Python 2.7: Using list.append in function arguments

Question

I am trying to write a code to print the path in a Binary Tree which add up to the sum passed to the function. To which the below code keeps on failing for a simple test case:

Last executed input: Binary Tree = [1], sum = 1

Runtime Error Message: Line 26: TypeError: object of type 'NoneType' has no len()

Stdout:None None

I cannot understand how the leftPath and rightPath are becoming None. I am not returning None at all.

'''
Created on Aug 12, 2015

@author: debpriyas
'''

class BTNode(object):
    '''
    classdocs
    '''
    def __init__(self,value, leftBTNode=None, rightBTNode=None):
        '''
        Constructor
        '''
        self.value = value
        self.left = leftBTNode
        self.right = rightBTNode
        # To deal with duplicate values in BST. 
        self.count = 1

def pathSum(root, sum):
    return pathSumHelper( root, sum, [])

def pathSumHelper(root, sum, path):

    if root == None:
        if sum == 0:
            return path
        else:
            return []

    leftPath = pathSumHelper(root.left, sum-root.value, path.append(root.value))
    rightPath = pathSumHelper(root.right, sum-root.value, path.append(root.value))
    print leftPath, rightPath
    if len(leftPath) == 0 and len(rightPath) == 0:
        return []

    if len(leftPath) == 0:
        return [rightPath]
    elif len(rightPath) == 0:
        return [leftPath]
    return [leftPath, rightPath]


if __name__ == '__main__':
    root = BTNode(1)

    print pathSum(root, 1)

Please try to convert your code snipped into a minimal, complete, and verifiable example. — das-g
– das-g, Commented Aug 22, 2015 at 21:42
@das-g: Changed code which can be ran on isolated system. The code was minimal and complete earlier as well. Now i guess its verifiable. — Debpriya Seal
– Debpriya Seal, Commented Aug 22, 2015 at 23:15

Alex Huszagh · Accepted Answer · 2015-08-23 00:53:30Z

2

The problem is here:

leftPath = pathSumHelper(root.left, sum-root.value, path.append(root.value))
rightPath = pathSumHelper(root.right, sum-root.value, path.append(root.value))

You are using path.append(root.value) and then using what is returned from the "append" function call as an argument in the pathSumHelper function, which is a NoneType (it modifies the object in-place, and returns None, which is why when you call it from the console, it returns nothing).

Rather, you need to use append prior to the function call and then use path in the function or do

path + [root.value]

in your function call so it will return an actual list.

I would recommend doing the following since you want to modify "path" in-place I am assuming.

path.append(root.value)
leftPath = pathSumHelper(root.left, sum-root.value, path)

Anything where the sum is 0, it will then return a Nonetype, which will cause a TypeError during the len call.

A simple, verifiable, reproducible example is:

>>>def a(x):
...    print(x)

>>>mylist = list(range(10))
>>> a(mylist.append(1))
None

>>> a(mylist + [5])
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 1, 5]

edited Aug 23, 2015 at 0:53

answered Aug 23, 2015 at 0:47

Alex Huszagh

14.8k3 gold badges42 silver badges70 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Debpriya Seal Over a year ago

I completely forgot that it's a method and would return a value. Now it makes sense. Now looking back it looks obvious. Thanks @Alexander Huszagh .

Alex Huszagh Over a year ago

Of course, if I've helped you and that answers your question and you feel there are no better answers, please feel free to accept it as an answer. Glad to help.

Debpriya Seal Over a year ago

How to accept the answer. I cannot upvote because of very less points. Is there any other way to accept the answer.

Alex Huszagh Over a year ago

See if this helps: meta.stackexchange.com/questions/23138/…

Collectives™ on Stack Overflow

Python 2.7: Using list.append in function arguments

1 Answer 1

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related