Theselectors module API is deliberately very narrow:

• Theregister() /modify() methods only acceptevents as a bitmask ofEVENT_READ |EVENT_WRITE
• Theselect() method only ever returns combinations ofEVENT_READ |EVENT_WRITE

This design makes it impossible to use many platform-specific events provided by the underlying OS APIs (poll,epoll,kqueue). For example,#79698 describes a use case forEPOLLEXCLUSIVE on Linux. The biggest missed opportunity, however, is the inability to fully exploitkqueue() on BSD / macOS, which would allow async handling of regular files, signals, and others (seeKQ_EVENT_*).

This limitation is relevant forasyncio, which uses theselectors module. For instance, on BSD / macOS,asyncio could be notified when a process terminates, enabling an optimization similar to#144047. Currenlyasyncio on Linux already takes advantage of this approach viaos.pidfd_open() +selectors.PollSelector (seesource), but can't do the same for BSD / macOS becauseselectors.KqueueSelector doesn't allow registering customselect.kevent() objects.

Whereas it would be relatively easy to add aregister_kevent() method, or to passEPOLL* events directly toregister(), it's less clear how theselect() method should behave in this case, since it's apparently bound to the promise of returning eitherEVENT_READ orEVENT_WRITE.