On Wed, 15 Jun 2016 at 11:39 Guido van Rossum <gu...@python.org> wrote:
> OK, so let's add a check on the return of __fspath__() and keep the check > on path-like or string/bytes. > I'll update the PEP. Ethan, do you want to leave a note on the os.fspath() issue to update the code and go through where we've used os.fspath() to see where we can cut out redundant type checks? > --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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