[Python-Dev] Defining a path protocol (was: When should pathlib stop being provisional?)

Brett Cannon brett at python.org
Wed Apr 6 13:26:36 EDT 2016

WIth Ethan volunteering to do the work to help make a path protocol a thing
-- and I'm willing to help along with propagating this through the stdlib
where I think Serhiy might be interested in helping as well -- and a
seeming consensus this is a good idea, it seems like this proposal has a
chance of actually coming to fruition.

Now we need clear details. :) Some open questions are:

   1. Name: __path__, __fspath__, or something else?
   2. Method or attribute? (changes what kind of one-liner you might use in
   libraries, but I think historically all protocols have been methods and the
   serialized string representation might be costly to build)
   3. Built-in? (name is dependent on #1 if we add one)
   4. Add the method/attribute to str? (I assume so, much like __index__()
   is on int, but I have not seen it explicitly stated so I would rather
   clarify it)
   5. Expand the C API to have something like PyObject_Path()?

Some people have asked for the pathlib PEP to have a more flushed out
reasoning as to why pathlib doesn't inherit from str. If Antoine doesn't
want to do it I can try to instil my blog post into a more succinct
paragraph or two and update the PEP myself.

Is this going to require a PEP or if we can agree on the points here are we
just going to do it? If we think it requires a PEP I'm willing to write it,
but I obviously have no issue if we skip that step either. :)

Oh, and we should resolve this before the next release of Python 3.4, 3.5,
or 3.6 so that pathlib can be updated in those releases.


On Wed, 6 Apr 2016 at 08:09 Ethan Furman <ethan at stoneleaf.us> wrote:

> On 04/05/2016 11:57 PM, Nick Coghlan wrote:
> > On 6 April 2016 at 16:53, Nathaniel Smith <njs at pobox.com> wrote:
> >> On Tue, Apr 5, 2016 at 11:29 PM, Nick Coghlan <ncoghlan at gmail.com>
> wrote:
> >>> I'd missed the existing precedent in DirEntry.path, so simply taking
> >>> that and running with it sounds good to me.
> >>
> >> This makes me twitch slightly, because NumPy has had a whole set of
> >> problems due to the ancient and minimally-considered decision to
> >> assume a bunch of ad hoc non-namespaced method names fulfilled some
> >> protocol -- like all .sum methods will have a signature that's
> >> compatible with numpy's, and if an object has a .log method then
> >> surely that computes the logarithm (what else in computing could "log"
> >> possibly refer to?), etc. This experience may or may not be relevant,
> >> I'm not sure -- sometimes these kinds of twitches are good guides to
> >> intuition, and sometimes they are just knee-jerk responses to an old
> >> and irrelevant problem :-)
> >>
> >> But you might want to at least think about
> >> how common it might be to have existing objects with unrelated
> >> attributes that happen to be called "path", and the bizarro problems
> >> that might be caused if someone accidentally passes one of them to a
> >> function that expects all .path attributes to be instances of this new
> >> protocol.
> >
> > sys.path, for example.
> >
> > That's why I'd actually prefer the implicit conversion protocol to be
> > the more explicitly named "__fspath__", with suitable "__fspath__ =
> > path" assignments added to DirEntry and pathlib. However, I'm also not
> > offering to actually *do* the work here, and the casting vote goes to
> > the folks pursuing the implementation effort.
> If we decide upon __fspath__ (or __path__) I will do the work on pathlib
> and scandir to add those attributes.
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