Stratos Ion Stratos Ion - 5 days ago 5
Python Question

Python threads are not closing after join

I am using a similar code as bellow to control an rgb led strip, random values come in every few seconds and 3 threads are being created to control the 3 individual colours.

import time
from ast import literal_eval
import threading
from random import randint
t=[]
myR =0
myG =0
myB =0
temp = 0
pins = [17,22,24]
myColours = [myR,myG,myB]
red_pin = 17
green_pin = 22
blue_pin = 24

def change_led_color(rgb):
global myR
global myG
global myB
y = 0
threads = []
for colour in rgb:
t = threading.Thread(target=leds,name=pins[y] ,args=(y,myColours[y],colour,pins[y]))
threads.append(t)
y += 1
for y in threads:
y.start()
for y in threads:
y.join()

def leds(index,start,end,pin):
temp=0
for i in range(start,end):
time.sleep(0.01)
temp = i
global myColours
print 'pin', pin, 'started at: ',start,' ended is: ', temp
myColours[index] = end

def set_colours():
print '..................................................................',t
print threading.activeCount(),'threads \n'
threading.Timer(2, set_colours).start()
change_led_color(t)
set_colours()

def get_data():
while True:
global t
t = (randint(0,255),randint(0,255),randint(0,255))
time.sleep(2)
threading.Thread(target=get_data).start()


The above runs fine but the responses are very weird, instead of getting all the three colours at the end of the threads, I sometimes get more than expected and most of the times at least one will be 0, like the thread never run!
I am assuming I'm misusing the threading in some way..

e.g. outcome

.................................................................. (187, 223, 42)
3 threads

pin 24 started at: 205 ended is: 0
pin 22 started at: 170 ended is: 222
pin 17 started at: 107 ended is: 186
.................................................................. (202, 115, 219)
3 threads

pin 22 started at: 223 ended is: 0
pin 17 started at: 187 ended is: 201
.................................................................. (244, 35, 194)
5 threads

pin 22 started at: 115 ended is: 0
pin 24 started at: 42 ended is: 218
pin 17 started at: 202 ended is: 243
pin 24 started at: 42 ended is: 193
.................................................................. (54, 25, 72)
3 threads

pin 17 started at: 244 ended is: 0
pin 22 started at: 35 ended is: 0
pin 24 started at: 194 ended is: 0

Answer

Have you considered GIL (Global Interpreter lock)? https://wiki.python.org/moin/GlobalInterpreterLock

Python has a GIL which doesn't allow multiple threads to run from a single process. When you run your code, it is run as a python process. You are trying to launch threads from same process which is not permitted in python.

If you want to perform some operations in parallel, multiprocessing package is a good option.
https://pymotw.com/2/multiprocessing/basics.html

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