Python3 Let threads finish when keyboardexception raised

Question

I'm looking for a way to let the threads finish their work and not take a new job if CTRL-C is raised. Theorical code:

import threading
from Queue import Queue
q = Queue(maxsize=0)
def work(q):
    print (line)
for line in file_of_100_line:
    q.put(line)
nbThreads = 2
for i in range(nbThreads):
    t = threading.Thread(target=work, args=(q))
    t.setDaemon(True)
    t.start()

Let the workers print the line if keyboard exception is raised (CTRL-C), and when all workers are done with their current task, quit.

Answer 1

The following should do what you want:

import signal
import threading
from time import sleep

keep_running = True

def signal_handler(signal, frame):
    global keep_running
    keep_running = False
    print("Received SIGINT\n")

signal.signal(signal.SIGINT,signal_handler)

def work():
    while keep_running:
        sleep(2)
    print("Worker finished\n")

nbThreads = 2
threads = []
for i in range(nbThreads):
    t = threading.Thread(target=work)
    t.setDaemon(True)
    t.start()
    threads.append(t)

while any([t.isAlive() for t in threads]):
    sleep(0.1)

Letting this program run yields

Received SIGINT

Worker finished
Worker finished

Explanation: When CTRL-C is pressed the program receives a signal.SIGINT which causes the registered handler signal_handler to be called setting the global variable keep_running to False. Meanwhile the main thread waits for all spawned threads to die. Thus these threads can finish whatever they were doing (in this case sleep-ing) before the main thread terminates.

Note: On Linux waiting for a signal could be achieved by calling signal.pause() (replacing the sleep(0.1)). On Windows this is not possible because this function does not exist. Therefore the above should be less platform-dependant.