Do we need to use root = None during in the function? (python BST, reference counting)

Question

I am practicing deleting node in BST and came across the following code.

def deleteNode(root, key):

    if root is None:
        return root

    if key < root.key:
        root.left = deleteNode(root.left, key)

    elif(key > root.key):
        root.right = deleteNode(root.right, key)

    else:
        if root.left is None:
            temp = root.right
            root = None
            return temp

        elif root.right is None:
            temp = root.left
            root = None
            return temp

        temp = minValueNode(root.right)
        root.key = temp.key
        root.right = deleteNode(root.right, temp.key)

    return root

My question is do we need the line of root = None? According to reference counting, I feel root will be automatically destroyed as the only reference like parent.child is detached. Any explanation is greatly appreciated.

Answer 1

Given that root = None is immediately followed by return temp, it is just (re-)binding a local variable that is immediately discarded again and has no effect whatsoever.

In the given code, the lines root = None can be removed without changing the result of running the code.

Presumably the code was translated too literally from a different language where assigning to a variable changes the contents of a memory location.

But in Python, the assignment operator = just binds a new name¹ to an already existing object (in this case it gives the name root to the object None) and does not change the object or any other object in memory.

---

¹ at least with "simple names" on the left-hand side, it is a bit more complicated when assigning to something like a[i].