Paho on_connect works if connection OK but is not called when error. Why? - paho

Question

Why does my paho-mqtt (1.5.1) on_connect work if connection OK but it is not called when an error. For testing I'm using Linux Lite 4.2 based on Ubuntu 18.04 LTS with Xfcs running in a VM (VBox).
class subscribemqtt:
.....
def on_connect(self, client, userdata, flags, rc):
print ("ZZZZZZZZZZZZZ in on_connect")
connectErrs = {.........}
self.connectRc = rc
self.connectReason = connectErrs[str(rc)]
print ("$$$$$$$$$$$$$", self.connectRc, self.connectReason)
return
def subscribe(self, arguments):
...........
self.client = paho.Client(self.config['CLIENT_ID'])
self.client.on_message = self.on_subscribe
self.client.on_connect = self.on_connect
print ("#############", self.on_connect)
print ("XXXXXXXXXXXX calling self.client.connect(...)
self.client.connect(self.config['HOST'],self.config['PORT'])
print ("YYYYYYYYYYYYY calling self.client.loop_start()")
self.client.loop_start()
print ("AAAAAAAAAAAAA", self.connected)
while not self.connected:
time.sleep(0.1)
print ("BBBBBBBBBBBBB", self.connected, self.connectRc)
When all the parameters are correct, on_connect gets called:
############# <bound method subscribemqtt.on_connect of <__main__.subscribemqtt object at 0x7f32065a6ac8>>
XXXXXXXXXXXX calling self.client.connect(self.config['HOST'],self.config['PORT']
YYYYYYYYYYYYY calling self.client.loop_start()
AAAAAAAAAAAAA**ZZZZZZZZZZZZZ in on_connect**
False$$$$$$$$$$$$$
0 Connection successful
BBBBBBBBBBBBB True 0
When I set the host address to an invalid address (to create an error to test my error handling) I get:
subscribemqtt.subscribe:topic= Immersion Dummy
############# <bound method subscribemqtt.on_connect of <__main__.subscribemqtt object at 0x7ffb942ae0b8>>
XXXXXXXXXXXX calling self.client.connect(self.config['HOST'],self.config['PORT']
Traceback (most recent call last):
File "/home/linuxlite/Desktop/EMS/sendroutines/subscribemqtt.py", line 275, in <module>
(retcode, reason, topic) = subscribeObj.subscribe([None, topic])
File "/home/linuxlite/Desktop/EMS/sendroutines/subscribemqtt.py", line 191, in subscribe
self.client.connect(self.config['HOST'],self.config['PORT'])
File "/usr/local/lib/python3.6/dist-packages/paho/mqtt/client.py", line 941, in connect
return self.reconnect()
File "/usr/local/lib/python3.6/dist-packages/paho/mqtt/client.py", line 1075, in reconnect
sock = self._create_socket_connection()
File "/usr/local/lib/python3.6/dist-packages/paho/mqtt/client.py", line 3546, in _create_socket_connection
return socket.create_connection(addr, source_address=source, timeout=self._keepalive)
File "/usr/lib/python3.6/socket.py", line 704, in create_connection
for res in getaddrinfo(host, port, 0, SOCK_STREAM):
File "/usr/lib/python3.6/socket.py", line 745, in getaddrinfo
for res in _socket.getaddrinfo(host, port, family, type, proto, flags):
socket.gaierror: [Errno -2] Name or service not known
>>>
Thanks for reading.
Alan
PS. I just tried:
try:
self.client.connect(self.config['HOST'],self.config['PORT'])
except:
print ("**************** Error exception calling self.client.connect")
And that works but my understanding is that on_connect should be called for errors.

Answer 1

From the docs:
on_connect()
on_connect(client, userdata, flags, rc)
Called when the broker responds to our connection request.
The important part there is "when the broker responds". But in the example you have shown the hostname provided can not be resolved, so broker never responds because it is never actually contacted.
on_connect() will be called if connection succeeds or if it fails because the username/password is wrong or a unavailable protocol version (e.g. requesting MQTTv5 from a broker that only supports v3)