基于s的Python单例设计模式

Question

我试图用Python实现一个singleton，在阅读了this post之后，我发现自己比以前更加困惑。答案太多了，其中许多人得到了他们应得的票数。现在的问题可能不是我有一个singleton，而是状态只需要初始化一次。我尝试了两个实现SingletonA和SingletonB，但我无法管理它来工作。对于我真正的问题，__init__函数非常重，所以我只需要做一次。到目前为止，我得到的是：

class ClassA:

    def __init__(self):

        # some state
        print("Creating state in A")
        self.X = 1.
        self.Y = 2.

class SingletonA(ClassA):
    _instance = None
    def __new__(cls, *args, **kwargs):
        if not cls._instance:
            cls._instance = super(SingletonA, cls).__new__(
                                cls, *args, **kwargs)
        return cls._instance


class ClassB:

    __shared_state = {}

    def __init__(self):

        if not bool(ClassB.__shared_state):

            # some state
            print("Creating state in B")
            self.X = 1.
            self.Y = 2.

        self.__dict__ = self.__shared_state


def singleton(cls):
    obj = cls()
    # Always return the same object
    cls.__new__ = staticmethod(lambda cls: obj)
    # Disable __init__
    try:
        del cls.__init__
    except AttributeError:
        pass
    return cls

@singleton
class SingletonB(ClassB):
    pass

if __name__ == "__main__":

    a1 = SingletonA()
    a2 = SingletonA()
    if (id(a1) == id(a2)):
        print("Same",a1.X, a1.Y)
    else:
        print("Different",a1.X, a1.Y)

    b1 = SingletonB()
    b2 = SingletonB()
    if (id(b1) == id(b2)):
        print("Same",b1.X, b1.Y)
    else:
        print("Different",b1.X, b1.Y)

现在打印：

^{pr2}$

我确实有一个singleton类，但我想避免创建state。在

Answer 1

如果我没听错，这就是你所需要的：

# edited according to discussion in comments
class C:
    _shared_dict = None
    def __init__(self):
        if self._shared_dict is None:
            print("initializing")
            self.x = 1 
            self.y = 2 
            self.__class__._shared_dict = self.__dict__
        else:
            self.__dict__ = self._shared_dict

a=C()
b=C()

print(id(a), a.x, a.y)
print(id(b), b.x, b.y)

所有实例将共享相同的数据，并且此数据将只计算一次。在

请注意，您可以在读取时将共享数据称为self._shared_dict，但写入时必须使用类属性self.__class__._shared_dict的全名。在

Answer 2

这无法工作，因为__init__方法是在对象创建之后通过__new__调用的。从Python Language Reference提取

If __new__() returns an instance of cls, then the new instance’s __init__() method will be invoked like __init__(self[, ...]), where self is the new instance and the remaining arguments are the same as were passed to __new__().

您应该有一个与__init__不同的特殊初始化方法，并在_instance创建时调用。在

代码可以是（不需要父类，所以我省略了它）：

class SingletonA:
    _instance = None
    def __init__(self):

        # some state
        print("Creating dummy state in SingletonA")

    def _init(self):

        # some state
        print("Creating state in SingletonA")
        self.X = 1.
        self.Y = 2.
    def __new__(cls, *args, **kwargs):
        if cls._instance is None:
            cls._instance = super(SingletonA, cls).__new__(
                cls, *args, **kwargs)
            cls._instance._init()
        return cls._instance

但实际上，您只需在声明时构建实例：

^{pr2}$

这两种方式都会导致以下输出（对于单件零件）：

Creating state in A
Creating dummy state in SingletonA
Creating dummy state in SingletonA
('Same', 1.0, 2.0)

完全删除__init__方法将导致一次初始化。在

注释版本可以是：

class ClassB:

    def __init__(self):

            # some state
            print("Creating state in B")
            self.X = 1.
            self.Y = 2.


def singleton(cls):
    obj = cls()
    # Always return the same object
    cls._instance = obj
    cls.__new__ = staticmethod(lambda cls: cls._instance)
    # Disable __init__
    cls.__init__ = (lambda self: None)
    return cls

@singleton
class SingletonB(ClassB):
    pass

简单地将单例实例存储在类本身中