Singleton pattern with twist

This problem was asked by Microsoft.

Implement the singleton pattern with a twist. First, instead of storing one instance, store two instances. And in every even call of getInstance(), return the first instance and in every odd call of getInstance(), return the second instance.

My Solution(Python):

class Singleton(object):
    """
    Simple Singleton class
    """
    _num_instances = 0
    _singleton_instance = None

    def __new__(cls):
        if Singleton._num_instances==0:
            print("Instance created")
            Singleton._num_instances += 1
            return super().__new__(cls)
        else:
            raise Exception("This is a singleton class")

    def __init__(self):
        print("Initializing")
        #this is not really required, but it's a good practice to store the only Singleton object somewhere, so you can retrieve it.
        Singleton._singleton_instance = self

    @staticmethod
    def getInstance():
        return Singleton._singleton_instance


def test():
    # Usage:
    s = Singleton() #prints Instance created
    t = Singleton() #raises Exception
    u = Singleton.getInstance() #returns the "s" instace



class SingleTon(object):
    """
    Twisted SingleTon class as the solution of the problem
    """
    _num_instances = 0
    _retrievals = 0
    _instances = {1: None, 2: None}

    def __new__(cls):
        if SingleTon._num_instances<2:
            SingleTon._num_instances += 1
            return super().__new__(cls)
        else:
            raise Exception("This is a twisted singleton class")

    def __init__(self):
        SingleTon._instances[SingleTon._num_instances] = self

    @staticmethod
    def getInstance():
        SingleTon._retrievals += 1
        return SingleTon._instances[SingleTon._retrievals%2 +1]


def main():
    # Usage
    print(SingleTon())
    # <__main__.SingleTon object at 0x10ae1c518>

    print(SingleTon())
    # <__main__.SingleTon object at 0x10ae1c550>

    # print(SingleTon())
    # Exception: This is a twisted singleton class

    print(SingleTon.getInstance())
    # odd call - 2nd instance
    # <__main__.SingleTon object at 0x10ae1c550>

    print(SingleTon.getInstance())
    # even call - 1st instance
    # <__main__.SingleTon object at 0x10ae1c518>

    print(SingleTon.getInstance())
    # odd call - 2nd instance
    # <__main__.SingleTon object at 0x10ae1c550>

    print(SingleTon.getInstance())
    # even call - 1st instance
    # <__main__.SingleTon object at 0x10ae1c518>


if __name__=='__main__':
    main()