subprocess.check_output while tee-ing output to stdout/stderr

subprocess_tee.py

def check_output_splitlines(args: List[str]) -> List[str]:
    """This is like subprocess.check_output().split('\n'), however it also
    copies all stderr and stdout to our own sys.stderr/stdout
    """
    stdout_buffer = io.StringIO(newline=None)

    # use pty instead of pipes so python subprocesses do not buffer
    stdout_ours, stdout_theirs = pty.openpty()
    stderr_ours, stderr_theirs = pty.openpty()

    proc = subprocess.Popen(args, bufsize=0, stdout=stdout_theirs, stderr=stderr_theirs)

    selector = selectors.DefaultSelector()
    selector.register(stdout_ours, selectors.EVENT_READ, (stdout_ours, sys.stdout))
    selector.register(stderr_ours, selectors.EVENT_READ, (stderr_ours, sys.stderr))

    os.close(stdout_theirs)
    os.close(stderr_theirs)

    registered = 2
    while registered:
        events = selector.select()
        for key, mask in events:
            our_pipe, stdio_file = key.data
            try:
                data = os.read(our_pipe, 8192).decode()
            except OSError as e:
                if e.errno != errno.EIO:
                    raise e
                data = ''
            if not data:
                selector.unregister(our_pipe)
                registered -= 1
                continue
            if stdio_file is sys.stdout:
                stdout_buffer.write(data)
            stdio_file.write(data)
            stdio_file.flush()

    proc.wait()
    if proc.returncode != 0:
        raise subprocess.CalledProcessError(proc.returncode, args, '', '')

    return stdout_buffer.getvalue().split('\n')