子类化列表，字典和字符串
1. 如果希望实现一个和内建列表行为相似的序列，可以使用子类list

   class CounterList(list):       def __init__(self, *args):           super(CounterList, self).__init__(*args)           self.counter = 0       def __getitem__(self, index):           self.counter += 1            return super(CounterList, self).__getitem__(index)