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}