使 isinstance(obj, cls) 与装饰类一起工作

Question

我有几个类需要执行以下操作：

当构造函数被调用时，如果一个相等的对象（也就是一个具有相同 id 的对象）已经存在，则返回那个对象。否则，创建一个新实例。 基本上，

>>> cls(id=1) is cls(id=1)
True

为了实现这一点，我编写了一个类装饰器，如下所示：

class Singleton(object):
    def __init__(self, cls):
        self.__dict__.update({'instances': {},
                                'cls': cls})

    def __call__(self, id, *args, **kwargs):
        try:
            return self.instances[id]
        except KeyError:
            instance= self.cls(id, *args, **kwargs)
            self.instances[id]= instance
            return instance

    def __getattr__(self, attr):
        return getattr(self.cls, attr)
    def __setattr__(self, attr, value):
        setattr(self.cls, attr, value)

这是我想要的，但是：

@Singleton
class c(object):
    def __init__(self, id):
        self.id= id

o= c(1)
isinstance(o, c) # returns False

我该如何解决这个问题？我找到了related question，但我似乎无法让这些解决方案适应我的用例。

---

我知道有人会要求我发布一些不起作用的代码，所以你去吧：

def Singleton(cls):
    instances= {}
    class single(cls):
        def __new__(self, id, *args, **kwargs):
            try:
                return instances[id]
            except KeyError:
                instance= cls(id, *args, **kwargs)
                instances[id]= instance
                return instance
    return single
# problem: isinstance(c(1), c) -> False

def Singleton(cls):
    instances= {}
    def call(id, *args, **kwargs):
        try:
            return instances[id]
        except KeyError:
            instance= cls(id, *args, **kwargs)
            instances[id]= instance
            return instance
    return call
# problem: isinstance(c(1), c) -> TypeError

Answer 1

作为使用装饰器创建单例类的替代解决方案，您可以改用元类来创建您的类。元类可用于向类添加功能，就像子类可以从其超类继承功能一样。元类的优点是名称 c 实际上将直接引用类 c 而不是 Singleton 对象或包装对 c 的构造函数的调用的函数。

例如：

class SingletonMeta(type):
    """SingletonMeta is a class factory that adds singleton functionality to a 
    class. In the following functions `cls' is the actual class, not 
    SingletonMeta."""

    def __call__(cls, id, *args, **kwargs):
        """Try getting a preexisting instance or create a new one"""
        return cls._instances.get(id) or cls._new_instance(id, args, kwargs)

    def _new_instance(cls, id, args, kwargs):
        obj = super(SingletonMeta, cls).__call__(*args, **kwargs)
        assert not hasattr(obj, "id"), "{} should not use 'id' as it is " \
            "reserved for use by Singletons".format(cls.__name__)
        obj.id = id
        cls._instances[id] = obj
        return obj        

    def __init__(cls, classname, bases, attributes):
        """Used to initialise `_instances' on singleton class"""
        super(SingletonMeta, cls).__init__(classname, bases, attributes)    
        cls._instances = {}

你可以这样使用它：

# python 2.x
class MySingleton(object):
    __metaclass__ = SingletonMeta

# python 3.x
class MySingleton(object, metaclass=SingletonMeta):
    pass

与你的装饰器对比使用：

class IDObject(object):
    def __str__(self):
        return "{}(id={})".format(type(self).__name__, self.id)

@Singleton
class A(IDObject):
    def __init__(self, id):
        self.id = id

class B(IDObject, metaclass=SingletonMeta):
    pass

format_str = """{4} class is {0}
an instance: {1}
{1} is {1} = {2}
isinstance({1}, {0.__name__}) = {3}"""
print(format_str.format(A, A(1), A(1) is A(1), isinstance(A(1), A), "decorator"))
print()
print(format_str.format(B, B(1), B(1) is B(1), isinstance(B(1), B), "metaclass"))

输出：

decorator class is <__main__.Singleton object at 0x7f2d2dbffb90>
an instance: A(id=1)
A(id=1) is A(id=1) = True
isinstance(A(id=1), A) = False

metaclass class is <class '__main__.B'>
an instance: B(id=1)
B(id=1) is B(id=1) = True
isinstance(B(id=1), B) = True

Answer 2

您可以在装饰器类中添加自定义 __instancecheck__ 钩子：

def __instancecheck__(self, other):
    return isinstance(other, self.cls)