bpo-23082: Better error message for PurePath.relative_to() from pathlib

Doc/library/pathlib.rst

@@ -551,7 +551,9 @@ Pure paths provide the following methods and properties:
         File "<stdin>", line 1, in <module>
         File "pathlib.py", line 694, in relative_to
           .format(str(self), str(formatted)))
-      ValueError: '/etc/passwd' does not start with '/usr'
+      ValueError: '/etc/passwd' is not in the subpath of '/usr' OR one path is relative and the other absolute.
+
+   NOTE: This function is part of :class:`PurePath` and works with strings. It does not check or access the underlying file structure.
 
 
 .. method:: PurePath.with_name(name)

Lib/pathlib.py

@@ -922,7 +922,8 @@ def relative_to(self, *other):
         cf = self._flavour.casefold_parts
         if (root or drv) if n == 0 else cf(abs_parts[:n]) != cf(to_abs_parts):
             formatted = self._format_parsed_parts(to_drv, to_root, to_parts)
-            raise ValueError("{!r} does not start with {!r}"
+            raise ValueError("{!r} is not in the subpath of {!r}"
+                    " OR one path is relative and the other is absolute."
                              .format(str(self), str(formatted)))
         return self._from_parsed_parts('', root if n == 1 else '',
                                        abs_parts[n:])

Misc/NEWS.d/next/Library/2020-04-20-22-08-36.bpo-23082.iX90Id.rst

@@ -0,0 +1 @@
+Updated the error message and docs of PurePath.relative_to() to better reflect the function behaviour.