code: def put(self, session):
          
          try:
              self._sessions.put_nowait(session)
          except queue.Full:
              try:
                  session.delete()
              except NotFound:
                  pass
docs: Return a session to the pool.
  
          Never blocks:  if the pool is full, the returned session is
          discarded.
  
          :type session: :class:`~google.cloud.spanner_v1.session.Session`
          :param session: the session being returned.