我不知道出了什么问题或导致错误的原因：
AttributeError: 'dict' object attribute 'update' is read-only
在以下代码中：
map = []
point1back = {}
point1fwd = {}
point1back.update = {'nextHop':point1Fwd, 'direction':1, 'distance':0}
point1fwd.update = {'nextHop':point1Fwd, 'direction':3, 'distance':160}
map.append(point1back)
map.append(point1fwd)