<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _upgrades(self, sid, transport):
 """Return the list of possible upgrades for a client connection."""

        if not self.allow_upgrades or self._get_socket(sid).upgraded or \
                self._async['websocket'] is None or transport == 'websocket':
            return []
        return ['websocket']