[Python-Dev] Defining a path protocol

[Koos Zevenhoven]

On Fri, Apr 8, 2016 at 8:34 PM, Brett Cannon <brett at python.org> wrote:
> On Fri, 8 Apr 2016 at 09:39 Ethan Furman <ethan at stoneleaf.us> wrote:
>> > I thought the whole point off all this is that not any old string can be
>> > a path! (whereas any int can be an index). Unless we go with Chris A's
>> > suggestion that this be a more generic lossless string protocol, rather
>> > than just for paths.
>>
>> That does seem to be a valid point against str.__fspath__.
>
> Yep, and I'm expecting we won't want that at this point. The fact that paths
> need strings for low-level OS stuff is a historical and technical detail, so
> no need to drag the entire str type into it if we can provide a reasonable
> helper function (for either the ABC or magic method solution).

I'm not sure I understand what these points are about. Anyway,
disallowing str or bytes as pathnames will break backwards
compatibility if done at some point in the future. There's no way
around that.

But regarding all this talk of mine about bytes is because it has not
been completely clear to me if something can break when converting a
bytes path to str. I did originally propose guaranteeing a str, but I
am so far only 85% convinced that that does not cause any problems. I
understand that fsencode(fsdecode(bytes_path)) should always be equal
to bytes_path. But can some other path operations fail when there are
surrogates in the strings? And again, not to forget DirEntry, which
may have a byte string path.

Either way, I suppose os.fspath should accept anything that has
__fspath__ or is a str or bytes (whether these have the dunder method
or not). Then the options are either to return Union[str, bytes] or to
always return str. And if the latter does not cause any problems, I
like it way better, and it seems others would do too. And in that case
it would probably be time to deprecate bytes paths on posix too (on
Windows, this is already the case).

But do we know that converting all paths to str does not cause any problems?

-Koos