Python threads exit with ctrl-c in Python

Question

I am having the Python Multi-threaded program as below. If I press ctrl+c within 5 seconds (approx), It is going inside the KeyboardInterrupt exception.

Running the code longer than 15 seconds failed to respond to ctrl+c. If I press ctrl+c after 15 seconds, It is not working. It is not throwing KeyboardInterrupt exception. What could be the reason ? I tested this on Linux.

#!/usr/bin/python

import os, sys, threading, time

class Worker(threading.Thread):
  def __init__(self):
    threading.Thread.__init__(self)
    # A flag to notify the thread that it should finish up and exit
    self.kill_received = False

  def run(self):
      while not self.kill_received:
          self.do_something()

  def do_something(self):
      [i*i for i in range(10000)]
      time.sleep(1)

def main(args):

    threads = []
    for i in range(10):
        t = Worker()
        threads.append(t)
        t.start()

    while len(threads) > 0:
        try:
            # Join all threads using a timeout so it doesn't block
            # Filter out threads which have been joined or are None
            threads = [t.join(1) for t in threads if t is not None and t.isAlive()]
        except KeyboardInterrupt:
            print "Ctrl-c received! Sending kill to threads..."
            for t in threads:
                t.kill_received = True

if __name__ == '__main__':
  main(sys.argv)

Antwane · Accepted Answer · 2017-02-15 11:34:28Z

After the first execution of

threads = [t.join(1) for t in threads if t is not None and t.isAlive()]

your variable threads contains

[None, None, None, None, None, None, None, None, None, None]

after the second execution, the same variable threads contains:

[]

At this point, len(threads) > 0 is False and you get out of the while loop. Your script is still running since you have 10 threads still active, but since you're not anymore in your try / except block (to catch KeyboardInterrupt), you can't stop using Ctrl + C

Add some prints to your script to see what I described:

#!/usr/bin/python

import os, sys, threading, time

class Worker(threading.Thread):
  def __init__(self):
    threading.Thread.__init__(self)
    # A flag to notify the thread that it should finish up and exit
    self.kill_received = False

  def run(self):
      while not self.kill_received:
          self.do_something()

  def do_something(self):
      [i*i for i in range(10000)]
      time.sleep(1)

def main(args):

    threads = []
    for i in range(10):
        t = Worker()
        threads.append(t)
        t.start()
        print('thread {} started'.format(i))

    while len(threads) > 0:
        print('Before joining')
        try:
            # Join all threads using a timeout so it doesn't block
            # Filter out threads which have been joined or are None
            threads = [t.join(1) for t in threads if t is not None and t.isAlive()]
            print('After join() on threads: threads={}'.format(threads))

        except KeyboardInterrupt:
            print("Ctrl-c received! Sending kill to threads...")
            for t in threads:
                t.kill_received = True
    print('main() execution is now finished...')

if __name__ == '__main__':
  main(sys.argv)

And the result:

$ python thread_test.py
thread 0 started
thread 1 started
thread 2 started
thread 3 started
thread 4 started
thread 5 started
thread 6 started
thread 7 started
thread 8 started
thread 9 started
Before joining
After join() on threads: threads=[None, None, None, None, None, None, None, None, None, None]
Before joining
After join() on threads: threads=[]
main() execution is now finished...

Actually, Ctrl + C doesn't stop to work after 15 seconds, but after 10 or 11 seconds. This is the time needed to create and start the 10 threads (less than a second) and to execute join(1) on each thread (about 10 seconds).

Hint from the doc:

As join() always returns None, you must call isAlive() after join() to decide whether a timeout happened – if the thread is still alive, the join() call timed out.

Gernot T2P · Accepted Answer · 2021-08-25 20:50:03Z

0

to follow up on the poster above, isAlive() got renamed to is_alive() tried on Python 3.9.6

full code:

#!/usr/bin/python

import os, sys, threading, time

class Worker(threading.Thread):
  def __init__(self):
    threading.Thread.__init__(self)
    # A flag to notify the thread that it should finish up and exit
    self.kill_received = False

  def run(self):
      while not self.kill_received:
          self.do_something()

  def do_something(self):
      [i*i for i in range(10000)]
      time.sleep(1)

def main(args):

    threads = []
    for i in range(10):
        t = Worker()
        threads.append(t)
        t.start()
        print('thread {} started'.format(i))

    while len(threads) > 0:
        print('Before joining')
        try:
            # Join all threads using a timeout so it doesn't block
            # Filter out threads which have been joined or are None
            threads = [t.join(1) for t in threads if t is not None and t.is_alive()]
            print('After join() on threads: threads={}'.format(threads))

        except KeyboardInterrupt:
            print("Ctrl-c received! Sending kill to threads...")
            for t in threads:
                t.kill_received = True
    print('main() execution is now finished...')

if __name__ == '__main__':
  main(sys.argv)

answered Aug 25, 2021 at 20:50

Gernot T2P

1

1 Comment

Trenton McKinney Over a year ago

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