def close(self):
		'''关闭与数据库的连接'''
		if VERBOSE:
			print 'Database connection CLOSE.'
		self.conn.close()
def __init__(
        self, minimum=None, maximum=None, choices=None, optional=False
    ):
        self._required = not optional
        self._minimum = minimum
        self._maximum = maximum
        self._choices = choices