Question

最近，我正在考虑如何在Python中实现Singleton模式。如果Singleton类没有子类，则以下代码可以正常工作。

class Singleton(object):
    _instance = None

    def __new__(cls, *args, **kwargs):
        if cls._instance is None:
            cls._instance = super(Singleton, cls).__new__(cls, *args, **kwargs)
        return cls._instance

但是，Singleton类可能有子类。

class SingletonSub1(Singleton):
    def __new__(cls, *args, **kwargs):
        return super(SingletonSub1, cls).__new__(cls, *args, **kwargs)

class SingletonSub2(Singleton):
    def __new__(cls, *args, **kwargs):
        return super(SingletonSub1, cls).__new__(cls, *args, **kwargs)

要求是系统中只能有一个实例，即Singleton，SingletonSub1或SingletonSub2。我该如何实现呢？我知道我绝对可以使用模块级变量来保存Singleton对象。但这真是一个糟糕的代码...

Answer 1

使用字典来保存每个子类的实例

class Singleton(object):
    _instances = { }

    def __new__(cls, *args, **kwargs):
        if cls._instances.get( cls, None ) is None:
            cls._instances[ cls ] = super(Singleton, cls).__new__(cls, *args, **kwargs)

        return Singleton._instances[ cls ]

Singleton with Python中的子类

1 个答案: