Singleton implementation in Python

singleton.py

def singleton(cls):
    class Singleton(cls):
        def __new__(cls, *args, **kwargs):
            try:
                return cls.__instance
            except AttributeError:
                cls.__instance = super(Singleton, cls).__new__(cls, *args, **kwargs)
                cls.instance = cls.__instance
                return cls.__instance
    t = Singleton
    t.__name__ = cls.__name__
    return t

if __name__ == '__main__':
    @singleton
    class First(object):
        pass

    @singleton
    class Another(object):
        pass

    @singleton
    class Third(list):
        pass

    s1 = First()
    s2 = First()
    assert s1 == s2
    assert s1 == s2.instance
    assert s2 == First.instance

    a1 = Another()
    a2 = Another()
    assert a1 == a2
    assert a1 == a2.instance
    assert a2 == Another.instance

    assert First.instance != Another.instance

    t1 = Third()
    assert len(t1) == 0
    assert t1 == Third.instance
    assert Third.instance != Another.instance