Home Assistant on Raspberry Pi 4 with Docker and MQTT Auto-Discovery

#!/usr/bin/env python3
# pi_mqtt_sensors.py - expose Raspberry Pi metrics to Home Assistant over MQTT
import json
import os
import socket
import time

import paho.mqtt.client as mqtt

BROKER, PORT = 'localhost', 1883
USER, PASSWORD = 'mqttuser', 'change-me'     # the mosquitto_passwd user
HOST = socket.gethostname()                   # e.g. pi4lab
STATE_TOPIC = f'{HOST}/state'
AVAIL_TOPIC = f'{HOST}/availability'

# Device block groups all entities under one device in Home Assistant
DEVICE = {'identifiers': [HOST], 'name': f'{HOST} Raspberry Pi 4',
          'manufacturer': 'Raspberry Pi Ltd', 'model': 'Raspberry Pi 4 Model B'}

SENSORS = {
    'cpu_temp': {'name': 'CPU temperature', 'unit_of_measurement': '\u00b0C',
                 'device_class': 'temperature', 'state_class': 'measurement'},
    'load1':    {'name': 'Load average 1 min', 'state_class': 'measurement'},
}

def read_values():
    with open('/sys/class/thermal/thermal_zone0/temp') as f:
        temp = int(f.read()) / 1000
    return {'cpu_temp': round(temp, 1), 'load1': round(os.getloadavg()[0], 2)}

# paho-mqtt 2.x needs a callback API version; 1.x does not have this attribute
try:
    client = mqtt.Client(mqtt.CallbackAPIVersion.VERSION2)
except AttributeError:
    client = mqtt.Client()
client.username_pw_set(USER, PASSWORD)
# Last will: broker publishes 'offline' if this script dies unexpectedly
client.will_set(AVAIL_TOPIC, 'offline', retain=True)
client.connect(BROKER, PORT, keepalive=60)
client.loop_start()                            # network loop in a background thread

# Publish one retained discovery config per sensor
for key, extra in SENSORS.items():
    config = {
        'unique_id': f'{HOST}_{key}',
        'state_topic': STATE_TOPIC,
        'value_template': '{{ value_json.' + key + ' }}',
        'availability_topic': AVAIL_TOPIC,
        'device': DEVICE,
        **extra,
    }
    client.publish(f'homeassistant/sensor/{HOST}_{key}/config',
                   json.dumps(config), retain=True)

client.publish(AVAIL_TOPIC, 'online', retain=True)

try:
    while True:
        values = read_values()
        client.publish(STATE_TOPIC, json.dumps(values))
        print(values)
        time.sleep(30)
except KeyboardInterrupt:
    client.publish(AVAIL_TOPIC, 'offline', retain=True)
finally:
    client.loop_stop()
    client.disconnect()