helper script to normalize and upload sensor readings from an arduino to thingspeak's MQTT broker

weatherStations.py

#!/usr/bin/python2.7
# normalize and publish weather sensor readings to thingspeak

from time import sleep
import datetime
import serial # install pySerial
import paho.mqtt.client as mqtt
import sys

SERIAL_PORT = sys.argv[2] if len(sys.argv) > 2 else '/dev/ttyUSB1'
MQTT_HOST = 'mqtt.thingspeak.com'
MQTT_TOPIC = 'channels/305027/publish/{}'.format(sys.argv[1])
SENDING_WINDOW = 15 #average how many messages before sending?

ser = serial.Serial(SERIAL_PORT, 115200) # Establish the connection on a specific port

# Set the PUBLISH messages to have a QoS value of 0.
# Set the connection RETAIN flag to 0.
# Set the connection CleanSession flag to 1.
client = mqtt.Client()
client.connect(MQTT_HOST)


def createPayload(fieldValues):
     payload = []
     for idx, val in enumerate(fieldValues):
          payload.append("field{}={}".format(idx+1, val))
     # we are assuming that there is no delay between reading from serial and creating the payload
     payload = '&'.join(payload) + "&created_at={}&timezone=America/Chicago".format(datetime.datetime.now().isoformat(), )
     return payload


def readNextValues():
     line = ser.readline() # QUESTION does this block untill a new line is available?
     rawValues = line.split(',')
     rawValues[-1] = rawValues[-1].replace('\r\n','')
     values = []
     for value in rawValues:
          values.append(float(value))
     print(values)
     return values

# averages an accumulated array of sensor values.
def averageReadings(valuesArr):
     averageSensorValues = [0] * len(valuesArr[0]) # init with 0
     for x in range(0,SENDING_WINDOW):
          for y in range(0,len(averageSensorValues)):
               averageSensorValues[y] += valuesArr[x][y]
     for x in range(0,len(averageSensorValues)):
          averageSensorValues[x] /= SENDING_WINDOW
          averageSensorValues[x] = round(averageSensorValues[x],2)
     return averageSensorValues

client.loop_start() # starts a spawns a background thread to keep the connection up 
while True:
     gatheredValues = []
     while len(gatheredValues) < SENDING_WINDOW:
          try: # guard against the unexpected
               gatheredValues.append(readNextValues())
          except Exception as e:
               print(e)
               continue # to next iteration

     # calculate the average of these readings.
     avgSensorValues = averageReadings(gatheredValues)
     # send it to server
     print('average is', avgSensorValues)
     client.publish(MQTT_TOPIC, createPayload(avgSensorValues))