spawning a process with subprocess

[bhunter]

On Nov 26, 1:50 pm, "Diez B. Roggisch" <de... at nospam.web.de> wrote:
> bhunter schrieb:
>
>
>
> > 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 have difficulties understanding what you are after here. To me it
> looks as if everything works as expected. I mean you periodically check
> on the liveness of the "thread" - which is what you describe above. All
> you are missing IMHO is the actual work in this program.
>
> So
>
> while True:
>      if do_work():
>          if thread.returncode:
>              break
>      else:
>          thread.kill()
>
> This assumes that your do_work()-method communicates the wish to end the
> sub-process using it's returnvalue.
>
> Diez

If the subprocess had finished, I expect that the returncode will not
be None, and the loop would break.  The process hasn't actually
started.  I know this because while this simple testcase just cats a
file, the real case submits a simulation job.  This job never starts
until after I ctrl-c the program.

Brian