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)