> My underlying question is why the Future has to set its loop in its constructor, instead of simply using get_event_loop() inside _schedule_callbacks().  This would always work.

So imagine a Future `fut` is completed. And we call `fut.add_done_callback()` in different contexts with different active event loops.  With your suggestion we'll schedule our callbacks in different loops.

Ideally you should use `loop.create_future()` when you can (and in libraries you usually can do that) to always make it explicit which loop your Future is attached to.