Python pass class variables as reference to class methods

Question

I have the following python code which sets the root of an AVL tree to a specified value. But it seems that setting the root variable of the class has no effect when done by passing it through a function in the same class.

class AVLTree:
    class AVLNode:
        def __init__(self, value) -> None:
            self.value = value

    def __init__(self) -> None:
        self._root = None

    def insert(self, value: int) -> None:
        return self._insert(value, self._root)

    def _insert(self, value, node):
        if node is None:
            node = value
        return


avl = AVLTree()
avl.insert(5)
print(avl._root)

Prints None

It seems that passing the class variable self._root as a parameter to a member method does not change it's value.

I read that python passes all class members by reference and only immutable types (int, etc.) as values.

Any idea why i am not able to modify the value of self._root in _insert function and how can i do that? Thank you

Python passes all objects the same way – there is no distinction between class members and immutable types. self._root evaluates to the object referred to by self._root, and that object is passed on. The object has no idea about any of its aliases, including self._root. You seem to want to have self._root refer both to the value of self._root (if node is None) and the expression self._root (node = value), is that correct? — MisterMiyagi
– MisterMiyagi, Commented Jul 6, 2020 at 14:34
None is a value. You have to pass an object which can be modified. — Pramote Kuacharoen
– Pramote Kuacharoen, Commented Jul 6, 2020 at 14:38
Related questions for separation of expressions and their values: In x = 1, are both x and 1 objects? and value reference semantics: Are python variables pointers? or else what are they? — MisterMiyagi
– MisterMiyagi, Commented Jul 6, 2020 at 14:43

chepner · Accepted Answer · 2020-07-06 14:41:05Z

1

You have complete control over how _insert is called. Simply don't call it with node=None. Something like

def insert(self, value: int) -> None:
    if self._root is None:
        self._root = self.AVLNode(value)
    else:
        self._insert(value, self._root)

def _insert(self, value, node):
    # Assume node is not None
    ...

(Unrelated, your AVLNode class also has to store pointers to its two children, and _insert needs to update those appropriately.)

edited Jul 6, 2020 at 14:41

answered Jul 6, 2020 at 14:35

chepner

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3 Comments

Help Helper Over a year ago

Hello, thanks for this, but after root gets initialized ` self._root = AVLNode(value)` then calling _insert (else clause).. then if i make changes to node (which i assume to be _root) then those changes don't persist .. this is what i am working with pastebin.com/j6bMQsai, after setting the root in insert as per your code, if i then pass the root to _insert then try to add children, they are never added.. i can't seem to work with root when passes to _insert as node. even tho its already initialized and is NOT none. Any clue? (root has no left or right child ever)

Help Helper Over a year ago

I think the "Unrelated" might be the solution to the issue in my comment above, thanks i will take a look!

chepner Over a year ago

Keep in mind that when you try to insert a value into an existing tree, it will fall into one of three cases: 1) It's already present at the root value 2) It will need to be inserted into the left subtree 3) It will need to be inserted into the right subtree. AVL trees complicate this a little bit in that any given insertion can cause a rotation that changes which node is considered the root, so as to maintain the height invariant.

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