GH-114610: Fix `pathlib.PurePath.with_stem('')` handling of file extensions (#114612)

Raise `ValueError` if `with_stem('')` is called on a path with a file
extension. Paths may only have an empty stem if they also have an empty
suffix.

1 files changed, 9 insertions, 1 deletions

diff --git a/Lib/pathlib/_abc.py b/Lib/pathlib/_abc.py
index 27c6b4e..44fea52 100644
--- a/Lib/pathlib/_abc.py
+++ b/Lib/pathlib/_abc.py
@@ -313,7 +313,14 @@ class PurePathBase:
 
     def with_stem(self, stem):
         """Return a new path with the stem changed."""
-        return self.with_name(stem + self.suffix)
+        suffix = self.suffix
+        if not suffix:
+            return self.with_name(stem)
+        elif not stem:
+            # If the suffix is non-empty, we can't make the stem empty.
+            raise ValueError(f"{self!r} has a non-empty suffix")
+        else:
+            return self.with_name(stem + suffix)
 
     def with_suffix(self, suffix):
         """Return a new path with the file suffix changed.  If the path
@@ -324,6 +331,7 @@ class PurePathBase:
         if not suffix:
             return self.with_name(stem)
         elif not stem:
+            # If the stem is empty, we can't make the suffix non-empty.
             raise ValueError(f"{self!r} has an empty name")
         elif suffix.startswith('.') and len(suffix) > 1:
             return self.with_name(stem + suffix)