单链表插入元素出错，疑问

ABC23

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

n1 = Node       
n2 = Node('A', None)
n3 = Node('B', n2)
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当我们调用n1.next = n3，为什么会出现AttributeError？？

ABC23

are u kidding me？？