链表实现报错：AttributeError: 'NoneType' object has no attribute 'next_node'

Hey there! Let's break down this frustrating error you're hitting when adding the second node to your linked list—this is one of the most common gotchas for folks new to implementing linked lists, so we’ll get this sorted out quickly.

What’s Causing This Error? #

This message means you’re trying to access the next_node attribute of a None object. In your case, when inserting the second node, somewhere in your code you’re treating a None value like it’s a valid Node instance, and trying to read/write its next_node property.

The Most Common Mistake (And How to Fix It) #

Let’s walk through a typical faulty implementation that triggers this error, then fix it.

Example of Broken Code

Suppose your linked list and node classes look like this, with an insert method that’s supposed to add nodes to the end:

class Node:
    def __init__(self, data):
        self.data = data
        self.next_node = None

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

    def insert(self, data):
        new_node = Node(data)
        # Wrong approach to find the end of the list
        current = self.head
        while current:
            current = current.next_node
        # Now current is None, so this line throws the error!
        current.next_node = new_node

Why This Fails

• When you insert the first node, self.head gets set to that node, and its next_node is None.
• When inserting the second node, the loop runs until current becomes None (since it keeps following next_node until there’s nothing left).
• Then you try to set current.next_node = new_node—but current is None, hence the AttributeError.

Corrected Insert Method

We need to stop at the last valid node (the one whose next_node is None), then attach the new node to it:

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

    def insert(self, data):
        new_node = Node(data)
        # Handle empty list case first
        if self.head is None:
            self.head = new_node
            return
        
        # Traverse to the last valid node
        current = self.head
        while current.next_node is not None:
            current = current.next_node
        
        # Now current is the last node—attach the new node here
        current.next_node = new_node

Other Things to Check #

If your insertion logic is for a different position (like inserting after a specific node), double-check these:

• Always verify that the node you’re trying to modify (e.g., the predecessor node for your new node) is not None before accessing its next_node.
• Make sure you’re not accidentally overwriting self.head or losing track of node pointers during insertion.

内容的提问来源于stack exchange，提问作者Pouya Jigsaw