Python subprocess: callback when cmd exits

Question

Hi,

I'm currently launching a programme using subprocess.Popen(cmd, shell=TRUE)

I'm fairly new to Python, but it 'feels' like there ought to be some api that lets me do something similar to:

subprocess.Popen(cmd, shell=TRUE,  postexec_fn=function_to_call_on_exit)

I am doing this so that function_to_call_on_exit can do something based on knowing that the cmd has exited (for example keeping count of the number of external processes currently running)

I assume that I could fairly trivially wrap subprocess in a class that combined threading with the Popen.wait() method, but as I've not done threading in Python yet and it seems like this might be common enough for an API to exist, I thought I'd try and find one first.

Thanks in advance :)

Answer 1

AFAIK there is no such API, at least not in subprocess module. You need to roll something on your own, possibly using threads.

Answer 2

multiprocessing?

Answer 3

You're right - there is no nice API for this. You're also right on your second point - it's trivially easy to design a function that does this for you using threading.

import threading
import subprocess

def popenAndCall(onExit, popenArgs):
    """
    Runs the given args in a subprocess.Popen, and then calls the function
    onExit when the subprocess completes.
    onExit is a callable object, and popenArgs is a list/tuple of args that 
    would give to subprocess.Popen.
    """
    def runInThread(onExit, popenArgs):
        proc = subprocess.Popen(*popenArgs)
        proc.wait()
        onExit()
        return
    thread = threading.Thread(target=runInThread, args=(onExit, popenArgs))
    thread.start()
    # returns immediately after the thread starts
    return thread

Even threading is pretty easy in Python, but note that if onExit() is computationally expensive, you'll want to put this in a separate process instead using multiprocessing (so that the GIL doesn't slow your program down). It's actually very simple - you can basically just replace all calls to threading.Thread with multiprocessing.Process since they follow (almost) the same API.