http://www.dingostick.com/2019/07/internet-of-things-air-conditioning.html

check_temp.py

# maximum allowable delta (°F) before we start the fans
TEMP_DELTA = 5.0

def check_temp():
    global VENTILATION_STATE
    c = db.cursor()

    # get the last ten minutes of data only
    c.execute("SELECT temp, room FROM temps WHERE (julianday('now') - julianday(timestamp))*86400 < 600 ORDER BY timestamp")
    inside_temps, outside = [], 0.0
    for temp, room in c.fetchall():
        if room == 'outside':
            outside = temp
        elif room == 'inside':
            inside_temps.append(temp)

    # average all inside temps seen
    inside = sum(inside_temps)/len(inside_temps)

    if inside - TEMP_DELTA > outside:
        # it's hot inside. let's cool it off
        if VENTILATION_STATE != 'on':
            print("Cooler outside (%f) than inside (%f). Turning on ventilation" % (outside, inside))
            VENTILATION_STATE == 'on'
            mqtt_client.publish('ventilate', payload='on')
    elif inside <= outside:
        # we're already cooler than outside
        if VENTILATION_STATE == 'on':
            # turn off ventilation
            print("Hotter outside (%f) than inside (%f). Turning off ventilation" % (outside, inside))
            VENTILATION_STATE == 'off'
            mqtt_client.publish('ventilate', payload='off')
    else:
        print("warmer outside, but not terrible")