Start IOLoop then set ready threading.Event.

def _start_io_loop(self):
        """Start IOLoop then set ready threading.Event."""

        def mark_as_ready():
            self._ready.set()

        if not self._io_loop:
            self._io_loop = ioloop.IOLoop()

        self._io_loop.add_callback(mark_as_ready)
        self._io_loop.start()