Fix error message for `ntpath.commonpath`

[nineteendo]

Bug report

Bug description:

>>> import ntpath
>>> ntpath.commonpath(["/foo", "foo"])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<frozen ntpath>", line 808, in commonpath
ValueError: Can't mix absolute and relative paths

/foo is not an absolute path, but a rooted path, which can't be mixed with not-rooted paths.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Windows

Linked PRs

• gh-117381: Fix error message for ntpath.commonpath #117382

[aisk]

I did some googling and found that Microsoft has a document which describes "relative rooted path": https://learn.microsoft.com/en-us/dotnet/api/system.io.path.ispathrooted?view=net-8.0#remarks

A relative rooted path specifies a drive, but its fully qualified path is resolved against the current directory.

So I think the relative rooted path should be something like C:foo (please notice that it's not C:/foo), but /foo still is an absolute path. And ntpath.isabs('/foo') returns True, so I think it's an absolute path.

[nineteendo]

ntpath.isabs('/foo') returns True

That was recently changed:

• GH-44626, GH-105476: Fix ntpath.isabs() handling of part-absolute paths #113829

The error message could also be "Can't mix rooted and not-rooted paths" as /foo is rooted and foo is not.

[zooba]

How does this change make life better or easier for those encountering this message?

At first glance, it appears to be getting into overly precise terminology that doesn't help educate or correct the caller.

[nineteendo]

The error message states: "Can't mix absolute and relative paths".
But /foo & foo are both relative according to ntpath.isabs():

Path	Rooted?	Absolute?
c:/foo	Yes	Yes
c:foo	Yes	No
/foo	Yes	No
foo	No	No

So, this error message is factually incorrect.
But /foo is rooted and foo is not, so that's what the new error message is in that case.
On a side note, it could be useful to add os.path.isrooted.