def _to_unicode(s):
    """
    decode a string as ascii or utf8 if possible (as required by the sftp
    protocol).  if neither works, just return a byte string because the server
    probably doesn't know the filename's encoding.
    """
    try:
        return s.encode('ascii')
    except (UnicodeError, AttributeError):
        try:
            return s.decode('utf-8')
        except UnicodeError:
            return s
