How to use Queue with threading properly

December 27, 2021

I am new to queue & threads kindly help with the below code , here I am trying to execute the function hd , I need to run the function multiple times but only after a single run has been completed

import queue
import threading
import time

fifo_queue = queue.Queue()

def hd():
    print("hi")
    time.sleep(1)
    print("done")


for i in range(3):
    cc = threading.Thread(target=hd)
    fifo_queue.put(cc)
    cc.start()

Current Output

hi
hi
hi
donedonedone

Expected Output

hi
done   
hi
done
hi
done

>Solution :

You can use a Semaphore for your purposes

A semaphore manages an internal counter which is decremented by each acquire() call and incremented by each release() call. The counter can never go below zero; when acquire() finds that it is zero, it blocks, waiting until some other thread calls release().

A default value of Semaphore is 1,

class threading.Semaphore(value=1)

so only one thread would be active at once:

import queue
import threading
import time

fifo_queue = queue.Queue()

semaphore = threading.Semaphore()


def hd():
    with semaphore:
        print("hi")
        time.sleep(1)
        print("done")


for i in range(3):
    cc = threading.Thread(target=hd)
    fifo_queue.put(cc)
    cc.start()

hi
done
hi
done
hi
done

As @user2357112supportsMonica mentioned in comments RLock would be more safe option

class threading.RLock

This class implements reentrant lock objects. A reentrant lock must be released by the thread that acquired it. Once a thread has acquired a reentrant lock, the same thread may acquire it again without blocking; the thread must release it once for each time it has acquired it.

import queue
import threading
import time

fifo_queue = queue.Queue()

lock = threading.RLock()


def hd():
    with lock:
        print("hi")
        time.sleep(1)
        print("done")


for i in range(3):
    cc = threading.Thread(target=hd)
    fifo_queue.put(cc)
    cc.start()