Mock a python object delegating magic methods

I have a class that mostly displays outwardly as a set of ids, but not entirely. Think this:

class PoundIds(object):
    ....
    @property
    def cats(self):
        return self.cats

    @property
    def dogs(self):
        return self.dogs

    @property
    def animal_ids(self):
        return set(dog.id for dog in self.dogs) | set(cat.id for cat in self.cats) | 

    def __contains__(self, id):
        return id in self.animal_ids

    def __iter__(self, id):
        return self.animal_ids.__iter__()

I am now testing this object, in a way that will only be using the list of ids it provides. Is there a way to mock out the object as a specific set of integers, passing magic methods through to the encapsulated set?