CODEWITHSUNDEEP
pythonlinked-list
madcrazydrumma
madcrazydrumma

Reputation: 1897

Error when creating a LinkedList

So for data structure practice in different languages, I decided to create a Linked List in python.

Now here is my code so far, it is still far from done, but I've encountered this error and I cant seem to point out to myself where it is coming from:

class LinkedNode:
    def __init__(self, value):
        self.value = value
        self.nextNode = None

class LinkedList:
    def __init__(self):
        self.headNode = None

    def isEmpty(self):
        return self.headNode == None

    def addToHead(self, value):
        head = LinkedNode(value)

    def removeFromHead(self):
        head = self.headNode
        self.headNode = self.headNode.nextNode
        return head

    def addToTail(self, value):
        if self.headNode == None:
            self.addToHead(value)
        else:
            tmp = self.headNode
            while tmp.nextNode != None:
                tmp = tmp.nextNode
            tmp.nextNode = LinkedNode(value)

    def empty(self):
        self.headNode = None

    def contains(self, value):
        tmp = self.headNode
        while tmp.nextNode != None:
            if tmp.value == value:
                return true
            tmp = tmp.nextNode
        return False

    def toString(self):
        tmp = self.headNode
        buffer = '['
        while tmp.nextNode != None:
            buffer = buffer + str(tmp.value) + ', '
            tmp = tmp.nextNode
        buffer = buffer + ']'

list = LinkedList()
list.addToHead(1)
list.addToHead(2)
list.addToTail(3)
list.toString()

Here is the error:

Traceback (most recent call last):

File "path/linkedList.py", line 52, in list.toString()

File "path/linkedList.py", line 44, in toString

while tmp.nextNode != None:

AttributeError: 'NoneType' object has no attribute 'nextNode' [Finished in 0.138s]

Upvotes: 0

Views: 181

Answers (2)

th3an0maly
th3an0maly

Reputation: 3510

  1. Firstly, to answer your question, this is exactly why you're getting the error:

You are doing head = LinkedNode(value), expecting the LinkedList's head to be overwritten. But instead, what happens is that you create a local variable named head and assign it the value of LinkedNode(value). As a result, head is always None. And you obviously can't do None.nextNode (because it ain't an object).

Solution: What you actually need is self.head = LinkedNode(value). self is a reference to the object you're working with.

  1. Secondly, there are several things wrong about your code. For instance:

a. By simply setting the head to the new node, you lose reference to the previously existing nodes.

Solution: What you need is:

def addToHead(self, value):
    tmp = LinkedNode(value)
    tmp.nextNode = self.headNode
    self.headNode = tmp

b. toString is a popular way of representing objects as strings, in other programming languages, but the pythonic way of doing it is by overriding the __str__() and __repr__() methods for the object.

Solution (I've stuck with your format, although it could use some changing ;):

def __repr__(self):
    tmp = self.headNode
    buffer = []
    while tmp is not None:
        buffer.append(tmp.value)
        tmp = tmp.nextNode
    return "[" + ", ".join(map(str, buffer)) + "]"

c. There are several things wrong about the toString method (headNode is not being printed, will fail headNode being None etc.), but I'm not gonna try and fix them, as we have the solution above.

Upvotes: 1

Joni
Joni

Reputation: 111389

Your addToHead method had no side effects. Possibly you meant

def addToHead(self, value): 
    head = LinkedNode(value)
    head.nextNode = self.headNode
    self.headNode = head

Also, toString will crash for empty lists.

Upvotes: 1

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