calculating depth of a binary tree in Python

Question

I am new to programming and am trying to calculate the depth of a binary tree in Python . I believe that my error is because depth is a method of the Node class and not a regular function. I am trying to learn OOP and was hoping to use a method. This might be a newbie error... Here is my code:

class Node:

    def __init__(self, item, left=None, right=None):
        """(Node, object, Node, Node) -> NoneType
        Initialize this node to store item and have children left and right.
        """
        self.item = item
        self.left = left
        self.right = right

    def depth(self):
        if self.left == None and self.right == None:
            return 1

        return max(depth(self.left), depth(self.right)) + 1

i receive this error:

>>>b = Node(100)

>>>b.depth()

1 

>>>a = Node(1, Node(2), Node(3))

>>>a.depth()

Traceback (most recent call last):
  File "C:\Program Files\Wing IDE 101 4.1\src\debug\tserver\_sandbox.py", line 1, in <module>
    # Used internally for debug sandbox under external interpreter
  File "C:\Program Files\Wing IDE 101 4.1\src\debug\tserver\_sandbox.py", line 15, in depth
builtins.NameError: global name 'depth' is not defined

Answer 1

def depth(self):
    if self.left == None and self.right == None:
        return 1

    return max(depth(self.left), depth(self.right)) + 1

should be

def depth(self):
    return max(self.left.depth() if self.left else 0, self.right.depth() if self.right else 0) + 1

A more readable version:

def depth(self):
    left_depth = self.left.depth() if self.left else 0
    right_depth = self.right.depth() if self.right else 0
    return max(left_depth, right_depth) + 1

The issue is that there is no function depth. It's a method of the Node object, so you would need to call it from the object itself (left and right). I shortened the code to self.left.depth() if self.left else 0 and self.right.depth() if self.right else 0 in order to remove the checks you previously have (they're implicit now) since I believe it is entirely possible that the left is None while the right is a Node or vice versa, which would cause the original code to throw an AttributeError since None does not have a method depth.

Edit

In response to the question about the <something> if <some condition> else <otherwise> block:

The line gives <something> if <some condition> is true-y (treated as true), and <otherwise> if <some condition> is false-y (treated as false)