Python subprocess: callback when cmd exits

Question

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

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

Even threading is pretty easy in Python, but note that if on_exit() 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.

Answer 2

There is concurrent.futures module in Python 3.2 (available via pip install futures for older Python < 3.2):

pool = Pool(max_workers=1)
f = pool.submit(subprocess.call, "sleep 2; echo done", shell=True)
f.add_done_callback(callback)

The callback will be called in the same process that called f.add_done_callback().

Full program

import logging
import subprocess
# to install run `pip install futures` on Python <3.2
from concurrent.futures import ThreadPoolExecutor as Pool

info = logging.getLogger(__name__).info

def callback(future):
    if future.exception() is not None:
        info("got exception: %s" % future.exception())
    else:
        info("process returned %d" % future.result())

def main():
    logging.basicConfig(
        level=logging.INFO,
        format=("%(relativeCreated)04d %(process)05d %(threadName)-10s "
                "%(levelname)-5s %(msg)s"))

    # wait for the process completion asynchronously
    info("begin waiting")
    pool = Pool(max_workers=1)
    f = pool.submit(subprocess.call, "sleep 2; echo done", shell=True)
    f.add_done_callback(callback)
    pool.shutdown(wait=False) # no .submit() calls after that point
    info("continue waiting asynchronously")

if __name__=="__main__":
    main()

Output

$ python . && python3 .
0013 05382 MainThread INFO  begin waiting
0021 05382 MainThread INFO  continue waiting asynchronously
done
2025 05382 Thread-1   INFO  process returned 0
0007 05402 MainThread INFO  begin waiting
0014 05402 MainThread INFO  continue waiting asynchronously
done
2018 05402 Thread-1   INFO  process returned 0

Answer 3

I modified Daniel G's answer to simply pass the subprocess.Popen args and kwargs as themselves instead of as a separate tuple/list, since I wanted to use keyword arguments with subprocess.Popen.

In my case I had a method postExec() that I wanted to run after subprocess.Popen('exe', cwd=WORKING_DIR)

With the code below, it simply becomes popenAndCall(postExec, 'exe', cwd=WORKING_DIR)

import threading
import subprocess

def popenAndCall(onExit, *popenArgs, **popenKWArgs):
    """
    Runs a subprocess.Popen, and then calls the function onExit when the
    subprocess completes.

    Use it exactly the way you'd normally use subprocess.Popen, except include a
    callable to execute as the first argument. onExit is a callable object, and
    *popenArgs and **popenKWArgs are simply passed up to subprocess.Popen.
    """
    def runInThread(onExit, popenArgs, popenKWArgs):
        proc = subprocess.Popen(*popenArgs, **popenKWArgs)
        proc.wait()
        onExit()
        return

    thread = threading.Thread(target=runInThread,
                              args=(onExit, popenArgs, popenKWArgs))
    thread.start()

    return thread # returns immediately after the thread starts