Are these arguments in Python being passed by value or by reference?

Question

I am wondering which of the arguments in the call self._insertInteral(value, self.root.rightChild) are by value and which are by reference? I am still learning Python and was reading about the pass by object methodology in Python. I think my misunderstanding on the topic may be the reason why my function for inserting into a binary tree does not result in the value being inserted.

Here is my code:

 class Node:
    def __init__(self, leftChild, rightChild, value):
        self.leftChild = leftChild
        self.rightChild = rightChild
        self.value = value

class BinaryTree:
    def __init__(self, root):
        self.root = root
    def _insertInternal(self, value, root):
        if root is None:
            root = Node(None, None, value)
            print 'new node, root.value = ' + str(root.value)
            return
        if root.value > value:
            self._insertInternal(value, root.leftChild)
        else:
            self._insertInternal(value, root.rightChild)

    def insert(self, value):
        print 'attempting to insert value = ' + str(value)
        if self.root is None:
            self.root = Node(None, None, value)
            return
        elif self.root.value > value:
            print str(self.root.value) + '>' + str(value)
            self._insertInternal(value, self.root.leftChild)
        else:
            print str(self.root.value) + '<' + str(value)
            self._insertInternal(value, self.root.rightChild)


if __name__ == '__main__':
    root = Node(None, None, 10)
    tree = BinaryTree(root)
    print tree.root.leftChild
    print tree.root.rightChild
    print tree.root.value


    tree.insert(5)

    print tree.root.leftChild
    print tree.root.rightChild
    print tree.root.value

I did checkout this post Understanding Python's call-by-object style of passing function arguments but was wondering about this example specifically.

No, its not the reason ... consider this ... where in your code do you actually (after finding the right place), add the node to the tree? — donkopotamus, May 24 '16 at 12:00
As the link you posted says, Python doesn't have pass-by-value or pass-by-reference, only pass-by-object. — interjay, May 24 '16 at 12:01
(and do not compare to None using `==`, use `is` (as in `if x is None: ...`) ) — donkopotamus, May 24 '16 at 12:01

score 2 · Accepted Answer · edited May 23 '17 at 12:16

2

Python is pass by assignment. Within your BinaryTree._insertInternal the assignment of root argument (also the local variable within the scobe of that method) is initially assigned the value of the root node (in this case, the value is an object reference), and the statement root = Node(None, None, value) is a new assignment, thus it becomes different to the initially passed in thus different to the instance's self.root.

edited May 23 '17 at 12:16

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answered May 24 '16 at 12:03

metatoaster

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Thanks for the link. I think I understand what is going on now. – Brady Sheehan May 24 '16 at 12:13

Are these arguments in Python being passed by value or by reference?

1 Answers1