How to check if subprocess has finished?

Question

I am starting a process by doing:

for i in range(1, processes + 1):
            hup = MyURLParser() //A class I made
            p = Process(target = hup.run)
            p.start()

After that, so that the main thread doesn't exit immediately I do:

 while (True):
        print("Main sleeping...")
        sleep(20)

Instead of doing this, how can I check that each of the child processes are still running from the main thread ? Then instead of having the infinite loop I can break out of the loop at the right time and do something else....

score 5 · Answer 1 · answered Jan 24 '18 at 11:33

Add all processes to a list, and join() each of them in turn:

processes = []
for i in range(1, processes + 1):
    hup = MyURLParser() //A class I made
    p = Process(target = hup.run)
    p.start()
    processes.append(p)

for p in processes:
    p.join()

The join() call blocks until the process is finished. It doesn't matter in which order you call them; calling join() on a process that's already complete will just return immediately.

You may also want to check out the other functionality in the multiprocessing module; in particular the Pool class might help you simplify this code.

grovina · Accepted Answer · 2018-01-24T12:44:05.983

4

p.is_alive() tells you whether the process is running.

To wait until it's ended, use p.join().

edited Jan 24 '18 at 12:44

answered Jan 24 '18 at 11:35

grovina

2,999
19
25

AndreyF · Answer 3 · 2018-01-24T11:45:09.340

1

you can use the join method (read more in docs):

First, keep the Process objects you create:

your_processes = []
...
   p.start()
   your_processes.append(p)

After starting all your processes, use join to wait each one of them to finish executing.:

for p in your_processes:
    p.join()
... #the rest of your code

Basically join makes the main process wait for process p to finish and only then to proceed to the next line of code.

edited Jan 24 '18 at 11:45

answered Jan 24 '18 at 11:33

AndreyF

1,798
1
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25

This approach won't work because processes do not share local or global variables. – Stephen Mar 21 '20 at 17:19
See here for more details: – Stephen Mar 21 '20 at 17:20
https://stackoverflow.com/questions/42490368/pythonappending-to-the-same-list-from-different-processes-using-multiprocessing – Stephen Mar 21 '20 at 17:20

klutt · Answer 4 · 2018-01-24T11:59:33.843

1

You can do like this:

P = []
for i in range(1, processes + 1):
            hup = MyURLParser() //A class I made
            p = Process(target = hup.run)
            p.start()
            P.append(p)

for p in P:
            p.join()

To only check if the process is alive, use .is_alive()

Read more here: https://docs.python.org/3/library/multiprocessing.html

edited Jan 24 '18 at 11:59

answered Jan 24 '18 at 11:34

klutt

30,332
17
55
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How to check if subprocess has finished?

4 Answers4