Problem with multiprocessing python 2.6

Question

I'm trying to build a simple program that will fire off a load of processes, and if the main process is killed, the sub processes will die. My code looks like this:

import time
def test_proc(name, conn):
    x = 0
    while True:
        print x
        x += 1
        conn.poll()



from multiprocessing import Process, Pipe

proc_name= ['a', 'b', 'c']
procs = []
for p in proc_name:
    parent_conn, child_conn = Pipe()
    p = Process(target=test_proc, args=(p, child_conn))
    procs.append(p)
    p.start()

while True:
    print [(p.is_alive(), 'Pid %s' %(p.pid)) for p in procs]
    time.sleep(1)

It works, but if I remove the print x on line 5 it doesn't. The processes will continue to run, why?

Also, I'd love to know if this is the right way of doing what I'm trying to achieve.

Answer 1

This works fine for me in Ubuntu:

>>> from time import sleep
>>> from multiprocessing import Process, Pipe
>>> 
>>> def test_proc(name, conn):
...     x = 0
...     while True:
...             #print x
...             x += 1
...             conn.poll()
... 
>>> def main():
...     proc_name= ['a', 'b', 'c']
...     procs = [Process(target=test_proc, args=Pipe()) for p in proc_name]
...     for p in procs:
...             p.start()
...     while True:
...             print [(p.is_alive(), 'Pid %s' %(p.pid)) for p in procs]
...             sleep(1)
... 
>>> main()
[(True, 'Pid 423'), (True, 'Pid 424'), (True, 'Pid 425')]
[(True, 'Pid 423'), (True, 'Pid 424'), (True, 'Pid 425')]
[(True, 'Pid 423'), (True, 'Pid 424'), (True, 'Pid 425')]
[(True, 'Pid 423'), (True, 'Pid 424'), (True, 'Pid 425')]
...

Are you using Windows, maybe? There are programming guidelines that relate to using multiprocessing with Windows. In particular, you need to provide an entry point by using if __name__ == '__main__': .

---

Later: actually, there is something I don't get. In your original code, you expected to kill the parent of the threads and have the threads keep running. How were you killing the parent -- main() in my code? And if the threads were performing no I/O, how did you know that the threads were still alive?

---

And later still: When I run the threads, I get this:

>>> main()
[(True, 'Pid 940'), (True, 'Pid 941'), (True, 'Pid 942')]
[(True, 'Pid 940'), (True, 'Pid 941'), (True, 'Pid 942')]
[(True, 'Pid 940'), (True, 'Pid 941'), (True, 'Pid 942')]
[(True, 'Pid 940'), (True, 'Pid 941'), (True, 'Pid 942')]
[(True, 'Pid 940'), (True, 'Pid 941'), (True, 'Pid 942')]

and this:

  PID TTY          TIME CMD
  911 pts/6    00:00:00 python
  940 pts/6    00:00:29 python
  941 pts/6    00:00:29 python
  942 pts/6    00:00:37 python
  944 pts/5    00:00:00 ps

And when I kill the main thread in python (Ctrl-C), I get this:

  PID TTY          TIME CMD
  911 pts/6    00:00:00 python
  940 pts/6    00:00:42 python <defunct>
  941 pts/6    00:00:50 python <defunct>
  942 pts/6    00:00:51 python <defunct>
  946 pts/5    00:00:00 ps

Is this unexpected or undesirable?