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

Ethan Furman ethan at stoneleaf.us
Wed Apr 6 20:57:21 EDT 2016

On 04/06/2016 05:43 PM, Chris Barker - NOAA Federal wrote:

fspath +0.1 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.

Which also means that I don't think we need a built-in function that calls it, either. How often do people need a stringified-path version of an arbitrary object?

Not often.

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.

If we were starting from scratch, I suppose path would return a Path object -- it would be a protocol one could use to duck-type a path.

Sure.

But since we have history, we are creating a protocol that conforms to the existing string-as-path protocol.

Yup.

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.

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.

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.

-- Ethan

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