在 Python 中销毁单例对象

Question

我在 Python 中有一个单例对象：

class Singleton(type):
    _instances = {}

    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
        return cls._instances[cls]

    @classmethod
    def destroy(cls):
        del cls._instances[cls]


class MockObject(metaclass=Singleton):

    def __init__(self, *args, **kwargs):
        # various things

我想在某个时候销毁对象，所以我在元类中写了一个类方法。但是，cls 指的是元类 Singleton 而不是 MockObject。有没有办法调用值为 MockObject 的 destroy 函数？

Answer 1

不要定义用于删除实例引用的自定义方法，而是使用 WeakValueDictionary.

现在，当任何地方不再有 MockObject 的引用时，它将自动从 Singleton._instances 中清除。

from weakref import WeakValueDictionary


class Singleton(type):
    _instances = WeakValueDictionary()

    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            # This variable declaration is required to force a
            # strong reference on the instance.
            instance = super(Singleton, cls).__call__(*args, **kwargs)
            cls._instances[cls] = instance
        return cls._instances[cls]


class MockObject(metaclass=Singleton):

    def __init__(self, *args, **kwargs):
        pass


if __name__ == '__main__':
    m = MockObject()
    print(dict(Singleton._instances))
    del m
    print(dict(Singleton._instances))

输出：

{<class '__main__.MockObject'>: <__main__.MockObject object at 0x104531128>}
{}

在 Python 中销毁单例对象

Destroying a Singleton object in Python

python

metaprogramming

python-3.x