python - Non blocking subprocess.call

Question

I'm trying to make a non blocking subprocess call to run a slave.py script from my main.py program. I need to pass args from main.py to slave.py once when it(slave.py) is first started via subprocess.call after this slave.py runs for a period of time then exits.

main.py
for insert, (list) in enumerate(list, start =1):

    sys.args = [list]
    subprocess.call(["python", "slave.py", sys.args], shell = True)


{loop through program and do more stuff..}

And my slave script

slave.py
print sys.args
while True:
    {do stuff with args in loop till finished}
    time.sleep(30)

Currently, slave.py blocks main.py from running the rest of its tasks, I simply want slave.py to be independent of main.py, once I've passed args to it. The two scripts no longer need to communicate.

I've found a few posts on the net about non blocking subprocess.call but most of them are centered on requiring communication with slave.py at some-point which I currently do not need. Would anyone know how to implement this in a simple fashion...?

Answer 1

You should use subprocess.Popen instead of subprocess.call.

Something like:

subprocess.Popen(["python", "slave.py"] + sys.argv[1:])

From the docs on subprocess.call:

Run the command described by args. Wait for command to complete, then return the returncode attribute.

(Also don't use a list to pass in the arguments if you're going to use shell = True).

---

Here's a MCVE¹ example that demonstrates a non-blocking suprocess call:

import subprocess
import time

p = subprocess.Popen(['sleep', '5'])

while p.poll() is None:
    print('Still sleeping')
    time.sleep(1)

print('Not sleeping any longer.  Exited with returncode %d' % p.returncode)

An alternative approach that relies on more recent changes to the python language to allow for co-routine based parallelism is:

# python3.5 required but could be modified to work with python3.4.
import asyncio

async def do_subprocess():
    print('Subprocess sleeping')
    proc = await asyncio.create_subprocess_exec('sleep', '5')
    returncode = await proc.wait()
    print('Subprocess done sleeping.  Return code = %d' % returncode)

async def sleep_report(number):
    for i in range(number + 1):
        print('Slept for %d seconds' % i)
        await asyncio.sleep(1)

loop = asyncio.get_event_loop()

tasks = [
    asyncio.ensure_future(do_subprocess()),
    asyncio.ensure_future(sleep_report(5)),
]

loop.run_until_complete(asyncio.gather(*tasks))
loop.close()

^{1Tested on OS-X using python2.7 & python3.6}