让isinstance(obj, cls)与装饰类一起工作

Question

我有几个类需要实现以下功能：

当调用构造函数时，如果已经存在一个相同的对象（也就是说，具有相同ID的对象），就返回那个对象。否则，就创建一个新的实例。

简单来说，就是：

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

为了实现这个功能，我写了一个类装饰器，代码如下：

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)

这个装饰器能达到我想要的效果，但：

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

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

我该如何修复这个问题呢？我找到了一些相关的问题，但我就是无法将那些解决方案适应到我的情况中。

---

我知道有人会让我贴出一些不工作的代码，所以这里是：

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

作为一种替代方案，你可以用元类来创建单例类，而不是使用装饰器。元类可以像子类那样为类添加功能。使用元类的好处是，类的名字 c 直接指向类 c 本身，而不是一个 Singleton 对象或者一个包装了构造函数调用的函数。

举个例子：

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

和你的装饰器相比：

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"))

输出结果：

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

你可以在你的装饰器类里添加一个自定义的 __instancecheck__ 钩子：

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