Return statement not executing

Question

So I wrote a code which is supposed to get the parent of a given node in a tree. Here is the pseudo code. Starting from the root,

def parent(self, child): 
    if right child exists:
        if the right child == child:
            return self
        else: self.right.parent(child)
    if left child exists:
        if the left child == child:
            print('f')
            return self
        else: self.left._get_parent(node)

I keep coming across this problem over and over and over again. For the if the left child == child: statement, the function DOES enter the if statement if it finds that either the left child == child or the right child == child.

However, the return statement does not execute this. I know this because when I wrote the if the left child == child: and wrote print('f') after it, it did print f, however, it did not return self. Does anyone know why and can anyone give a solution as to how to fix this problem?

Additionally, does anyone know how to return two statements at once NOT as tuples or lists? For example, if I want to return 1 and 2,

def x(n):
    return 1, 2

This would return (1, 2).. is there any way for it to not return it as a tuple? And just to return it normally. I'm asking this because when it comes to recursion, I want to call the same function on 1 AS WELL AS 2, and not on the tuple (1, 2).

1, 2 is a tuple. The parenthesis are syntactic sugar, really. Returning a tuple is the way to return multiple values. Just index the return value or use tuple unpacking. — Martijn Pieters
– Martijn Pieters, Commented Apr 5, 2013 at 15:36

Martijn Pieters · Accepted Answer · 2013-04-05 15:40:07Z

2

Your code discards the return values of recursive calls in the else: branches. You need more return statements:

if right child exists:
    if the right child == child:
        return self
    else: 
        return self.right.parent(child)
if left child exists:
    if the left child == child:
        print('f')
        return self
    else:
        return self.left._get_parent(node)

In Python, the expression 1, 2 creates a tuple, and that is the canonical way to return multiple values from functions. That is how multiple value returns work, by returning a tuple.

Simply unpack the returned value:

def foo():
   return 1, 2

value1, value2 = foo()

or use indexing:

values = foo()
values[0], values[1]

answered Apr 5, 2013 at 15:40

Martijn Pieters

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2 Comments

Sly Cooper Over a year ago

THE FUNCTION YOU WROTE DOES NOT WORK. IT ONLY RETURNS THE PARENT FOR THE RIGHT CHILD, BUT NOT THE LEFT CHILD.

Martijn Pieters Over a year ago

WHY ARE WE SHOUTING? All I did was add return statements to your code. If your code is not returning the left child, you need to verify your algorithm.

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