Stop the workers (will block until they are finished).

def stop(self):
        """Stop the workers (will block until they are finished)."""
        if self.running and self.num_workers:
            for worker in self.workers:
                self.queue.put(None)
            for worker in self.workers:
                worker.join()
            # Free up references held by workers
            del self.workers[:]
            self.queue.join()
        self.running = False