spawning a process with subprocess

[bhunter]

Wow, everyone.  Great comments.  Thanks so much!

A few points on all of the above, just so I don't look too stupid:

* The problem with the testcase, I believe, was the size of the file
and the output pipe filling up, as Nick suggested.  When run on a
smaller file, with Jordan's suggestions, it works fine.  With a larger
file, it's necessary to do as Nick says.  If the size of the file is
unknown, its best to use this case as the default.  This seems
unfortunate to me, because it's quite a bit of code to do something
that should be fairly straightforward--at least, that's what I think.

* Using poll() and checking for None and not non-zero:  Yes, I had
both of those originally in my testcase, but when I re-wrote and re-
wrote it after it initially didn't work those initial concepts got
dropped.  Thanks for reminding me.

* Yes, I should use proc instead of thread as a variable.  Good point,
Ove.  But your solution works on small files but chokes on larger
files, too.

Thanks again...and just to reiterate, I really think this could be
more straightforward for the rest of us if Popen could do all of this
on its own.

Brian



On Nov 27, 5:13 am, Ove Svensson <ove.svens... at jeppesen.com> wrote:
> bhunter <brian.p.hun... at gmail.com> writes:
> > 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.
>
> Reading documentation for subprocess, it mentions that
>
>     On UNIX, with shell=False (default): In this case, the Popen class
>     uses os.execvp() to execute the child program.  args should normally
>     be a sequence.  A string will be treated as a sequence with the string
>     as the only item (the program to execute).
>
>     On UNIX, with shell=True: If args is a string, it specifies the
>     command string to execute through the shell.  If args is a sequence,
>     the first item specifies the command string, and any additional items
>     will be treated as additional shell arguments.
>
> Since you have specified shell = True, and since you pass a sequence as
> args, you will efficiently invoke the cat process through the shell and
> then pass somefile as an extra argument to she shell (not the cat command)
> That is probably not what you intended.
>
> This can be solved by either
> - Not splitting the cmd, in which case you will pass the whole cmd
>   string to the shell for execution
> - Or setting shell to False. This is what I would have done, since
>   I can't see any reason for going via the shell. Please note that
>   if setting shell to False, you must then split the cmd.
>
> Please also note that your test for the returncode might not work
> since a normal returncode is 0. Your code will only detect non-0
> values.
>
> Also, it is good practice to call wait() on the subprocess in order
> to avoid zombie-processes.
>
> Finally, I find it somewhat misleading to use the name thread for
> the variable used to represent a sub-process. Threads and processes
> are not exactly the same
>
> Hence, the following code should works as expected
>
> cmd = "cat somefile"
> proc = subprocess.Popen(
>     args      = cmd.split(),
>     shell     = False,
>     stdin     = None,
>     stdout    = subprocess.PIPE,
>     stderr    = subprocess.STDOUT,
>     close_fds = True)
>
> while True:
>     rc = proc.poll()
>     if rc != None: break
>     print rc
>     time.sleep(1)
>
> lno = 1
> for lin in proc.stdout:
>     print '%i: %s' % (lno,lin.rstrip('\n'))
>     lno += 1
>
> rc = proc.wait()
> print "rc = %i" % rc
>
> /Ove