Issue #27186: Update os.fspath()/PyOS_FSPath() to check the return

type of __fspath__().

As part of this change, also make sure that the pure Python
implementation of os.fspath() is tested.

Lib/test/test_os.py

@@ -3112,55 +3112,59 @@ class TestScandir(unittest.TestCase):
 
 
 class TestPEP519(unittest.TestCase):
-    "os.fspath()"
+
+    # Abstracted so it can be overridden to test pure Python implementation
+    # if a C version is provided.
+    fspath = staticmethod(os.fspath)
+
+    class PathLike:
+        def __init__(self, path=''):
+            self.path = path
+        def __fspath__(self):
+            return self.path
 
     def test_return_bytes(self):
         for b in b'hello', b'goodbye', b'some/path/and/file':
-            self.assertEqual(b, os.fspath(b))
+            self.assertEqual(b, self.fspath(b))
 
     def test_return_string(self):
         for s in 'hello', 'goodbye', 'some/path/and/file':
-            self.assertEqual(s, os.fspath(s))
-
-    def test_fsencode_fsdecode_return_pathlike(self):
-        class PathLike:
-            def __init__(self, path):
-                self.path = path
-            def __fspath__(self):
-                return self.path
+            self.assertEqual(s, self.fspath(s))
 
+    def test_fsencode_fsdecode(self):
         for p in "path/like/object", b"path/like/object":
-            pathlike = PathLike(p)
+            pathlike = self.PathLike(p)
 
-            self.assertEqual(p, os.fspath(pathlike))
+            self.assertEqual(p, self.fspath(pathlike))
             self.assertEqual(b"path/like/object", os.fsencode(pathlike))
             self.assertEqual("path/like/object", os.fsdecode(pathlike))
 
-    def test_fspathlike(self):
-        class PathLike:
-            def __init__(self, path=''):
-                self.path = path
-            def __fspath__(self):
-                return self.path
+    def test_pathlike(self):
+        self.assertEqual('#feelthegil', self.fspath(self.PathLike('#feelthegil')))
+        self.assertTrue(issubclass(self.PathLike, os.PathLike))
+        self.assertTrue(isinstance(self.PathLike(), os.PathLike))
 
-        self.assertEqual('#feelthegil', os.fspath(PathLike('#feelthegil')))
-        self.assertTrue(issubclass(PathLike, os.PathLike))
-        self.assertTrue(isinstance(PathLike(), os.PathLike))
-
-        message = 'expected str, bytes or os.PathLike object, not'
-        for fn in (os.fsencode, os.fsdecode):
-            for obj in PathLike(None), None:
-                with self.assertRaisesRegex(TypeError, message):
-                    fn(obj)
+        with self.assertRaises(TypeError):
+            self.fspath(self.PathLike(42))
 
     def test_garbage_in_exception_out(self):
         vapor = type('blah', (), {})
         for o in int, type, os, vapor():
-            self.assertRaises(TypeError, os.fspath, o)
+            self.assertRaises(TypeError, self.fspath, o)
 
     def test_argument_required(self):
         with self.assertRaises(TypeError):
-            os.fspath()
+            self.fspath()
+
+
+# Only test if the C version is provided, otherwise TestPEP519 already tested
+# the pure Python implementation.
+if hasattr(os, "_fspath"):
+    class TestPEP519PurePython(TestPEP519):
+
+        """Explicitly test the pure Python implementation of os.fspath()."""
+
+        fspath = staticmethod(os._fspath)
 
 
 if __name__ == "__main__":