subprocess woes

[Chris Rebert]

On Tue, Sep 15, 2009 at 12:26 PM, kj <no.email at please.post> wrote:
> I'm trying to write a function, sort_data, that takes as argument
> the path to a file, and sorts it in place, leaving the last "sentinel"
> line in its original position (i.e. at the end).  Here's what I
> have (omitting most error-checking code):
>
> def sort_data(path, sentinel='.\n'):
>    tmp_fd, tmp = tempfile.mkstemp()
>    out = os.fdopen(tmp_fd, 'wb')
>    cmd = ['/usr/local/bin/sort', '-t', '\t', '-k1,1', '-k2,2']
>    p = Popen(cmd, stdin=PIPE, stdout=out)
>    in_ = file(path, 'r')
>    while True:
>        line = in_.next()
>        if line != sentinel:
>            p.stdin.write(line)
>        else:
>            break
>    in_.close()
>    p.stdin.close()
>    retcode = p.wait()
>    if retcode != 0:
>        raise CalledProcessError(retcode, cmd)
>    out.write(sentinel)
>    out.close()
>    shutil.move(tmp, path)
>
>
> This works OK, except that it does not catch the stderr from the
> called sort process.  The problem is how to do this.  I want to to
> avoid having to create a new file just to capture this stderr
> output.  I would like instead to capture it to an in-memory buffer.
> Therefore I tried using a StringIO object as the stderr parameter
> to Popen, but this resulted in the error "StringIO instance has no
> attribute 'fileno'".
>
> How can I capture stderr in the scenario depicted above?

Use a pipe by setting stderr=PIPE?:

p = Popen(cmd, stdin=PIPE, stdout=out, stderr=PIPE)
#...
error_output = p.stderr.read()

Or am I missing something?

Cheers,
Chris
--
http://blog.rebertia.com