Python multiprocessing queue not sending data to parent process

Question

So I've got a bluetooth connection from an arduino reading a joystick and sending the axis readout over bluetooth to my raspberry pi (4b running ubuntu 20.10). I've confirmed it's receiving this data too.

Now I try to run this bluetooth communcication in a separate process using the python multiprocessing module. to access the data from the arduino, I give the function a queue from the parent main process to put the data in there. Then in the main function I continuously try to read from this queue and process the data there.

The queue in the parent process always remains empty, however, and as such I can't process the data any further.

How can I get the data from the bluetooth process back to the main process?

main.py
#!/usr/bin/env python3
import time
import logging
import multiprocessing as mp
import bluetoothlib

logging.basicConfig(level=logging.DEBUG)

logging.info("creating queue")
global q
q = mp.Queue()      

def main():
    try:
        logging.info("starting bluetooth process")
        p = mp.Process(target=bluetoothlib.serlistener, args=(q,))
        p.start()
    except:
        logging.error("unable to start bluetooth listener")
        
    logging.info("start reading from queue")
    while True:
        #logging.info(q.qsize())
        if not q.empty():
            mss = q.get()
            logging.info(mss)
            #do something with data
        elif q.empty():
            logging.info("queue empty")
            
        time.sleep(1)
            

main()

bluetoothlib.py
#!/usr/bin/env python3
import os
import serial
import io

def serlistener(q):
    print ("creating connection")
    btConn = serial.Serial("/dev/rfcomm0", 57600, timeout=1)
    btConn.flushInput()
    sio = io.TextIOWrapper(io.BufferedRWPair(btConn, btConn, 1),encoding="utf-8")
    
    print ("connection created, starting listening")
    while btConn.is_open:
        try:
            mss = sio.readline()
            q.put(mss)
        except:
            print("error")                                                                              
            break

Why are you using 'global queue';then passing the object q to the serlistener function; and then calling 'queue' for get and put? It seems like you are calling global queue in the main thread; but expected messages in the q object. — thelizardking34
– thelizardking34, Commented Apr 30, 2021 at 15:54
@thelizardking34 oh whoops forgot to fix that here, I did fix it in the actual program without any success, I'll fix it in the question now — Alexander de Haan
– Alexander de Haan, Commented Apr 30, 2021 at 15:57
@thelizardking34 yep it did do that at least. now with the global things fixed, it works! Thanks for your help! — Alexander de Haan
– Alexander de Haan, Commented Apr 30, 2021 at 16:21

Alexander de Haan · Accepted Answer · 2021-04-30 16:19:56Z

1

At thelizardking34's suggestion, I relooked at the global stuff I'd been messing with and after correcting it, the code as now given in the question works.

Thanks to thelizardking34!

answered Apr 30, 2021 at 16:19

Alexander de Haan

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1 Comment

thelizardking34 Over a year ago

It might be worth adding your global declaration back into post with a comment on what was causing the issue, for future posterity :D

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