gh-93156 - fix negative indexing into absolute `pathlib.PurePath().parents` (GH-93273)

When a `_PathParents` object has a drive or a root, the length of the
object is *one less* than than the length of `self._parts`, which resulted
in an off-by-one error when `path.parents[-n]` was fed through to
`self._parts[:-n - 1]`. In particular, `path.parents[-1]` was a malformed
path object with spooky properties.

This is addressed by adding `len(self)` to negative indices.
(cherry picked from commit f32e6b48d12834ba3bde01ec21c14da33abd26d6)

Co-authored-by: Barney Gale <barney.gale@gmail.com>

3 files changed, 9 insertions, 0 deletions

diff --git a/Lib/pathlib.py b/Lib/pathlib.py
index 621fba0..97b23ca 100644
--- a/Lib/pathlib.py
+++ b/Lib/pathlib.py
@@ -528,6 +528,8 @@ class _PathParents(Sequence):
 
         if idx >= len(self) or idx < -len(self):
             raise IndexError(idx)
+        if idx < 0:
+            idx += len(self)
         return self._pathcls._from_parsed_parts(self._drv, self._root,
                                                 self._parts[:-idx - 1])
 
diff --git a/Lib/test/test_pathlib.py b/Lib/test/test_pathlib.py
index 555c7ee..bf3fc5f 100644
--- a/Lib/test/test_pathlib.py
+++ b/Lib/test/test_pathlib.py
@@ -463,6 +463,9 @@ class _BasePurePathTest(object):
         self.assertEqual(par[0], P('/a/b'))
         self.assertEqual(par[1], P('/a'))
         self.assertEqual(par[2], P('/'))
+        self.assertEqual(par[-1], P('/'))
+        self.assertEqual(par[-2], P('/a'))
+        self.assertEqual(par[-3], P('/a/b'))
         self.assertEqual(par[0:1], (P('/a/b'),))
         self.assertEqual(par[:2], (P('/a/b'), P('/a')))
         self.assertEqual(par[:-1], (P('/a/b'), P('/a')))
@@ -471,6 +474,8 @@ class _BasePurePathTest(object):
         self.assertEqual(par[::-1], (P('/'), P('/a'), P('/a/b')))
         self.assertEqual(list(par), [P('/a/b'), P('/a'), P('/')])
         with self.assertRaises(IndexError):
+            par[-4]
+        with self.assertRaises(IndexError):
             par[3]
 
     def test_drive_common(self):
diff --git a/Misc/NEWS.d/next/Library/2022-05-26-23-10-55.gh-issue-93156.4XfDVN.rst b/Misc/NEWS.d/next/Library/2022-05-26-23-10-55.gh-issue-93156.4XfDVN.rst
new file mode 100644
index 0000000..165baa0
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2022-05-26-23-10-55.gh-issue-93156.4XfDVN.rst
@@ -0,0 +1,2 @@
+Accessing the :attr:`pathlib.PurePath.parents` sequence of an absolute path
+using negative index values produced incorrect results.