CODEWITHSUNDEEP
pythonlinked-list
Gary Kong
Gary Kong

Reputation: 1

Why am I getting an AttributeError when trying to traverse a linked list?

I am trying to write a search() function to search a linked list for a node with the requested value and return the node.

Below is my code:

class Node:
    def __init__(self, value):
        self.value = value
        self.next = None

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

    def to_list(self):
        out = []
        node = self.head
        while node:
            out.append(node.value)
            node = node.next
        return out

    def search(self, value):
        """ Search the linked list for a node with the requested value  and return the node. """
        # Traverse through the list until a number is found
        node = self.head
        print(node)
        while node != value:
            node = node.next
        return node

# Test search
linked_list = LinkedList()
linked_list.prepend(2) # Method not shown for brevity
linked_list.prepend(1)
linked_list.append(4) # Method not shown for brevity
linked_list.append(3)
linked_list.to_list()
assert linked_list.search(1).value == 1, f"list contents: {linked_list.to_list()}"
assert linked_list.search(4).value == 4, f"list contents: {linked_list.to_list()}"

Running this code gives me the following error:

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-58-d70897699f08> in <module>()
      6 linked_list.append(3)
      7 linked_list.to_list()
----> 8 assert linked_list.search(1).value == 1, f"list contents: {linked_list.to_list()}"
      9 assert linked_list.search(4).value == 4, f"list contents: {linked_list.to_list()}"

<ipython-input-54-c7f29bb4a2be> in search(self, value)
      5     print(node)
      6     while node != value:
----> 7         node = node.next
      8     return node
      9 

AttributeError: 'NoneType' object has no attribute 'next'

Can anyone point out why I am getting this AttributeError? Had used very similar traversal code in the to_list method so am not sure why I"m encountering this issue in the search method.

Upvotes: 0

Views: 126

Answers (1)

Serge Ballesta
Serge Ballesta

Reputation: 148965

The text while node != value is plain wrong. The node variable should contain Node objects or None. As no node object can be the integer 1 (even if its value attribute could), you end with None and raise the error.

What you want is:

while node and (node.value != value):
    ...

Upvotes: 1

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