    def _maybe_purge(self):
        """purge expired values _if_ it's been purge_interval since the last purge

        Called on every get/set, to keep the expired values clear.
        """
        if not self.purge_interval > 0:
            return
        now = time.monotonic()
        if self._last_purge < (now - self.purge_interval):
            self.purge_expired()
purge expired values _if_ it's been purge_interval since the last purge

Called on every get/set, to keep the expired values clear.
jupyterhub/services/auth.py