class SingletonMeta(type): """ The Singleton class can be implemented in different ways in Python. Some possible methods include: base class, decorator, metaclass. We will use the metaclass because it is best suited for this purpose. """ _instances = {} def __call__(cls, *args, **kwargs): """ Possible changes to the value of the `__init__` argument do not affect the returned instance. """ if cls not in cls._instances: instance = super().__call__(*args, **kwargs) cls._instances[cls] = instance return cls._instances[cls] class Singleton(metaclass=SingletonMeta): def some_business_logic(self): """ Finally, any singleton should define some business logic, which can be executed on its instance. """ # ... if __name__ == "__main__": # The client code. s1 = Singleton() s2 = Singleton() if id(s1) == id(s2): print("Singleton works, both variables contain the same instance.") else: print("Singleton failed, variables contain different instances.")

def produce_instance(cls, *args, **kwargs):
    if cls not in instances:
        instances[cls] = cls(*args, **kwargs)
    return instances[cls]


@singleton
class MySingletonClass:
    def __init__(self, arg):
        self.value = arg