code: def _get_jwt_for_audience(self, audience):
          
          token, expiry = self._cache.get(audience, (None, None))
  
          if token is None or expiry < _helpers.utcnow():
              token, expiry = self._make_jwt_for_audience(audience)
              self._cache[audience] = token, expiry
  
          return token
docs: Get a JWT For a given audience.
  
          If there is already an existing, non-expired token in the cache for
          the audience, that token is used. Otherwise, a new token will be
          created.
  
          Args:
              audience (str): The intended audience.
  
          Returns:
              bytes: The encoded JWT.