Class HistorySavingThread

IPython/core/history.py:1122–1194

This thread takes care of writing history to the database, so that the UI isn't held up while that happens. It waits for the HistoryManager's save_flag to be set, then writes out the history cache. The main thread is responsible for setting the flag when the cache size reaches a def

class HistorySavingThread(threading.Thread):
    """This thread takes care of writing history to the database, so that
    the UI isn't held up while that happens.

    It waits for the HistoryManager's save_flag to be set, then writes out
    the history cache. The main thread is responsible for setting the flag when
    the cache size reaches a defined threshold."""

    save_flag: threading.Event
    daemon: bool = True
    _stop_now: bool = False
    enabled: bool = True
    history_manager: ref[HistoryManager]
    _stopped = False

    def __init__(self, history_manager: HistoryManager) -> None:
        super(HistorySavingThread, self).__init__(name="IPythonHistorySavingThread")
        self.history_manager = ref(history_manager)
        self.enabled = history_manager.enabled
        self.save_flag = threading.Event()

    @only_when_enabled
    def run(self) -> None:
        atexit.register(self.stop)
        # We need a separate db connection per thread:
        try:
            hm: ReferenceType[HistoryManager]
            with hold(self.history_manager) as hm:
                if hm() is not None:
                    self.db = sqlite3.connect(
                        str(hm().hist_file),  # type: ignore [union-attr]
                        **cast(dict[str, t.Any], hm().connection_options),  # type: ignore [union-attr]
                    )
            while True:
                self.save_flag.wait()
                with hold(self.history_manager) as hm:
                    if hm() is None:
                        self._stop_now = True
                    if self._stop_now:
                        self.db.close()
                        return
                    self.save_flag.clear()
                    if hm() is not None:
                        hm().writeout_cache(self.db)  # type: ignore [union-attr]

        except Exception as e:
            print(
                (
                    "The history saving thread hit an unexpected error (%s)."
                    "History will not be written to the database."
                )
                % repr(e)
            )
        finally:
            atexit.unregister(self.stop)

    def stop(self) -> None:
        """This can be called from the main thread to safely stop this thread.