When calling kevent(), selectors.KqueueSelector.select sets the "maxevents" argument to len(self._fd_to_key). So if there are no fds registered, it passes maxevents=0.

It turns out that the kevent() API has a special case behavior for maxevents=0: it returns immediately, ignoring timeout. I have no idea why kevent() works this way, but it's specifically called out in the man page:

   The nevents argument determines the size of eventlist.  When
   nevents is zero, kevent() will return immediately even if there is a
   timeout specified unlike select(2).

https://www.freebsd.org/cgi/man.cgi?query=kqueue&sektion=2

It happens that asyncio doesn't run into this because asyncio always has at least one fd registered with its selector, but this causes problems for other users of the selectors module, e.g.:
  https://github.com/dabeaz/curio/issues/156

I believe fix would just be to add some code like: "if max_ev == 0: max_ev = 1" to selectors.KqueueSelector.select.

This looks like it's the cause of https://bugs.python.org/issue25680

New changeset ba1bcffe5cafc1bb0ac6fdf9ecef51e75e342707 by Russell Davis in branch 'master':
bpo-29255: Wait in KqueueSelector.select when no fds are registered (GH-19508)
https://github.com/python/cpython/commit/ba1bcffe5cafc1bb0ac6fdf9ecef51e75e342707