单链表、reversed函数返回值（为何打印为空）

def __reversed__(self):
                if self.head is None or self.head.next is None:
                        return None
                curl = self.head
                prev = None
                while curl:
                        temp = curl.next
                        curl.next = prev
                        prev = curl
                        curl = temp
                return prev


lyst = LinkedList()
head = lyst.Node()
node1 = lyst.Node(1)
node2 = lyst.Node(2)
node3 = lyst.Node(3)
lyst.head = head
head.next = node1
node1.next = node2
node2.next = node3

lyst1 = LinkedList(reversed(lyst))
lyst1.printNode()
print("=========================")
lyst.printNode()

----------------------------------------------------------------------------------------------------------


2
1
None
=========================

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为什么在reversed后lyst就没有了（printNode时没有结点。）？？？

另，顺便问下。如果要在LinkedList类中实现reverse()（不是__reversed__()），应该返回什么？（返回self吗）