python - error in sqlalchemy after replacing count query with exists query -
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class sourcetoport(base): """""" __tablename__ = 'source_to_port' id = column(integer, primary_key=true) port_no = column(integer) src_address = column(string,index=true) #---------------------------------------------------------------------- def __init__(self, src_address,port_no): """""" self.src_address = src_address self.port_no = port_no def act_like_switch (self, packet, packet_in): """ implement switch-like behavior. """ # learn port source mac #print "recieved port ",packet_in.in_port , "source ",packet.src # create session #session = sessionmaker(bind=engine) #session = session() self.mac_to_port[packet.src]=packet_in.in_port #if self.mac_to_port.get(packet.dst)!=none: print "count dst",session.query(sourcetoport).filter_by(src_address=str(packet.dst)).count(),str(packet.dst) #if session.query(sourcetoport).filter_by(src_address=str(packet.dst)).count(): if session.query(exists().where(sourcetoport.src_address == str(packet.dst))).scalar() not none: #send packet print "got info database" q_res = session.query(sourcetoport).filter_by(src_address=str(packet.dst)).first() self.send_packet(packet_in.buffer_id, packet_in.data,q_res.port_no, packet_in.in_port) #create flow modification message msg = of.ofp_flow_mod() #set fields match incoming packet msg.match = of.ofp_match.from_packet(packet) #send rule switch not query controller again. msg.actions.append(of.ofp_action_output(port=q_res.port_no)) #push rule self.connection.send(msg) else: #flood packet out don't know node. print "flooding first packet" self.send_packet(packet_in.buffer_id, packet_in.data, of.ofpp_flood, packet_in.in_port) #self.matrix[(packet.src,packet.dst)]+=1 entry = sourcetoport(src_address=str(packet.src) , port_no=packet_in.in_port) #add record session object session.add(entry) #add record session object session.commit() i have peice of code.i replaced
#if session.query(sourcetoport).filter_by(src_address=str(packet.dst)).count(): with
if session.query(exists().where(sourcetoport.src_address == str(packet.dst))).scalar() not none: getting following error. file "/home/karthik/pox/tutorial.py", line 86, in act_like_switch self.send_packet(packet_in.buffer_id, packet_in.data,q_res.port_no, packet_in.in_port) attributeerror: 'nonetype' object has no attribute 'port_no' ^cinfo:core:going down... the above piece of code used work count query.why working exists query.
as francis-avila explained in brilliant answer other question there issue in logic exists()...scalar() not none. returns true or false - return not none. sorry mistake while suggesting how use exists in sqlalchemy yesterday.
other that code correct , should work after changing logic of using exists() query results from:
if session.query(exists().where(sourcetoport.src_address == str(packet.dst))).scalar() not none: to
if session.query(exists().where(sourcetoport.src_address == str(packet.dst))).scalar(): approach using exists()...one() described here , handling exception work too. keep in mind handling exception more expensive operation (use more cpu cycles) in python doing condition check. if application not performance critical - using try/catch exception handling fine.
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