[cclauss]

• os.path.commonprefix deprecated in Python 3.15 #364
• https://docs.python.org/3/library/os.path.html#os.path.commonpath
• https://docs.python.org/3/whatsnew/3.15.html#pending-removal-in-future-versions

[gengjiawen]

Thanks for picking this up! Unfortunately os.path.commonpath() isn't a drop-in replacement here. When the only shared ancestor is the filesystem root, it returns the root with a trailing separator (/, C:\, \\srv\share\), so .split(os.path.sep) yields an extra empty component and prefix_len is off by one. The result is missing one .. and drops the first component of path:

path	relative_to	main	this PR
/a/b	/c	../a/b	b ❌
/a	/	../a	'' ❌
C:\work\proj	C:\Users\me\node-gyp	..\..\..\work\proj	..\..\proj ❌
\\srv\share\a	\\srv\share\b	..\a	'' ❌
/a/b	/a/c	../b	../b ✅

For example, a .gyp file under /tmp with --depth under /home (or C:\work vs C:\Users on Windows) would get wrong relative paths written into the generated build files. CI doesn't catch this because RelativePath has no unit tests and none of the fixtures cross a top-level directory.

path_split and relative_to_split are already lists of components, so the simplest fix is to count the shared leading components directly (see the inline suggestion). I compared it with the old commonprefix() for every pair of paths up to depth 3 under /, C:\ and \\srv\share\, and the results are identical, so behavior is unchanged.

Could you also add a regression test to pylib/gyp/common_test.py? This fails on the current PR and passes on main and with the suggestion:

class TestRelativePath(unittest.TestCase):
    def test_common_parent(self):
        assert gyp.common.RelativePath("/a/b", "/a/c") == os.path.join("..", "b")

    def test_only_root_in_common(self):
        """Paths whose only common ancestor is the filesystem root."""
        assert gyp.common.RelativePath("/a/b", "/c") == os.path.join("..", "a", "b")

    def test_same_path(self):
        assert gyp.common.RelativePath("/a/b", "/a/b") == ""

Nit: please retitle the PR to a Conventional Commits title such as fix: avoid deprecated os.path.commonprefix() in RelativePath. The squash commit uses the commit title only while the PR has a single commit; after more pushes it uses the PR title, and release-please silently drops titles it can't parse. The current title also says commonpath, which won't be accurate after this change.

[gengjiawen]

Thanks for the update! LGTM — verified locally (pytest + ruff), and the new tests run on all CI platforms.