[Tutor] Unsupported operand types for +: 'NoneType' and 'int' - HUH?!

[Neil Schemenauer]

Eladio Ventura wrote:
> I try to make a function which calculates the total number of nodes
> and get the following error:
> 
> TypeError: unsupported operand types for +: 'NoneType' and 'int'

You are trying to add an integer to None.

> 
> The culprit is the following function:
> 
> def size(self, tree):
> 	if tree == None: return
> 	else:
> 		return self.size(tree.left) + 1 + self.size(tree.right)


Your problem is with the line "if tree == None: return".  You want:

    if tree is None:
        return 0

Note that a bare return statement is the same as "return None".


> class Tree:
>     def __init__(self, cargo=None, left=None, right=None):
>         self.cargo = cargo
>         self.left  = left
>         self.right = right
> 
>     def build123a(self):
>         root  = Tree(2)
>         left   = Tree(1)
>         right  = Tree(3)
>         root.left = left
>         root.right = right
>         return root
> 
>     def printTreeInorder(self, tree):
>         if tree == None: return
>         self.printTreeInorder(tree.left)
>         print tree.cargo,
>         self.printTreeInorder(tree.right)
> 
>     def size(self, tree):
>         if tree == None: return
>         else:
>             return self.size(tree.left) + 1 + self.size(tree.right)

There is no reason for build123a(), printTreeInorder() and size() to be
methods.  They would be simpler as functions since you don't use "self".
Eg:

class Tree:
    ...

def build123a():
    left = Tree(1)
    right = Tree(3)
    root = Tree(2, left, right)
    return root

def printTreeInorder(tree):
    ...

HTH,

  Neil