[Python-Dev] Pathlib enhancements - acceptable inputs and outputs for __fspath__ and os.fspath()

[Random832]

On Wed, Apr 13, 2016, at 11:28, Ethan Furman wrote:
> On 04/13/2016 08:17 AM, Random832 wrote:
> > On Wed, Apr 13, 2016, at 10:21, Nick Coghlan wrote:
> 
> >> I'd expect the main consumers to be os and os.path, and would honestly
> >> be surprised if we needed many explicit invocations above that layer,
> >> other than in pathlib itself.
> >
> > I made a toy implementation to try this out, and making os.open support
> > it does not get you builtin open "for free" as I had suspected; builtin
> > open has its own type checks in _iomodule.c.
> 
> Yup, it will take some effort to make this work.

A corner case just occurred to me...

For functions that will continue to accept str/bytes (and functions that
accept some other type such as Number or file-like objects), what should
be done with an object that is one of these, *and* has an __fspath__
method, *and* this method returns a value other than the object's own
value? Basically, should the protocol check be done unconditionally
(before attempting to use the argument as a string) or only if the
argument is not a string (there's an efficiency argument for this). Or
should it be left "unspecified", with the understanding that such
objects are badly behaved and may not be handled consistently across
different functions / python implementations / cpython versions?

Also, should the os.fspath (or whatever we call it) function itself
accept str/bytes, even if these are not going to implement the protocol?