When is this function called during serialization? I can't see anywhere where the method is called in the PR itself?

It would be called by the user if they want to serialize PrimitiveJob - example here

Would it be better to make this happen in one of the serialization methods that python has built-in? I believe this could be executed on one of the methods, avoiding that function call. You could include the lines here and override one of the reserved methods for serialization: __reduce__, __getstate__, __getnewargs__, depending on your needs.

Note: This might require you to also implement a __setstate__ method.

Thank you for your suggestion. I think the following should work (to remove non-pickable object _future) by replacing _prepare_dump with the following.

    def __getstate__(self):
        _ = self.result()
        _ = self.status()
        state = self.__dict__.copy()
        state["_future"] = None
        return state

    def __setstate__(self, state):
        self.__dict__.update(state)
        self._future = None

I made a patch to this PR kt474#1.