停止子流程多次启动流程

Question

I have a pythonic question: lets say I am using the subprocess library in python to run a program from the terminal. I need to call the subprocess in a loop. How can I: 1) I stop subprocess from initiating the program if it has already started. Or... 2) How can I find out if the program is still running so that I don't initiate a new subprocess?

Answer 1

You can check if the process is running using Process.poll . It will return None if the process is currently running. Otherwise, it will return the exit code of the sub-process. For example:

import subprocess
import time

p = None
while True:
   if p and p.poll() is None:
       print("Process is currently running. Do nothing.")
   else:  # No process running
       if p:  # We've run the process at least once already, so get the result.
           ret = p.returncode
           print("Last run returned {}".format(ret))
       p = subprocess.Popen(["some_command", "some arg"])
   time.sleep(3)  # Wait a few seconds before iterating again.