class Node:
    def __init__(self, Val, Prv = None, Nxt = None):
        self.Value = Val
        self.Previous, self.Next = Prv, Nxt
    def setPrevious(self, Prv = None):
        self.Previous = Prv
    def setNext(self, Nxt = None):
        self.Next = Nxt

firstNode = Node(1)
firstNode.Next = Node(2,firstNode)
firstNode.Next.Next = Node(3,firstNode.Next)
firstNode.Next.setPrevious(firstNode)
firstNode.Next.Next.setPrevious(firstNode.Next)

secondNode = firstNode.Next
firstNode.Next = secondNode.Next
secondNode.Next.Previous = firstNode