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" <ncogh...@gmail.com> wrote: > On 15 June 2016 at 10:59, Brett Cannon <br...@python.org> wrote: > > > > > > On Wed, 15 Jun 2016 at 09:48 Guido van Rossum <gu...@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 | ncogh...@gmail.com | Brisbane, Australia >
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