我的python printlinkedlist函数打印内存而不是链表

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

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

    def append(self, data):
        new_node = Node(data)
        if self.head is None:
            self.head = new_node
        else:
            current = self.head
            while current.next:
                current = current.next
            current.next = new_node

    def __str__(self):
        current = self.head
        nodes = []
        while current:
            nodes.append(str(current.data))
            current = current.next
        return ' -> '.join(nodes)

# 示例用法
linked_list = LinkedList()
linked_list.append(1)
linked_list.append(2)
linked_list.append(3)
print(linked_list)  # 输出：1 -> 2 -> 3