python的特殊方法介绍 __repr__、__str____len__、__getitem__、__setitem__、__delitem__、__contains____iter__、__reversed__、__next____call____enter__、__exit____new__、__init__、__del____getattr__、__getattribute__、__setattr__、__delattr__、__dir____get__、__set__、__delete__