    def purge_expired(self):
        """Purge all expired values"""
        if not self.max_age > 0:
            return
        now = self._last_purge = time.monotonic()
        cutoff = now - self.max_age
        for key in list(self.timestamps):
            timestamp = self.timestamps[key]
            if timestamp < cutoff:
                del self[key]
Purge all expired values
jupyterhub/services/auth.py