python运行一个诡异的地方，求指点

env ：
Python 2.7.2 (default, Oct 11 2012, 20:14:37)
[GCC 4.2.1 Compatible Apple Clang 4.0 (tags/Apple/clang-418.0.60)] on darwin

问题，copyedNode.neighbors.append(copyingNode)这一行会把copyingNode的neighbors同样append一个copyingNode自身，不懂求指教



class Node():
index = 0
neighbors = []
def __init__ (self, arg) :
self.index = arg

def description(self):
out = []
out.append (self.index)
for item in self.neighbors:
out.append(item.index)
print (out)

def allNode (self, checkArray, out) :
tempNodesArray = self.neighbors[:]
tempNodesArray.append(self)
for node in tempNodesArray:
if node in checkArray:
pass
else:
checkArray.append(node)
if node in out:
pass
else:
out.append(node)
node.allNode(checkArray, out)
def copy(self):
queue = [self]
copyNode = Node(self.index)
nodesMap = {self:copyNode}

while len(queue) > 0:
node = queue.pop()
copyedNode = nodesMap[node]
for subNode in node.neighbors :
if subNode in nodesMap :
copyedNode.neighbors.append(nodesMap[subNode])
else:
copyingNode = Node(subNode.index)
copyedNode.neighbors.append(copyingNode)
copyingNode.neighbors = []
nodesMap[subNode] = copyingNode
queue.append(subNode)
copyedNode.description()
return copyNode

#///////// test code below //////////
#create nodes in graph
nodes = []
for i in range (0, 7) :
node = Node(i)
nodes.append(node)

#create relation ships among all nodes
for node in nodes:
tempNodesArray = nodes[:]
tempNodesArray.remove(node)
node.neighbors = tempNodesArray

print("old graph")
for node in nodes:
node.description()

print("new graph")
newnode = nodes[0].copy()