Python Multiprocessing atexit Error “Error in atex

Question

I am trying to run a simple multiple processes application in Python. The main thread spawns 1 to N processes and waits until they all done processing. The processes each run an infinite loop, so they can potentially run forever without some user interruption, so I put in some code to handle a KeyboardInterrupt:

#!/usr/bin/env python
import sys
import time
from multiprocessing import Process

def main():
    # Set up inputs..

    # Spawn processes
    Proc( 1).start()
    Proc( 2).start()

class Proc ( Process ):
    def __init__ ( self, procNum):
        self.id = procNum
        Process.__init__(self)

    def run ( self ):
        doneWork = False

        while True:

            try:
                # Do work...
                time.sleep(1)
                sys.stdout.write('.')

                if doneWork:
                    print "PROC#" + str(self.id) + " Done."
                    break

            except KeyboardInterrupt:
                print "User aborted."
                sys.exit()

# Main Entry
if __name__=="__main__":
    main()

The problem is that when using CTRL-C to exit, I get an additional error even though the processes seem to exit immediately:

......User aborted.
Error in atexit._run_exitfuncs:
Traceback (most recent call last):
  File "C:\Python26\lib\atexit.py", line 24, in _run_exitfuncs
    func(*targs, **kargs)
  File "C:\Python26\lib\multiprocessing\util.py", line 281, in _exit_function
    p.join()
  File "C:\Python26\lib\multiprocessing\process.py", line 119, in join
    res = self._popen.wait(timeout)
  File "C:\Python26\lib\multiprocessing\forking.py", line 259, in wait
    res = _subprocess.WaitForSingleObject(int(self._handle), msecs)
KeyboardInterrupt
Error in sys.exitfunc:
Traceback (most recent call last):
  File "C:\Python26\lib\atexit.py", line 24, in _run_exitfuncs
    func(*targs, **kargs)
  File "C:\Python26\lib\multiprocessing\util.py", line 281, in _exit_function
    p.join()
  File "C:\Python26\lib\multiprocessing\process.py", line 119, in join
    res = self._popen.wait(timeout)
  File "C:\Python26\lib\multiprocessing\forking.py", line 259, in wait
    res = _subprocess.WaitForSingleObject(int(self._handle), msecs)
KeyboardInterrupt

I am running Python 2.6 on Windows. If there is a better way to handle multiprocessing in Python, please let me know.

Answer 1

Rather then just forcing sys.exit(), you want to send a signal to your threads to tell them to stop. Look into using signal handlers and threads in Python.

You could potentially do this by changing your while True: loop to be while keep_processing: where keep_processing is some sort of global variable that gets set on the KeyboardInterrupt exception. I don't think this is a good practice though.