利用Python中的元类实现缓存实例（cached instance）

关于缓存实例（cached instance），可以参考Python中的弱引用

import weakref
class Cached(type):
    def __init__(self, *args, **kwargs):
        super(Cached, self).__init__(*args, **kwargs)
        # 建立一个value为弱引用的对象
        self.__cache = weakref.WeakValueDictionary()
    def __call__(self, *args):
        if args in self.__cache:
            return self.__cache[args]
        else:
            # 注意，这里的self为Spam.Spam已经通过元类创建了__cache对象
            obj = super(Cached, self).__call__(*args)
            self.__cache[args] = obj
            return obj
class Spam(metaclass=Cached):
    def __init__(self, name):
        print('Creating Spam({!r})'.format(name))
        self.name = name
if __name__ == '__main__':
    a = Spam('foo')
    b = Spam('bar')
    print('a is b:', a is b)
    c = Spam('foo')
    print('a is c:', a is c)