subprocess woes

[kj]

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?

TIA!

kynn