[Python-Dev] proposed os.fspath() change

[Guido van Rossum]

OK, so let's add a check on the return of __fspath__() and keep the check
on path-like or string/bytes.

--Guido (mobile)
On Jun 15, 2016 11:29 AM, "Nick Coghlan" <ncoghlan at gmail.com> wrote:

> On 15 June 2016 at 10:59, Brett Cannon <brett at python.org> wrote:
> >
> >
> > On Wed, 15 Jun 2016 at 09:48 Guido van Rossum <guido at python.org> wrote:
> >>
> >> These are really two separate proposals.
> >>
> >> I'm okay with checking the return value of calling obj.__fspath__;
> that's
> >> an error in the object anyways, and it doesn't matter much whether we do
> >> this or not (though when approving the PEP I considered this and
> decided not
> >> to insert a check for this). But it doesn't affect your example, does
> it? I
> >> guess it's easier to raise now and change the API in the future to avoid
> >> raising in this case (if we find that raising is undesirable) than the
> other
> >> way around, so I'm +0 on this.
> >
> > +0 from me as well. I know in some code in the stdlib that has been
> ported
> > which prior to adding support was explicitly checking for str/bytes this
> > will eliminate its own checking (obviously not a motivating factor as
> it's
> > pretty minor).
>
> I'd like a strong assertion that the return value of os.fspath() is a
> plausible filesystem path representation (so either bytes or str), and
> *not* some other kind of object that can also be used for accessing
> the filesystem (like a file descriptor or an IO stream)
>
> >> The other proposal (passing anything that's not understood right
> through)
> >> is more interesting and your use case is somewhat compelling. Catching
> the
> >> exception coming out of os.fspath() would certainly be much messier. The
> >> question remaining is whether, when this behavior is not desired (e.g.
> when
> >> the caller of os.fspath() just wants a string that it can pass to
> open()),
> >> the condition of passing that's neither a string not supports __fspath__
> >> still produces an understandable error. I'm not sure that that's the
> case.
> >> E.g. open() accepts file descriptors in addition to paths, but I'm not
> sure
> >> that accepting an integer is a good idea in most cases -- it either
> gives a
> >> mystery "Bad file descriptor" error or starts reading/writing some
> random
> >> system file, which it then closes once the stream is closed.
> >
> > The FD issue of magically passing through an int was also a concern when
> > Ethan brought this up in an issue on the tracker. My argument is that FDs
> > are not file paths and so shouldn't magically pass through if we're
> going to
> > type-check anything or claim os.fspath() only works with paths (FDs are
> > already open file objects). So in my view  either we go ahead and
> type-check
> > the return value of __fspath__() and thus restrict everything coming out
> of
> > os.fspath() to Union[str, bytes] or we don't type check anything and be
> > consistent that os.fspath() simply does is call __fspath__() if present.
> >
> > And just  because I'm thinking about it, I would special-case the FDs,
> not
> > os.PathLike (clearer why you care and faster as it skips the override of
> > __subclasshook__):
> >
> > # Can be a single-line ternary operator if preferred.
> > if not isinstance(filename, int):
> >     filename = os.fspath(filename)
>
> Note that the LZMA case Ethan cites is one where the code accepts
> either an already opened file-like object *or* a path-like object, and
> does different things based on which it receives.
>
> In that scenario, rather than introducing an unconditional "filename =
> os.fspath(filename)" before the current logic, it makes more sense to
> me to change the current logic to use the new protocol check rather
> than a strict typecheck on str/bytes:
>
>     if isinstance(filename, os.PathLike): # Changed line
>         filename = os.fspath(filename)    # New line
>         if "b" not in mode:
>             mode += "b"
>         self._fp = builtins.open(filename, mode)
>         self._closefp = True
>         self._mode = mode_code
>     elif hasattr(filename, "read") or hasattr(filename, "write"):
>         self._fp = filename
>         self._mode = mode_code
>     else:
>         raise TypeError(
>              "filename must be a path-like or file-like object"
>               )
>
> I *don't* think it makes sense to weaken the guarantees on os.fspath
> to let it propagate non-path-like objects.
>
> Cheers,
> Nick.
>
> --
> Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
>
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