def _copy(master_fd, master_read=_read, stdin_read=_read):
    """Parent copy loop.
    Copies
            pty master -> standard output   (master_read)
            standard input -> pty master    (stdin_read)"""
    fds = [master_fd, STDIN_FILENO]
    while True:
        rfds, wfds, xfds = select(fds, [], [])
        if master_fd in rfds:
            data = master_read(master_fd)
            if not data:  # Reached EOF.
                return
            else:
                os.write(STDOUT_FILENO, data)
        if STDIN_FILENO in rfds:
            data = stdin_read(STDIN_FILENO)
            if not data:
                fds.remove(STDIN_FILENO)
            else:
                _writen(master_fd, data)
Parent copy loop.
    Copies
            pty master -> standard output   (master_read)
            standard input -> pty master    (stdin_read)