[Python-Dev] Defining a path protocol

Chris Barker chris.barker at noaa.gov
Wed Apr 6 23:50:23 EDT 2016

On Wed, Apr 6, 2016 at 5:57 PM, Ethan Furman <ethan at stoneleaf.us> wrote:

> But not a big deal. I think this is pretty much for occasional use by
> library authors, so not a big deal what it is named.
> It's mostly for the stdlib itself.  I imagine that most libraries would
> just take what they are given and pass it along to open or os.stat or
> whatever.

Exactly -- so we really don't need a builtin shortcut.

> Which makes me think: str() calls __str__ on an arbitrary object, and
>> creates a new string object.
>> But fspath(), if it exists, would call __fspath__ on an arbitrary
>> object, and create a new string -- not a new Path. That may be
>> confusing...
> It would be more along the lines of pickle -- give me the standard
> serialized form of this Path, please.

well, give me the standard serialized-path of this arbitrary object, yes?

> So are we imagining that future libs will be written that only take
>> objects with a __fspath__ method? In which case, do we need to add it
>> to str? In which case, this is all kind of pointless.
> We are imagining that future libraries that have to muck about with paths
> will work with Path objects, either by accepting them or converting to them
> as the (possibly) stringified paths are passed in -- and when necessary
> those libs can pass either the Path obj or the stringified path to the
> stdlib.

if that's the case, we don't need the __fspath__ protocol -- the reason for
the protocol is that we imagine there may be any number of third-party
objects to represent/work-with paths, that aren't strings or stdlib Path

Or maybe all future libs will continue to accept either an str or an
>> object with __fspath__.  In which case, this is pretty pointless, too.
> The point is to allow future programs to work with Path and be able to
> work with the stdlib as seamlessly and painlessly as possible.

again, we don't need a new protocol for that -- we only need the protocol
if we want arbitrary future programs to work with arbitrary path

which I suppose we do -- there are already other path implimentaitons out
there (though at least some are strings :-) )

> I guess what I'm wondering is if we are stuck with str-paths as the
>> lingua-Franca for paths forever. In which case, we should embrace that
>> and just call str() on anything passed in as a path argument.
> Nah.  That's inviting trouble and pain, and we're trying to get away from
> that.
> Sure, then open(3.5) will give you a file not found error, or maybe
>> create a file with a weird name, but really? Who's going to make that
>> mistake and not figure it out really quickly?
> Well, since the 3.5 was actually in my_var, and could have been written
> before it was read, it could easily be days, weeks, or even months --
> probably after the last guy quit, you took the job, the server died, and
> you had to restore from backup -- at which point you'll see all the really,
> really strange file names and wonder what they are.  And of course,
> whatever logic was determining those weird names is now out of sync because
> of the server swap.
> And, yeah, I've seen weirder things happen.

People can totally screw up path variables as strings or Path objects too
-- I'm having trouble seeing that this is all that more likely -- after
all, python is a dynamic language -- if we wanted full type safety, we
wouldn't be using python...

Speaking of which, how is this going to work with the new type system? Do
we need an ABC, rather than just a protocol?

But as long as we get to the stdlib taking Path objects, I'm happy :-)



Christopher Barker, Ph.D.

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Chris.Barker at noaa.gov
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