Python 状态下的单例设计模式

Question

我正在尝试在 Python 中实现一个单例，在阅读 this post 之后，我发现自己比以前更加困惑。答案太多了，其中许多都获得了相当多的选票。现在的问题可能不是我有一个单例，而是状态只需要初始化一次。我尝试了几个实现 SingletonA 和 SingletonB 但我无法让它工作。对于我的实际问题，__init__ 函数非常繁重，所以我只需要执行一次。这是我目前所拥有的：

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)

正在打印：

$ python singleton.py 
Creating state in B
Creating state in B
Same 1.0 2.0
Creating state in A
Creating state in A
Same 1.0 2.0

指出我确实有一个单身人士class，但我想避免创建状态。

Answer 1

如果我没理解错的话，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，并且该数据将只计算一次。

请注意，您可以在读取时将共享数据引用为self._shared_dict，但在写入时必须使用class属性self.__class__._shared_dict的全名。

Answer 2

这行不通，因为 __init__ 方法是在对象创建后通过 __new__ 调用的。摘自 Python Language Reference

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__().

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

代码可以是（不需要父class，所以我省略了）：

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，您可以在声明时简单地构建实例：

class SingletonA:
    def __init__(self):

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

    def _init(self):

        # some state
        print("Creating state in A")
        self.X = 1.
        self.Y = 2.
    def __new__(cls, *args, **kwargs):
        return cls._instance

SingletonA._instance = super(SingletonA, SingletonA).__new__(SingletonA)
SingletonA._instance._init()

两种方式都会导致以下输出（对于 SingletonA 部分）：

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

完全删除 __init__ 方法会导致一次初始化。

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 class 本身

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