docs: 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()