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.

def get(self):
        """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.
        """
        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