Please consider my patches instead; it seems our patches crossed.  Merging is now difficult because I already submitted my version to Tulip.  Your version still has a bug: it uses select.kevent(...) twice, where it should just use kevent(...).  Also my version makes fewer syscalls when unregistering a FD that has both read and write events registered.

Regarding the _Key return value: I think it's asking for trouble if the signature of the base class differs from that of the subclass.  The return value may even be useful occasionally.

Given that no spurious FD events are now reported by the unittests, I'm not sure that it is useful to log and ignore them; it may be better to have the exception be raised, as it might expose an app bug, and in my experience it usually ends up in an infinite busy-wait loop once it happens.