GH-113528: Deoptimise pathlib._abc.PurePathBase.parts (#113883)

Implement `parts` using `_stack`, which itself calls `pathmod.split()`
repeatedly. This avoids use of `_tail`, which will be moved to `PurePath`
shortly.

Author: barneygale

Lib/pathlib/__init__.py

@@ -195,6 +195,15 @@ def __ge__(self, other):
             return NotImplemented
         return self._parts_normcase >= other._parts_normcase
 
+    @property
+    def parts(self):
+        """An object providing sequence-like access to the
+        components in the filesystem path."""
+        if self.drive or self.root:
+            return (self.drive + self.root,) + tuple(self._tail)
+        else:
+            return tuple(self._tail)
+
     @property
     def parent(self):
         """The logical parent of the path."""

Lib/pathlib/_abc.py

@@ -381,10 +381,10 @@ def is_relative_to(self, other):
     def parts(self):
         """An object providing sequence-like access to the
         components in the filesystem path."""
-        if self.drive or self.root:
-            return (self.drive + self.root,) + tuple(self._tail)
-        else:
-            return tuple(self._tail)
+        anchor, parts = self._stack
+        if anchor:
+            parts.append(anchor)
+        return tuple(reversed(parts))
 
     def joinpath(self, *pathsegments):
         """Combine this path with one or several arguments, and return a