def close(self):
		'''关闭与数据库的连接'''
		if VERBOSE:
			print 'Database connection CLOSE.'
		self.conn.close()
def sort(cls, dataset, by=[], reverse=False):
        cols = [dataset.get_dimension(d, strict=True).name for d in by]
        return dataset.data.sort_values(by=cols, ascending=not reverse)