def __init__(self, data):
            self.data = data
            self.left = None
            self.right = None

        # Return String Representation of BST (taken from https://stackoverflow.com/a/62407035)
        def __repr__(self):
            lines = []
            if self.right:
                found = False
                for line in repr(self.right).split("\n"):
                    if line[0] != " ":
                        found = True
                        line = " ┌─" + line
                    elif found:
                        line = " | " + line
                    else:
                        line = "   " + line
                    lines.append(line)
            lines.append(str(self.data))
            if self.left:
                found = False
                for line in repr(self.left).split("\n"):
                    if line[0] != " ":
                        found = True
                        line = " └─" + line
                    elif found:
                        line = "   " + line
                    else:
                        line = " | " + line
                    lines.append(line)
            return "\n".join(lines)

And here is my main.py with various tests while I coded the BST

import DataStructures

bst = DataStructures.BST(4, 2, 6, 3, 1, 5, 8, 10)

# bst.insert(10)
# bst.insert(5)
# bst.search(0)
# print(bst.getMax())
# bst.inOrder()
# print(bst.getHeight())
# print(bst.getBalance())
# print(bst)
bst.delNode(8)
print(bst)
bst.inOrder()

After trying for a while myself and getting nowhere I attempted implementing code from other stackoverflow questions and various other outlets but couldn't get it to overwrite the correct node. In my main.py example only the 8 node should be removed but I always end up with all of the right tree deleted

Output from main.py:

4
 |  ┌─3
 └─2
    └─1
1 2 3 4

Please post a minimal reproducible example. We don't need the methods that are unrelated to adding and deleting nodes, like getHeight() and getBalance(). — Barmar
– Barmar, Commented May 24, 2024 at 17:22
Some of the branches in delNode() return a value, some don't. You should be consistent -- does this return something or does it modify in place? — Barmar
– Barmar, Commented May 24, 2024 at 17:24
What's supposed to happen when deleting "root"? Assigning to the local variable node has no effect. — Barmar
– Barmar, Commented May 24, 2024 at 17:28

Barmar · Accepted Answer · 2024-05-24 17:29:46Z

When you recurse into the left or right nodes, you're not returning anything. If this came from a call that was recursing into a left or right node, it will set that node to None. You always need to return something.

    def delNode(self, value, node = "root"):
        if node == "root":
            return self.root
        if node is None:
            return node
        if value < node.data:
            node.left = self.delNode(value, node.left)
            return node
        elif value > node.data:
            node.right = self.delNode(value, node.right)
            return node
        else:
            if node.left is None:
                return node.right
            elif node.right is None:
                return node.left
            node.data = self.getMin(node.right)
            node.right = self.delNode(node.data, node.right)
            return node

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