code: def bind(self, database):
          
          self._database = database
  
          for _ in xrange(self.size):
              session = self._new_session()
              session.create()
              self.put(session)
docs: Associate the pool with a database.
  
          :type database: :class:`~google.cloud.spanner_v1.database.Database`
          :param database: database used by the pool:  used to create sessions
                           when needed.