[Python-ideas] os.path.commonprefix: Yes that old chestnut.

[Serhiy Storchaka]

On 24.03.15 13:56, Paul Moore wrote:
> On 23 March 2015 at 21:33, Gregory P. Smith <greg at krypto.org> wrote:
>> +1 pathlib would be the appropriate place for the correctly behaving
>> function to appear.
>
> OK, so here's a question. What actual use cases exist for a
> common_prefix function? The reason I ask is that I'm looking at some
> of the edge cases, and the obvious behaviour isn't particularly clear
> to me.
>
> For example, common_prefix('a/b/file.c', 'a/b/file.c'). The common
> prefix is obviously 'a/b/file.c' - but I can imagine people *actually*
> wanting the common *directory* containing both files. But taken
> literally, that's only possible if you check the filesystem, so it
> would no longer be a PurePath operation.
>
> And what about common_prefix('foo/bar', '../here/foo')? Or
> common_prefix('bar/baz', 'foo/../bar/baz')? Pathlib avoids collapsing
> ... because the meaning could change in the face of symlinks. I believe
> the same applies here. Maybe you need to call resolve() before doing
> the common prefix operation (but that gives an absolute path).
>
> With the above limitations, would a common_prefix function actually
> help typical use cases? In my experience, doing list operations on
> pathobj.parts is often simple enough that I don't need specialised
> functions like common_prefix...
>
> Getting the edge cases right is fiddly enough for common_prefix that a
> specialised function is a reasonable idea, but only if the "obvious"
> behaviour is clear. If there's a lot of conflicting possibilities,
> maybe a recipe in the docs would be a better option.

Your are asking the same questions that were asked about 
os,path.commonpath(). You can look at discussions on the tracker, in 
Python-Dev and in Python-Ideas.

http://bugs.python.org/issue10395
http://mail.python.org/pipermail/python-dev/2000-July/005897.html
http://mail.python.org/pipermail/python-dev/2000-August/008385.html
http://comments.gmane.org/gmane.comp.python.ideas/17719