spawning a process with subprocess

kyosohma at kyosohma at
Mon Nov 26 18:10:38 CET 2007

On Nov 26, 10:54 am, bhunter <brian.p.hun... at> wrote:
> Hi,
> I've used subprocess with 2.4 several times to execute a process, wait
> for it to finish, and then look at its output.  Now I want to spawn
> the process separately, later check to see if it's finished, and if it
> is look at its output.  I may want to send a signal at some point to
> kill the process.  This seems straightforward, but it doesn't seem to
> be working.
> Here's my test case:
> import subprocess, time
> cmd = "cat somefile"
> thread = subprocess.Popen(args=cmd.split(), shell=True,
> stdout=subprocess.PIPE, stdin=subprocess.PIPE,
> stderr=subprocess.STDOUT, close_fds=True)
> while(1):
>       time.sleep(1)
>       if(thread.returncode):
>          break
>       else:
>          print thread.returncode
> print "returncode = ", thread.returncode
> for line in thread.stdout:
>    print "stdout:\t",line
> This will just print the returncode of None forever until I Ctrl-C it.
> Of course, the program works fine if I call thread.communicate(), but
> since this waits for the process to finish, that's not what I want.
> Any help would be appreciated.

I've read that this sort of thing can be a pain. I'm sure someone will
post and have other views though. I have had some success using
Python's threading module though. There's a pretty good walkthrough
here (it uses wxPython in its example):

Other places of interest include:

If I were doing something like this, I would have the process write
it's output to a file and periodically check to see if the file has

Hopefully someone with more knowledge will come along soon.


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