def _purge_expired(self):
        """Removes expired keys from the cache."""

        now = timeutils.utcnow_ts()
        for timeout in sorted(self._keys_expires.keys()):

            # NOTE(flaper87): If timeout is greater
            # than `now`, stop the iteration, remaining
            # keys have not expired.
            if now < timeout:
                break

            # NOTE(flaper87): Unset every key in
            # this set from the cache if its timeout
            # is equal to `timeout`. (The key might
            # have been updated)
            for subkey in self._keys_expires.pop(timeout):
                try:
                    if self._cache[subkey][0] == timeout:
                        del self._cache[subkey]
                except KeyError:
                    continue
dataset/ETHPy150Open openstack/neutron/neutron/openstack/common/cache/_backends/memory.py/MemoryBackend._purge_expired/original