[Python-Dev] Defining a path protocol (original) (raw)

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 objects.

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 implementations.

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 myvar, 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 :-)

-CHB

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Christopher Barker, Ph.D. Oceanographer

Emergency Response Division NOAA/NOS/OR&R (206) 526-6959 voice 7600 Sand Point Way NE (206) 526-6329 fax Seattle, WA 98115 (206) 526-6317 main reception

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