def stop(self, grace_period=None):
        """Signals the background thread to stop.

        This does not terminate the background thread. It simply queues the
        stop signal. If the main process exits before the background thread
        processes the stop signal, it will be terminated without finishing
        work. The ``grace_period`` parameter will give the background
        thread some time to finish processing before this function returns.

        :type grace_period: float
        :param grace_period: If specified, this method will block up to this
            many seconds to allow the background thread to finish work before
            returning.

        :rtype: bool
        :returns: True if the thread terminated. False if the thread is still
            running.
        """
        if not self.is_alive:
            return True

        with self._operational_lock:
            self._queue.put_nowait(_WORKER_TERMINATOR)

            if grace_period is not None:
                print("Waiting up to %d seconds." % (grace_period,), file=sys.stderr)

            self._thread.join(timeout=grace_period)

            # Check this before disowning the thread, because after we disown
            # the thread is_alive will be False regardless of if the thread
            # exited or not.
            success = not self.is_alive

            self._thread = None

            return success