GH-110488: Fix two small issues in pathlib.PurePath.with_name() (#110651)

Ensure that `PurePath('foo/a').with_name('.')` raises `ValueError`

Ensure that `PureWindowsPath('foo/a').with_name('a:b')` does not raise
`ValueError`.

Author: barneygale

Lib/pathlib.py

@@ -603,8 +603,7 @@ def with_name(self, name):
         if not self.name:
             raise ValueError("%r has an empty name" % (self,))
         m = self.pathmod
-        drv, root, tail = m.splitroot(name)
-        if drv or root or not tail or m.sep in tail or (m.altsep and m.altsep in tail):
+        if not name or m.sep in name or (m.altsep and m.altsep in name) or name == '.':
             raise ValueError("Invalid name %r" % (name))
         return self._from_parsed_parts(self.drive, self.root,
                                        self._tail[:-1] + [name])

Lib/test/test_pathlib.py

@@ -614,6 +614,7 @@ def test_with_name_common(self):
         self.assertRaises(ValueError, P('.').with_name, 'd.xml')
         self.assertRaises(ValueError, P('/').with_name, 'd.xml')
         self.assertRaises(ValueError, P('a/b').with_name, '')
+        self.assertRaises(ValueError, P('a/b').with_name, '.')
         self.assertRaises(ValueError, P('a/b').with_name, '/c')
         self.assertRaises(ValueError, P('a/b').with_name, 'c/')
         self.assertRaises(ValueError, P('a/b').with_name, 'c/d')
@@ -631,6 +632,7 @@ def test_with_stem_common(self):
         self.assertRaises(ValueError, P('.').with_stem, 'd')
         self.assertRaises(ValueError, P('/').with_stem, 'd')
         self.assertRaises(ValueError, P('a/b').with_stem, '')
+        self.assertRaises(ValueError, P('a/b').with_stem, '.')
         self.assertRaises(ValueError, P('a/b').with_stem, '/c')
         self.assertRaises(ValueError, P('a/b').with_stem, 'c/')
         self.assertRaises(ValueError, P('a/b').with_stem, 'c/d')
@@ -1235,8 +1237,10 @@ def test_with_name(self):
         self.assertRaises(ValueError, P('c:').with_name, 'd.xml')
         self.assertRaises(ValueError, P('c:/').with_name, 'd.xml')
         self.assertRaises(ValueError, P('//My/Share').with_name, 'd.xml')
-        self.assertRaises(ValueError, P('c:a/b').with_name, 'd:')
-        self.assertRaises(ValueError, P('c:a/b').with_name, 'd:e')
+        self.assertEqual(str(P('a').with_name('d:')), '.\\d:')
+        self.assertEqual(str(P('a').with_name('d:e')), '.\\d:e')
+        self.assertEqual(P('c:a/b').with_name('d:'), P('c:a/d:'))
+        self.assertEqual(P('c:a/b').with_name('d:e'), P('c:a/d:e'))
         self.assertRaises(ValueError, P('c:a/b').with_name, 'd:/e')
         self.assertRaises(ValueError, P('c:a/b').with_name, '//My/Share')
 
@@ -1249,8 +1253,10 @@ def test_with_stem(self):
         self.assertRaises(ValueError, P('c:').with_stem, 'd')
         self.assertRaises(ValueError, P('c:/').with_stem, 'd')
         self.assertRaises(ValueError, P('//My/Share').with_stem, 'd')
-        self.assertRaises(ValueError, P('c:a/b').with_stem, 'd:')
-        self.assertRaises(ValueError, P('c:a/b').with_stem, 'd:e')
+        self.assertEqual(str(P('a').with_stem('d:')), '.\\d:')
+        self.assertEqual(str(P('a').with_stem('d:e')), '.\\d:e')
+        self.assertEqual(P('c:a/b').with_stem('d:'), P('c:a/d:'))
+        self.assertEqual(P('c:a/b').with_stem('d:e'), P('c:a/d:e'))
         self.assertRaises(ValueError, P('c:a/b').with_stem, 'd:/e')
         self.assertRaises(ValueError, P('c:a/b').with_stem, '//My/Share')
 

Misc/NEWS.d/next/Library/2023-10-10-22-54-56.gh-issue-110488.2I7OiZ.rst

@@ -0,0 +1,4 @@
+Fix a couple of issues in :meth:`pathlib.PurePath.with_name`: a single dot
+was incorrectly considered a valid name, and in :class:`PureWindowsPath`, a
+name with an NTFS alternate data stream, like ``a:b``, was incorrectly
+considered invalid.