def _prepare_threads(self):
        """
        Threads are created only when get_project is called, and terminate
        before it returns. They are there primarily to parallelise I/O (i.e.
        fetching web pages).
        """
        self._threads = []
        for i in range(self.num_workers):
            t = threading.Thread(target=self._fetch)
            t.setDaemon(True)
            t.start()
            self._threads.append(t)
Threads are created only when get_project is called, and terminate
        before it returns. They are there primarily to parallelise I/O (i.e.
        fetching web pages).