Binary Search Tree Delete method Deletes Whole Subtree

Question

I have been learning data structures and have been trying to implement the delete method in my BST class. What I have noticed is that instead of deleting one node it will delete the whole subtree that the node is a part of.

    class BST:
    def __init__(self, data):
        self.data = data
        self.left = None
        self.right = None

    def addchild(self, child):
        if child == self.data:
            return
        if child < self.data:
            if self.left:
                self.left.addchild(child)
            else:
                self.left = BST(child)
        else:
            if self.right:
                self.right.addchild(child)
            else:
                self.right = BST(child)

    def search(self, data):
        if self.data == data:
            return True
        if data < self.data:
            if self.left:
                return self.left.search(data)
            else:
                return False
        else:
            if self.right:
                return self.right.search(data)
            else:
                return False

    def iot(self):
        vals = []

        if self.left:
            vals += self.left.iot()

        vals.append(self.data)

        if self.right:
            vals += self.right.iot()

        return vals

    def findmax(self):
        if self.right:
            return self.right.findmax()
        else:
            return self.data

    def findmin(self):
        if self.left:
            return self.left.findmin()
        else:
            return self.data

    def delete(self, data):
        if data < self.data:
            if self.left:
                self.left = self.left.delete(data)
        elif data > self.data:
            if self.right:
                self.right = self.right.delete(data)
        else:
            if self.left is None and self.right is None:
                return None
            elif self.left is None:
                return self.right
            elif self.right is None:
                return self.left

            minval = self.right.findmin()
            self.data = minval
            self.right = self.right.delete(minval)


def buildtree(arr):
    r = BST(arr[0])

    for i in range(1, len(arr)):
        r.addchild(arr[i])
    return r

So when input with these values and deleting the number 1 from the tree it will print this

bst = buildtree([34, 7, 8, 1, 3, 4, 5, 10, 65, 98, 100, 203])

print(bst.iot())

bst.delete(1)

print(bst.iot())

1st print statement output: [1, 3, 4, 5, 7, 8, 10, 34, 65, 98, 100, 203]
2nd print statement output: [34, 65, 98, 100, 203]

When I debug in Pycharm, before leaving the delete method and after all steps are executed the tree will show that the left subtree is intact and everything is how it should be. However once I leave the method the left subtree becomes None.

Any help would be appreciated.

Answer 1

Here's a strong hint:

You are missing a return statement on final block of you code:

        minval = self.right.findmin()
        self.data = minval
        self.right = self.right.delete(minval)

Answer 2

A debug answer: In your example 1 is going to be the leftmost leaf in the tree. So look what's happening: When you start the method delete you say that

if data < self.data:
        if self.left:
            self.left = self.left.delete(data)

So basically the left son of the root will hold whatever comes at the end of the process. If you will follow the left branch until you get 1, you will see that you get the the case of

else:
        if self.left is None and self.right is None:
            return None

This means that the root left son is None, and this explains your bug