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MQTT from Python

Scope: a complete miniature system in Python — one script that publishes sensor-style readings, one that subscribes and reacts — run for real against the guide's broker. Verified with paho-mqtt 2.1.0 on Python 3.12.

Setup

The client library is paho-mqtt, the Eclipse project's Python client. Keep it in a virtual environment so it never tangles with system packages:

python3 -m venv mqtt-env
source mqtt-env/bin/activate
pip install paho-mqtt

paho-mqtt version: 2.1.0

The publisher

examples/sensor_publisher.py stands in for a sensor. It connects, then publishes five readings, two seconds apart, to home/kitchen/temperature — the same topic the wildcard chapter played with. The readings step upward on purpose, so the watcher below gets one hot enough to react to; in a real setup a sensor read would replace the list, and nothing else in the script would change.

The shape to notice: build a client, set an on_connect callback, connect(), loop_start() (that runs the network in a background thread), then publish() whenever you please.

python3 sensor_publisher.py

Connected to broker.
Published 21.5 to home/kitchen/temperature
Published 22.3 to home/kitchen/temperature
Published 23.1 to home/kitchen/temperature
Published 23.9 to home/kitchen/temperature
Published 24.7 to home/kitchen/temperature
Done.

The watcher

examples/temperature_watcher.py subscribes to the same topic and prints each reading; above 24 °C it also prints the reaction — the spot where a real setup would open a vent, start a fan, or send a notification. It subscribes inside on_connect, which matters: if the connection ever drops and reconnects, the subscription is re-made automatically. loop_forever() then just sits in the message stream until Ctrl+C.

Start it in its own terminal, then run the publisher:

python3 temperature_watcher.py

Connected. Subscribing to home/kitchen/temperature
home/kitchen/temperature: 21.5 °C
home/kitchen/temperature: 22.3 °C
home/kitchen/temperature: 23.1 °C
home/kitchen/temperature: 23.9 °C
home/kitchen/temperature: 24.7 °C
  -> Too hot (over 24 °C): this is where you'd open the vent or send an alert.

Two terminals, two small programs, no server code, no polling: the sensor publishes when it has something, the watcher is told the instant it lands. That is the whole pattern from What MQTT Is, wearing code.

Notes from the lab

  • Payloads arrive as bytes. The watcher does float(msg.payload.decode()) — MQTT payloads are untyped, so converting is the receiver's job.
  • Buffering surprise: when the watcher's output was redirected to a file in the lab, its prints vanished when the program was stopped — Python buffers output that isn't going to a terminal. In a terminal (as above) it's line-buffered and fine; when scripting output to files, use python3 -u for unbuffered output.
  • The paho 2.x callback signatures (with CallbackAPIVersion.VERSION2) differ from the 1.x examples all over the internet — if a snippet from a forum throws callback errors, that version drift is the first suspect.