code: def get(self):
          
          try:
              session = self._sessions.get_nowait()
          except queue.Empty:
              session = self._new_session()
              session.create()
          else:
              if not session.exists():
                  session = self._new_session()
                  session.create()
          return session
docs: Check a session out from the pool.
  
          :rtype: :class:`~google.cloud.spanner_v1.session.Session`
          :returns: an existing session from the pool, or a newly-created
                    session.