Tell all the threads to terminate (by sending a sentinel value) and
        wait for them to do so.

def _wait_threads(self):
        """
        Tell all the threads to terminate (by sending a sentinel value) and
        wait for them to do so.
        """
        # Note that you need two loops, since you can't say which
        # thread will get each sentinel
        for t in self._threads:
            self._to_fetch.put(None)    # sentinel
        for t in self._threads:
            t.join()
        self._threads = []