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

Question

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.

seems like you may be at the end? Where node.next becomes none, so you try to access None.next which is not possible? — Mahrkeenerh
– Mahrkeenerh, Commented Oct 11, 2021 at 13:54
Tip: represent an empty list with self.head = Node(0) rather than self.head = None. Now all other methods can assume that self.head.next refers to the head (if any) of the list, rather than having to cater to self.head being a Node or None. The dummy node can be used to store metadata (such as the length) about the list. — chepner
– chepner, Commented Oct 11, 2021 at 14:14

Serge Ballesta · Accepted Answer · 2021-10-11 14:00:50Z

1

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):
    ...

answered Oct 11, 2021 at 14:00

Serge Ballesta

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