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