absolute path to a file

[tom arnall]

Thanks. Hope you found a solution to the problem.

On Tue, Aug 20, 2019, 2:51 AM Cameron Simpson <cs at cskk.id.au> wrote:

> Please remember to CC the list.
>
> On 19Aug2019 22:06, Paul St George <email at paulstgeorge.com> wrote:
> >On 19/08/2019 14:16, Cameron Simpson wrote:
> [...]
> >>There's a remark on that web page I mentioned that suggests that the
> >>leading '//' indicates the filename is relative to the Blender model,
> >>so the context directory for the '//' is likely
> >>/Users/Lion/Desktop/test8.
> >
> >Yes. That makes sense. The reason I was testing with two images, one
> >at /Users/Lion/Desktop/test8/image01.tif and the other at
> >/Users/Lion/Desktop/images/image02.tif is that I cannot rely on images
> >being in the same folder as the Blender file.
> >
> >So, let's assume the context directory is /Users/Lion/Desktop/test8
> >and see how we get on below.
> [...]
> >>realpath needs a UNIX path. Your //image01.tif isn't a UNIX path, it
> >>is a special Blender path. First you need to convert it. By replacing
> >>'//' with the blend file's directory. Then you can call realpath. If
> >>you still need to.
> >
> >Understood. Now. Thanks!
> >>
> >>[...snip...]
> >
> >Did you just [...snip...] yourself?
>
> Yes. It keeps the surrounding context manageable. In this way you know
> to which text I am referring, without having to wade through paragraphs
> to guess what may be relevant.
>
> >>    from os.path import dirname
> >>
> >>    # Get this from somewhere just hardwiring it for the example.
> >>    # Maybe from your 'n' object below?
> >>    blend_file = '/Users/Lion/Desktop/test8/tifftest8.blend'
> >Is this setting a relative path?
> >>
> >>    blender_image_file = n.image.filename
> >>
> >>    unix_image_file = unblenderise(blender_image_file,
> dirname(blend_file))
> >>
> >>Now you have a UNIX path. If blend_file is an absolute path,
> >>unix_image_path will also be an absolute path. But if blend_file is
> >>a relative path (eg you opened up "tifftest8.blend") unix_image_path
> >>will be a relative path.
> >
> >Does unix_image_path = unix_image_file?
>
> Yeah, sorry,  my mistake.
>
> >Two possibilities here.
> >blend_file (and so unix_image_file) is an absolute path OR blend_file
> >(and so unix_image_file) is a relative path.
> >
> >I just want to check my understanding. If I supply the path to
> >blend_file then it is absolute, and if I ask Python to generate the
> >path to blend_file from within Blender it is relative. Have I got it?
>
> Not quite. What seems to be the situation is:
>
> You've got some object from Blender called "n.image", which has a
> ".file" attribute which is a Blender reference to the image file of the
> form "//image01.tif".
>
> I presume that Blender has enough state inside "n" or "n.image" to
> locate this in the real filesystem; maybe it has some link to the
> Blender model of your blend file, and thus knows the path to the blend
> file and since //image01.tif is a reference relative to the blend file,
> it can construct the UNIX path to the file.
>
> You want to know the UNIX pathname to the image file (maybe you want to
> pass it to some unrelated application to view the file or something).
>
> So you need to do what Blender would do if it needs a UNIX path (eg to
> open the file).
>
> The formula for that is dirname(path_to_blendfile) with n.image.file[2:]
> appended to it. So that's what we do with unblenderise(): if the
> filename is a "Blender relative name", rip off the "//" and prepend the
> blend file directory path. That gets you a UNIX path which you can hand
> to any function expecting a normal operating system pathname.
>
> Whether that is an absolute path or a relative path is entirely "does
> the resulting path start with a '/'"?
>
> An absolute path starts with a slash and is relative to the root of the
> filesystem. You can use such a path regardless of what your current
> working directory is, because it doesn't use the working directory.
>
> A relative path doesn't start with a slash and is relative to the
> current working directory. It only works if you're in the right working
> directory.
>
> _Because_ a relative path depends on the _your_ working directory,
> usually we pass around absolute paths if we need to tell something else
> about a file, because that will work regardless if what _their_ working
> directory may be.
>
> So, you may well want to turn a relative path into an absolute path...
>
> >If I decided not to supply the path and so ended up with a relative
> >UNIX path, I could now use realpath or abspath to find the absolute
> >path. Have I still got it?
>
> This is correct.
>
> Abspath may even call realpath to do its work, unsure.
>
> >It works very well. So thank you! I tested it with a Blend file that
> >had two images, one in the same folder as the Blend file and the other
> >was in a folder on the Desktop called 'images'.
> >
> >The initial results were:
> >Plane uses image01.tif saved at //image01.tif which is at
> >/Users/Lion/Desktop/test8/image01.tif
> >Plane uses image02.tif saved at //../images/image02.tif which is at
> >/Users/Lion/Desktop/test8/../images/image02.tif
> >BUT as you say, this was easily sorted by using os.path.realpath or
> >os.path.abspath. Both worked equally well.
>
> Yep. Abspath does some things in a purely lexical way: it resolves '.'
> and '..' components in the path even if they don't exist. For example it
> resolves 'a/b/c/d/../../../../foo' for me even though here 'a/b/c/d'
> doesn't exist: it see the '.. and strips off the preceeding component,
> repeatedly. So it starts with 'foo' after that pahse  and prepends the
> working directory path.
>
> The system realpath is not documented to work if some of the path is
> fictitious, but the Python realpath seems to work even then, maybe it
> falls back to abspath or something.
>
> Abspath takes a relative path and returns you an absolute path with the
> '.' and '..' components stripped out.
>
> Realpath follows symlinks and stuff and returns you the direct absolute
> path _after_ all that - one that doesn't traverse symlinks.
>
> For your purposes the results are equally functional.
>
> >Plane uses image01.tif saved at //image01.tif which is at
> >/Users/Lion/Desktop/test8/image01.tif
> >Plane uses image02.tif saved at //../images/image02.tif which is at
> >/Users/Lion/Desktop/images/image02.tif
> >
> >So, everything now works. Thank you again. I am just a little unclear
> >on the absolute and relative, hence my questions.
>
> Hope the above xaplinations help.
>
> Cheers,
> Cameron Simpson <cs at cskk.id.au>
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