code: def put(self, session):
          
          self._sessions.put_nowait((_NOW() + self._delta, session))
docs: Return a session to the pool.
  
          Never blocks:  if the pool is full, raises.
  
          :type session: :class:`~google.cloud.spanner_v1.session.Session`
          :param session: the session being returned.
  
          :raises: :exc:`six.moves.queue.Full` if the queue is full.