GH-114610: Fix pathlib._abc.PurePathBase.with_suffix('.ext') handling of stems (#114613)

Raise `ValueError` if `with_suffix('.ext')` is called on a path without a
stem. Paths may only have a non-empty suffix if they also have a non-empty
stem.

ABC-only bugfix; no effect on public classes.

Lib/pathlib/_abc.py

@@ -299,10 +299,13 @@ class PurePathBase:
         has no suffix, add given suffix.  If the given suffix is an empty
         string, remove the suffix from the path.
         """
+        stem = self.stem
         if not suffix:
-            return self.with_name(self.stem)
+            return self.with_name(stem)
+        elif not stem:
+            raise ValueError(f"{self!r} has an empty name")
         elif suffix.startswith('.') and len(suffix) > 1:
-            return self.with_name(self.stem + suffix)
+            return self.with_name(stem + suffix)
         else:
             raise ValueError(f"Invalid suffix {suffix!r}")
 

Lib/test/test_pathlib/test_pathlib.py

@@ -327,13 +327,6 @@ class PurePathTest(test_pathlib_abc.DummyPurePathTest):
         self.assertRaises(ValueError, P('a/b').with_stem, '')
         self.assertRaises(ValueError, P('a/b').with_stem, '.')
 
-    def test_with_suffix_empty(self):
-        # Path doesn't have a "filename" component.
-        P = self.cls
-        self.assertRaises(ValueError, P('').with_suffix, '.gz')
-        self.assertRaises(ValueError, P('.').with_suffix, '.gz')
-        self.assertRaises(ValueError, P('/').with_suffix, '.gz')
-
     def test_relative_to_several_args(self):
         P = self.cls
         p = P('a/b')

Lib/test/test_pathlib/test_pathlib_abc.py

@@ -977,9 +977,8 @@ class DummyPurePathTest(unittest.TestCase):
     def test_with_suffix_empty(self):
         P = self.cls
         # Path doesn't have a "filename" component.
-        self.assertEqual(P('').with_suffix('.gz'), P('.gz'))
-        self.assertEqual(P('.').with_suffix('.gz'), P('..gz'))
-        self.assertEqual(P('/').with_suffix('.gz'), P('/.gz'))
+        self.assertRaises(ValueError, P('').with_suffix, '.gz')
+        self.assertRaises(ValueError, P('/').with_suffix, '.gz')
 
     def test_with_suffix_seps(self):
         P = self.cls