gh-112405: Optimise pathlib.Path.relative_to (#112406)

Author: AlexWaygood

Lib/pathlib.py

@@ -14,6 +14,7 @@
 import warnings
 from _collections_abc import Sequence
 from errno import ENOENT, ENOTDIR, EBADF, ELOOP, EINVAL
+from itertools import chain
 from stat import S_ISDIR, S_ISLNK, S_ISREG, S_ISSOCK, S_ISBLK, S_ISCHR, S_ISFIFO
 
 try:
@@ -445,7 +446,7 @@ def relative_to(self, other, /, *_deprecated, walk_up=False):
             other = self.with_segments(other, *_deprecated)
         elif not isinstance(other, PurePath):
             other = self.with_segments(other)
-        for step, path in enumerate([other] + list(other.parents)):
+        for step, path in enumerate(chain([other], other.parents)):
             if path == self or path in self.parents:
                 break
             elif not walk_up:

Misc/NEWS.d/next/Library/2023-11-25-20-29-28.gh-issue-112405.cOtzxC.rst

@@ -0,0 +1 @@
+Optimize :meth:`pathlib.PurePath.relative_to`. Patch by Alex Waygood.