Patrick, I am most grateful for your comments. You obviously know a lot more 
about MQTT than I.
I think that I'll have a little play with the software that I have working, and 
then start cleaning it up using some of your suggestions.  And perhaps read a 
bit more of the MQTT v5 spec.

Much thanks,

On 27/08/2021 12:23, Patrick Wigmore wrote:
Hmm. I don't think I'm able to solve this.

Referring to the full MQTT subscriber/car control program you sent me
off-list, this is some of the context I was missing:

It is using paho-mqtt, f.k.a. Mosquitto, and message is a MQTTMessage
object, with message.payload being arbitrary bytes. It looks like this
payload is indeed the unadulterated MQTT payload, as sent over the

So, the payload is bytes, rather than a text string, and there isn't
any intervening code that's not shown and does additional encoding of
the payload.

on_message is run by paho-mqtt as an on_message callback function when
an MQTT PUBLISH message is received in a subscribed topic, which I
believe is what you intend.

The publisher is using the Arduino MQTT Client, which wraps lwmqtt.

This much is probably obvious if you've been immersed in it for a

So, regardless of the payload contents:

* Given that message.payload is a Python Bytes object, which it should
   be, print(message.payload) should always work. Bytes that don't
   encode printable characters print as hexadecimal within an escape

* Testing whether the payload equals some specific bytes should at
   least not cause a crash, even if the test doesn't evaluate to True/
   False as expected.

* The if/elif/else structure looks fine.

So, unless I'm missing a syntax error or typo somewhere, I don't think
there's anything wrong with your non-working on_message!

Just to confirm to myself how Python's print handles unprintable
     >>> print(b'\x00\x00hello\xff\x68\x65\x6c\x6c\x6f')

I'll assume that all the functions that do the GPIO (e.g. motorleft)
are correct. That could be a wrong assumption, but I'll assume it
anyway because it seems likely.

Given that you were interpreting the same bytes (b'Pot-left' and
b'Pot-rt') successfully before, all I can think is that the code at
the publishing end must have changed too, to get it to send different

Since the subscriber crashes, there must be something about the way
the publishing code was changed that causes it to send messages that
paho-mqtt can't handle.

Because paho-mqtt shouldn't care about the contents of the payload,
this would seem to suggest that there is a mistake in the way the MQTT
packets are being constructed at the publisher.

I'm having a hard time imagining that you would have intentionally
significantly altered the code at the publishing end, though, so it is
probably something quite subtle; perhaps an issue that was already
there but is only tripped up by something else that changed.

But I feel like I'm probably missing something obvious here. Normally
when I try to make these kinds of deductions on the list, someone
comes along and points out a glaring error in my logic, so no doubt I
am making a similar error right now!


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