`makeThingDoerForTest` is a standalone function (or static method) that never gets called in production code. I don't see why it needs to be in an entirely separate module. If you want, you could put it in a separate thing_test module, but then you'd have to expose a more powerful constructor for thing_test to access. I'd rather hide that inside the thing module, and I see no benefit to splitting this functionality across two modules just for the sake of not mixing my peas with my potatoes.