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, Peter 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 network. 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 while. 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 sequence. * 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 bytes: >>> print(b'\x00\x00hello\xff\x68\x65\x6c\x6c\x6f') b'\x00\x00hello\xffhello' 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 bytes. 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! Patrick
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