def encode_to_py3bytes_or_py2str(s):
    """ takes anything and attempts to return a py2 string or py3 bytes.  this
    is typically used when creating command + arguments to be executed via
    os.exec* """

    fallback_encoding = "utf8"

    if IS_PY3:
        # if we're already bytes, do nothing
        if isinstance(s, bytes):
            pass
        else:
            s = str(s)
            try:
                s = bytes(s, DEFAULT_ENCODING)
            except UnicodeEncodeError:
                s = bytes(s, fallback_encoding)
    else:
        # attempt to convert the thing to unicode from the system's encoding
        try:
            s = unicode(s, DEFAULT_ENCODING)
        # if the thing is already unicode, or it's a number, it can't be
        # coerced to unicode with an encoding argument, but if we leave out
        # the encoding argument, it will convert it to a string, then to unicode
        except TypeError:
            s = unicode(s)

        # now that we have guaranteed unicode, encode to our system encoding,
        # but attempt to fall back to something
        try:
            s = s.encode(DEFAULT_ENCODING)
        except:
            s = s.encode(fallback_encoding, "replace")
    return s