How can I handle KeyboardInterrupt events with python’s multiprocessing Pools? Here is a simple example:

from multiprocessing import Pool
from time import sleep
from sys import exit

def slowly_square(i):
    sleep(1)
    return i*i

def go():
    pool = Pool(8)
    try:
        results = pool.map(slowly_square, range(40))
    except KeyboardInterrupt:
        # **** THIS PART NEVER EXECUTES. ****
        pool.terminate()
        print "You cancelled the program!"
        sys.exit(1)
    print "\nFinally, here are the results: ", results

if __name__ == "__main__":
    go()

When running the code above, the KeyboardInterrupt gets raised when I press ^C, but the process simply hangs at that point and I have to kill it externally.

I want to be able to press ^C at any time and cause all of the processes to exit gracefully.

This is a Python bug. When waiting for a condition in threading.Condition.wait(), KeyboardInterrupt is never sent. Repro:

import threading
cond = threading.Condition(threading.Lock())
cond.acquire()
cond.wait(None)
print "done"

The KeyboardInterrupt exception won’t be delivered until wait() returns, and it never returns, so the interrupt never happens. KeyboardInterrupt should almost certainly interrupt a condition wait.

Note that this doesn’t happen if a timeout is specified; cond.wait(1) will receive the interrupt immediately. So, a workaround is to specify a timeout. To do that, replace

    results = pool.map(slowly_square, range(40))

with

    results = pool.map_async(slowly_square, range(40)).get(9999999)

or similar.

From what I have recently found, the best solution is to set up the worker processes to ignore SIGINT altogether, and confine all the cleanup code to the parent process. This fixes the problem for both idle and busy worker processes, and requires no error handling code in your child processes.

import signal

...

def init_worker():
    signal.signal(signal.SIGINT, signal.SIG_IGN)

...

def main()
    pool = multiprocessing.Pool(size, init_worker)

    ...

    except KeyboardInterrupt:
        pool.terminate()
        pool.join()

Explanation and full example code can be found at http://noswap.com/blog/python-multiprocessing-keyboardinterrupt/ and http://github.com/jreese/multiprocessing-keyboardinterrupt respectively.

For some reasons, only exceptions inherited from the base Exception class are handled normally. As a workaround, you may re-raise your KeyboardInterrupt as an Exception instance:

from multiprocessing import Pool
import time

class KeyboardInterruptError(Exception): pass

def f(x):
    try:
        time.sleep(x)
        return x
    except KeyboardInterrupt:
        raise KeyboardInterruptError()

def main():
    p = Pool(processes=4)
    try:
        print 'starting the pool map'
        print p.map(f, range(10))
        p.close()
        print 'pool map complete'
    except KeyboardInterrupt:
        print 'got ^C while pool mapping, terminating the pool'
        p.terminate()
        print 'pool is terminated'
    except Exception, e:
        print 'got exception: %r, terminating the pool' % (e,)
        p.terminate()
        print 'pool is terminated'
    finally:
        print 'joining pool processes'
        p.join()
        print 'join complete'
    print 'the end'

if __name__ == '__main__':
    main()

Normally you would get the following output:

staring the pool map
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
pool map complete
joining pool processes
join complete
the end

So if you hit ^C, you will get:

staring the pool map
got ^C while pool mapping, terminating the pool
pool is terminated
joining pool processes
join complete
the end

Usually this simple structure works for Ctrl–C on Pool :

def signal_handle(_signal, frame):
    print "Stopping the Jobs."

signal.signal(signal.SIGINT, signal_handle)

As was stated in few similar posts:

Capture keyboardinterrupt in Python without try-except

The voted answer does not tackle the core issue but a similar side effect.

Jesse Noller, the author of the multiprocessing library, explains how to correctly deal with CTRL+C when using multiprocessing.Pool in a old blog post.

import signal
from multiprocessing import Pool


def initializer():
    """Ignore CTRL+C in the worker process."""
    signal.signal(signal.SIGINT, signal.SIG_IGN)


pool = Pool(initializer=initializer)

try:
    pool.map(perform_download, dowloads)
except KeyboardInterrupt:
    pool.terminate()
    pool.join()

It seems there are two issues that make exceptions while multiprocessing annoying. The first (noted by Glenn) is that you need to use map_async with a timeout instead of map in order to get an immediate response (i.e., don’t finish processing the entire list). The second (noted by Andrey) is that multiprocessing doesn’t catch exceptions that don’t inherit from Exception (e.g., SystemExit). So here’s my solution that deals with both of these:

import sys
import functools
import traceback
import multiprocessing

def _poolFunctionWrapper(function, arg):
    """Run function under the pool

    Wrapper around function to catch exceptions that don't inherit from
    Exception (which aren't caught by multiprocessing, so that you end
    up hitting the timeout).
    """
    try:
        return function(arg)
    except:
        cls, exc, tb = sys.exc_info()
        if issubclass(cls, Exception):
            raise # No worries
        # Need to wrap the exception with something multiprocessing will recognise
        import traceback
        print "Unhandled exception %s (%s):\n%s" % (cls.__name__, exc, traceback.format_exc())
        raise Exception("Unhandled exception: %s (%s)" % (cls.__name__, exc))

def _runPool(pool, timeout, function, iterable):
    """Run the pool

    Wrapper around pool.map_async, to handle timeout.  This is required so as to
    trigger an immediate interrupt on the KeyboardInterrupt (Ctrl-C); see
    http://stackoverflow.com/questions/1408356/keyboard-interrupts-with-pythons-multiprocessing-pool

    Further wraps the function in _poolFunctionWrapper to catch exceptions
    that don't inherit from Exception.
    """
    return pool.map_async(functools.partial(_poolFunctionWrapper, function), iterable).get(timeout)

def myMap(function, iterable, numProcesses=1, timeout=9999):
    """Run the function on the iterable, optionally with multiprocessing"""
    if numProcesses > 1:
        pool = multiprocessing.Pool(processes=numProcesses, maxtasksperchild=1)
        mapFunc = functools.partial(_runPool, pool, timeout)
    else:
        pool = None
        mapFunc = map
    results = mapFunc(function, iterable)
    if pool is not None:
        pool.close()
        pool.join()
    return results

I found, for the time being, the best solution is to not use the multiprocessing.pool feature but rather roll your own pool functionality. I provided an example demonstrating the error with apply_async as well as an example showing how to avoid using the pool functionality altogether.

http://www.bryceboe.com/2010/08/26/python-multiprocessing-and-keyboardinterrupt/

I’m a newbie in Python. I was looking everywhere for answer and stumble upon this and a few other blogs and youtube videos. I have tried to copy paste the author’s code above and reproduce it on my python 2.7.13 in windows 7 64- bit. It’s close to what I wanna achieve.

I made my child processes to ignore the ControlC and make the parent process terminate. Looks like bypassing the child process does avoid this problem for me.

#!/usr/bin/python

from multiprocessing import Pool
from time import sleep
from sys import exit


def slowly_square(i):
    try:
        print "<slowly_square> Sleeping and later running a square calculation..."
        sleep(1)
        return i * i
    except KeyboardInterrupt:
        print "<child processor> Don't care if you say CtrlC"
        pass


def go():
    pool = Pool(8)

    try:
        results = pool.map(slowly_square, range(40))
    except KeyboardInterrupt:
        pool.terminate()
        pool.close()
        print "You cancelled the program!"
        exit(1)
    print "Finally, here are the results", results


if __name__ == '__main__':
    go()

The part starting at pool.terminate() never seems to execute.

You can try using the apply_async method of a Pool object, like this:

import multiprocessing
import time
from datetime import datetime


def test_func(x):
    time.sleep(2)
    return x**2


def apply_multiprocessing(input_list, input_function):
    pool_size = 5
    pool = multiprocessing.Pool(processes=pool_size, maxtasksperchild=10)

    try:
        jobs = {}
        for value in input_list:
            jobs[value] = pool.apply_async(input_function, [value])

        results = {}
        for value, result in jobs.items():
            try:
                results[value] = result.get()
            except KeyboardInterrupt:
                print "Interrupted by user"
                pool.terminate()
                break
            except Exception as e:
                results[value] = e
        return results
    except Exception:
        raise
    finally:
        pool.close()
        pool.join()


if __name__ == "__main__":
    iterations = range(100)
    t0 = datetime.now()
    results1 = apply_multiprocessing(iterations, test_func)
    t1 = datetime.now()
    print results1
    print "Multi: {}".format(t1 - t0)

    t2 = datetime.now()
    results2 = {i: test_func(i) for i in iterations}
    t3 = datetime.now()
    print results2
    print "Non-multi: {}".format(t3 - t2)

Output:

100
Multiprocessing run time: 0:00:41.131000
100
Non-multiprocessing run time: 0:03:20.688000

An advantage of this method is that results processed before interruption will be returned in the results dictionary:

>>> apply_multiprocessing(range(100), test_func)
Interrupted by user
{0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

Strangely enough it looks like you have to handle the KeyboardInterrupt in the children as well. I would have expected this to work as written… try changing slowly_square to:

def slowly_square(i):
    try:
        sleep(1)
        return i * i
    except KeyboardInterrupt:
        print 'You EVIL bastard!'
        return 0

That should work as you expected.