Instantiate only unique objects of a class

I'm trying to create a class that only creates an instance if the arguments passed in during instantiation are a unique combination. If the combination of arguments have previously been passed in, then return the instance that has already been previously created. I'd like for this class to be inherited by other classes so they inherit the same behavior. This is my first attempt at a solution,

The base/parent class to be inherited:

class RegistryType(type):
    def __init__(cls, name, bases, namespace, *args):
        cls.instantiated_objects = {}


class AdwordsObject(object, metaclass=RegistryType):
    api = AdWordsAPI()

    def __new__(cls, *args):
        object_name = '-'.join(args)
        if object_name in cls.instantiated_objects:
            return cls.instantiated_objects[object_name]
        else:
            obj = super(AdwordsObject, cls).__new__(cls)
            cls.instantiated_objects[object_name] = obj
            # cls.newt_connection.commit()
            return obj

And this is how it's being used in the child class:

class ProductAdGroup(AdwordsObject):
    # init method only called if object being instantiated hasn't already been instantiated
    def __init__(self, product_name, keyword_group):
        self.name = '-'.join([product_name, keyword_group])

    @classmethod
    def from_string(cls, name: str):
        arguments = name.split('-')
        assert len(arguments) == 2, 'Incorrect ad group name convention. ' \
                                    'Use: Product-KeywordGroup'
        ad_group = cls(*arguments)
        return ad_group

I've ran the program with this setup but it seems like a new dict is being created every time ProductAdGroup() is being created so the memory is exploding... even though the program returns the instance that had already been previously instantiated.

Is there anyway to fix this? Thanks!!!