polling for output from a subprocess module

[jakub.hrozek at gmail.com]

On 4 Ún, 11:49, Thomas Bellman <bell... at lysator.liu.se> wrote:
> jakub.hro... at gmail.com wrote:
> >       try:
> >             test = Popen(test_path,
> >                                   stdout=PIPE,
> >                                   stderr=PIPE,
> >                                   close_fds=True,
> >                                   env=test_environ)
> >             while test.poll() == None:
> >                 ready = select.select([test.stderr], [], [])
> >                 if test.stderr in ready[0]:
> >                     t_stderr_new = test.stderr.readlines()
> >                     if t_stderr_new != []:
> >                         print "STDERR:", "\n".join(t_stderr_new)
> >                         t_stderr.extend(t_stderr_new)
> [...]
> > The problem is, that it seems that all the output from the subprocess
> > seems to be coming at once. Do I need to take a different approach?
>
> The readlines() method will read until it reaches end of file (or
> an error occurs), not just what is available at the moment.  You
> can see that for your self by running:
>
>     $ python -c 'import sys; print sys.stdin.readlines()'
>
> The call to sys.stdin.readlines() will not return until you press
> Ctrl-D (or, I think, Ctrl-Z if you are using MS-Windows).
>
> However, the os.read() function will only read what is currently
> available.  Note, though, that os.read() does not do line-based
> I/O, so depending on the timing you can get incomplete lines, or
> multiple lines in one read.
>

Right, I didn't realize that. I'll try the os.read() method. Reading
what's available (as opposed to whole lines) shouldn't be an issue in
this specific case. Thanks for the pointer!