20220406-day26:__getitem__、__setitem__、__delitem__,增加用括号[]调用的方法

class Foo(object):

    def __getitem__(self, item):
        pass

    def __setitem__(self, key, value):
        pass

    def __delitem__(self, key):
        pass


obj = Foo("嘻嘻哈哈", 19)

obj["x1"]
obj['x2'] = 123
del obj['x3']