Pointers in Python on variables with None value

Question

Hi everybody!

I have a method that creates a new node in a tree - either left or right. If the value is lower than my current value it is inserted on the left, otherwise on the right side.

I want to refactor this code, so that I first see on which side I have to insert my element, and then insert it. Before I implemented this twice: Once for the left side and once for the right side.

It currently looks like this:

def neu(self, sortByValue, secondValue):
    child = self.left if(sortByValue.lower() < self.value[0].lower()) else self.right

    if(child == None):
        child = MyTree(sortByValue,secondValue)
    else: child.neu(sortByValue,secondValue)

My problem is, though, that self.left is None, and self.right is None. So when I create child as a variable and set it to MyTree(...), self.left and self.right are not receiving the value.

Is there anything I can do to improve this? Thanks!

Answer 1

In Python variables are names not locations. For example:

>>> a = 1
>>> b = a
>>> a = 2
>>> print b
1

In your code you're simply rebinding the name child to a different value (your new node) and that has no affect on the previously bound value (None).

Here's a reworking of your code that should do what you want (untested):

def neu(self, sortByValue, secondValue):
    def child(node):
        if(node is None):
            return MyTree(sortByValue, secondValue)
        else:
            child.neu(sortByValue, secondValue)
            return node

    if(sortByValue.lower() < self.value[0].lower()):
        self.left = child(self.left)
    else:
        self.right = child(self.right)

Answer 2

Hallo ;-)

self.left or self.right don't receive the value because you assign to child which just holds a copy of the destination value and no reference to it.

You want to have a pointer- This doesn't exist directly in Python.

You could express this using a class wrapper, but I think it's more comprehensible when you just write both possibilities in the if clause.

Answer 3

Why are you using a tree?

I would use a dictionary:

Initialization:

tree = {}

Adding new node:

tree[sortByValue] = secondValue

Extracting things

print tree[sortByValue]

Not the straight answer but a more pythonic way of doing it