GH-113528: Deoptimise `pathlib._abc.PurePathBase` (#113559)

Apply pathlib's normalization and performance tuning in `pathlib.PurePath`, but not `pathlib._abc.PurePathBase`.

With this change, the pathlib ABCs do not normalize away alternate path separators, empty segments, or dot segments. A single string given to the initialiser will round-trip by default, i.e. `str(PurePathBase(my_string)) == my_string`. Implementors can set their own path domain-specific normalization scheme by overriding `__str__()`

Eliminating path normalization makes maintaining and caching the path's parts and string representation both optional and not very useful, so this commit moves the `_drv`, `_root`, `_tail_cached` and `_str` slots from `PurePathBase` to `PurePath`. Only `_raw_paths` and `_resolving` slots remain in `PurePathBase`. This frees the ABCs from the burden of some of pathlib's hardest-to-understand code.

1 files changed, 41 insertions, 0 deletions

diff --git a/Lib/test/test_pathlib/test_pathlib.py b/Lib/test/test_pathlib/test_pathlib.py
index 6e42122..04e6280 100644
--- a/Lib/test/test_pathlib/test_pathlib.py
+++ b/Lib/test/test_pathlib/test_pathlib.py
@@ -140,6 +140,8 @@ class PurePathTest(test_pathlib_abc.DummyPurePathTest):
         # The empty path points to '.'
         p = self.cls('')
         self.assertEqual(str(p), '.')
+        # Special case for the empty path.
+        self._check_str('.', ('',))
 
     def test_parts_interning(self):
         P = self.cls
@@ -300,6 +302,40 @@ class PurePathTest(test_pathlib_abc.DummyPurePathTest):
                 self.assertEqual(q, p)
                 self.assertEqual(repr(q), r)
 
+    def test_name_empty(self):
+        P = self.cls
+        self.assertEqual(P('').name, '')
+        self.assertEqual(P('.').name, '')
+        self.assertEqual(P('/a/b/.').name, 'b')
+
+    def test_stem_empty(self):
+        P = self.cls
+        self.assertEqual(P('').stem, '')
+        self.assertEqual(P('.').stem, '')
+
+    def test_with_name_empty(self):
+        P = self.cls
+        self.assertRaises(ValueError, P('').with_name, 'd.xml')
+        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, '.')
+
+    def test_with_stem_empty(self):
+        P = self.cls
+        self.assertRaises(ValueError, P('').with_stem, 'd')
+        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, '.')
+
+    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')
@@ -313,6 +349,11 @@ class PurePathTest(test_pathlib_abc.DummyPurePathTest):
         with self.assertWarns(DeprecationWarning):
             p.is_relative_to('a', 'b')
 
+    def test_match_empty(self):
+        P = self.cls
+        self.assertRaises(ValueError, P('a').match, '')
+        self.assertRaises(ValueError, P('a').match, '.')
+
 
 class PurePosixPathTest(PurePathTest):
     cls = pathlib.PurePosixPath