On Tue, 14 Jul 2026 at 02:25, Rob Cliffe via Python-list
<[email protected]> wrote:
> It seems to me to be useful to distinguish between
> cases where the attempt fails because the file is owned by someone else
> the case where the attempt only fails because the file is read-only.
> We already have a FileExistsError subclass of OSError, which AIUI allows
> you to distinguish
> between the cases where trying to create an file (say)
> fails because the file already exists
> fails because you do not have permission to write to the parent
> directory.
> This distinction is useful information.
The question is, HOW do you distinguish? FileExistsError is not a
variant form of PermissionsError; it, as per the documentation,
corresponds to the EEXIST error return. OSError subclasses tell you
about different errno values. Prior to FileNotFoundError's
introduction in Python 3.3 (along with a number of others), the way to
recognize that a file wasn't found was:
except OSError as e:
if e.errno != errno.ENOENT: raise
# it's "File Not Found", proceed accordingly
This is very different from what you're asking for, which is a
separate and independent check, and introduces TOCTOU issues.
> In the same way, surely it is useful to have more information
> about why trying to delete a file failed.
Only if the OS provides it.
> I was talking about a read-only file, not a read-only directory.
> That may be another ball game - one I haven't thought about.
> (I wasn's suggesting a DirectoryReadOnly Exception, though there might
> be some merit in the idea.)
Yes, and on Linux, a read-only file can be removed without error. So
the distinction is important. That's why the error is simply "you
aren't allowed to do that", without trying to crystal-ball the reason
for that.
> You jest of course. My point was that such logic may be written not
> just by me,
> but by many others. Reinventing the wheel.
I'm unconvinced that it is, though. I've never implemented that logic,
and among those who have, I think you'll find that there are subtly
different variations for different situations. You're specifically
wanting to check the *file's* permissions, not its containing
directory, and not whether the file system *as a whole* is read only.
That's correct for your situation, but not for everyone's.
ChrisA
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