python - python单例类可以继承吗？

Question

我的想法是任何一个对象都应该存在于单例类的所有子类中。我一直在尝试的代码和结果矩阵如下所示。在子类的情况下，矩阵似乎工作正常。我走错路了吗？它是否得到了父类对象和子类对象的情况？

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

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

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

class C(B):
    def __new__(cls, *args, **kwargs):
        super(B, cls).__new__(cls, *args, **kwargs)


if __name__ == '__main__':
    s1=Singleton()
    s2=Singleton()
    if(id(s1)==id(s2)):
        print "Same"
    else:
        print "Different"

'''
I got a result matrix for s1 and s2
           |************ s2 **************************|           
s1         |Singleton() |A()      | B()     | C()     |
===========|==========================================|
Singleton()|Same        |Different|Different|Different|
A()        |Different   |Same     |Same     |Same     |
B()        |Different   |Same     |Same     |Same     |
C()        |Different   |Same     |Same     |Same     |
'''

Answer 1

所有子类的 __new__() 方法都没有明确的 return 语句，因此它们都返回 None。这就是为什么他们的新实例都是一样的。使用此代码，它们也将与 Singleton() 相同:

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

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

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

class C(B):
    def __new__(cls, *args, **kwargs):
        return super(C, cls).__new__(cls, *args, **kwargs)

甚至不需要为子类定义__new__():

class A(Singleton):
    pass

class B(Singleton):
    pass

class C(B):
    pass