Popen pipe hang

[schickb]

On May 12, 7:35 pm, Jean-Paul Calderone <exar... at divmod.com> wrote:
> >from subprocess import Popen, PIPE
> >from array import array
>
> >arr = array('B')
> >arr.fromstring("hello\n")
>
> >src = Popen( ["cat"], stdin=PIPE, stdout=PIPE)
> >dst = Popen( ["cat"], stdin=src.stdout)
> >arr.tofile(src.stdin)
> >src.stdin.close()
> >dst.wait()
>
> Alas, you haven't actually closed src's standard input.  Though you did
> close src.stdin, you didn't close the copy of it which was inherited by
> dst!  So though the file descriptor is no longer open in your main process,
> it remains open due to the reference dst has to it.  You can fix this by
> having Popen close all file descriptors except 0, 1, and 2 before it execs
> cat - pass close_fds=True to the 2nd Popen call and you should get the
> behavior you want.
>

Thanks, that did the trick. Although I assume that by passing
close_fds=True the second Popen is actually closing src.stdout (rather
than src.stdin as mentioned)? With close_fds=True, is Python buffering
the data even though bufsize=0 is the default? Otherwise I don't see
how it could close the fd before executing the process.