[Python-Dev] When should pathlib stop being provisional?

[Nathaniel Smith]

On Apr 6, 2016 07:44, "Steven D'Aprano" <steve at pearwood.info> wrote:
>
> On Wed, Apr 06, 2016 at 11:30:32AM +0200, Petr Viktorin wrote:
>
> > Python was in a similar situation with the .next method on iterators,
> > which changed to __next__ in Python 3. PEP 3114 (which explains this
> > change) says:
> >
> > > Code that nowhere contains an explicit call to a next method can
> > > nonetheless be silently affected by the presence of such
> > > a method. Therefore, this PEP proposes that iterators should have
> > > a __next__ method instead of a next method (with no change in
> > > semantics).
> >
> > How well does that apply to path/__path__?
>
> I think it's potentially the same. Possibly there are fewer existing
> uses of "obj.path" out there which conflict with this use, but there's
> at least one in the std lib: sys.path.
>
>
> > That PEP also introduced the next() builtin. This suggests that a
> > protocol with __path__/__fspath__ would need a corresponding
> > path()/fspath() builtin.
>
> Not necessarily. Take a look at (say) dir(object()) and you'll see a few
> dunders that don't correspond to built-ins:
>
> __reduce__  and __reduce_ex__ are used by pickle;
> __sizeof__ is used by sys.getsizeof;
> __subclasshook__ is used by the ABC system;
>
> Another example is __trunc__ used by math.trunc().
>
> So any such fspath function should stand on its own as a useful
> feature, not just because there's a dunder method __fspath__.

An even more precise analogy is provided by __index__, whose semantics are
to provide safe casting to integer (the name is a historical accident), as
opposed to __int__'s tendency to cast things to integer willy-nilly,
including things that really shouldn't be silently accepted as integers.
Basically __index__ is to __int__ as __(fs)path__ would be to __str__.

There's an operator.index but no builtins.index.

-n
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