pythonGH-110109: pathlib ABCs: do not vary path syntax by host OS.

Change the value of `pathlib._abc.PurePathBase.pathmod` from `os.path` to
`posixpath`.

User subclasses of `PurePathBase` and `PathBase` previously used the host
OS's path syntax, e.g. backslashes as separators on Windows. This is wrong
in most use cases, and likely to catch developers out unless they test on
both Windows and non-Windows machines.

In this patch we change the default to POSIX syntax, regardless of OS. This
is somewhat arguable (why not make all aspects of syntax abstract and
individually configurable?) but an improvement all the same.

This change has no effect on `PurePath`, `Path`, nor their subclasses. Only
private APIs are affected.

Lib/pathlib/__init__.py

@@ -59,6 +59,7 @@ class PurePath(_abc.PurePathBase):
         # path. It's set when `__hash__()` is called for the first time.
         '_hash',
     )
+    pathmod = os.path
 
     def __new__(cls, *args, **kwargs):
         """Construct a PurePath from one or several strings and or existing

Lib/pathlib/_abc.py

@@ -1,7 +1,6 @@
 import functools
 import io
 import ntpath
-import os
 import posixpath
 import sys
 import warnings
@@ -204,7 +203,7 @@ class PurePathBase:
         # work from occurring when `resolve()` calls `stat()` or `readlink()`.
         '_resolving',
     )
-    pathmod = os.path
+    pathmod = posixpath
 
     def __init__(self, *paths):
         self._raw_paths = paths

Lib/test/test_pathlib/test_pathlib.py

@@ -2,8 +2,10 @@
 import os
 import sys
 import errno
+import ntpath
 import pathlib
 import pickle
+import posixpath
 import socket
 import stat
 import tempfile
@@ -44,6 +46,47 @@
 class PurePathTest(test_pathlib_abc.DummyPurePathTest):
     cls = pathlib.PurePath
 
+    def test_concrete_class(self):
+        if self.cls is pathlib.PurePath:
+            expected = pathlib.PureWindowsPath if os.name == 'nt' else pathlib.PurePosixPath
+        else:
+            expected = self.cls
+        p = self.cls('a')
+        self.assertIs(type(p), expected)
+
+    def test_concrete_pathmod(self):
+        if self.cls is pathlib.PurePosixPath:
+            expected = posixpath
+        elif self.cls is pathlib.PureWindowsPath:
+            expected = ntpath
+        else:
+            expected = os.path
+        p = self.cls('a')
+        self.assertIs(p.pathmod, expected)
+
+    def test_different_pathmods_unequal(self):
+        p = self.cls('a')
+        if p.pathmod is posixpath:
+            q = pathlib.PureWindowsPath('a')
+        else:
+            q = pathlib.PurePosixPath('a')
+        self.assertNotEqual(p, q)
+
+    def test_different_pathmods_unordered(self):
+        p = self.cls('a')
+        if p.pathmod is posixpath:
+            q = pathlib.PureWindowsPath('a')
+        else:
+            q = pathlib.PurePosixPath('a')
+        with self.assertRaises(TypeError):
+            p < q
+        with self.assertRaises(TypeError):
+            p <= q
+        with self.assertRaises(TypeError):
+            p > q
+        with self.assertRaises(TypeError):
+            p >= q
+
     def test_constructor_nested(self):
         P = self.cls
         P(FakePath("a/b/c"))

Lib/test/test_pathlib/test_pathlib_abc.py

@@ -48,6 +48,9 @@ def test_magic_methods(self):
         self.assertIs(P.__gt__, object.__gt__)
         self.assertIs(P.__ge__, object.__ge__)
 
+    def test_pathmod(self):
+        self.assertIs(self.cls.pathmod, posixpath)
+
 
 class DummyPurePath(pathlib._abc.PurePathBase):
     def __eq__(self, other):
@@ -93,37 +96,6 @@ def test_constructor_common(self):
         P('a/b/c')
         P('/a/b/c')
 
-    def test_concrete_class(self):
-        if self.cls is pathlib.PurePath:
-            expected = pathlib.PureWindowsPath if os.name == 'nt' else pathlib.PurePosixPath
-        else:
-            expected = self.cls
-        p = self.cls('a')
-        self.assertIs(type(p), expected)
-
-    def test_different_pathmods_unequal(self):
-        p = self.cls('a')
-        if p.pathmod is posixpath:
-            q = pathlib.PureWindowsPath('a')
-        else:
-            q = pathlib.PurePosixPath('a')
-        self.assertNotEqual(p, q)
-
-    def test_different_pathmods_unordered(self):
-        p = self.cls('a')
-        if p.pathmod is posixpath:
-            q = pathlib.PureWindowsPath('a')
-        else:
-            q = pathlib.PurePosixPath('a')
-        with self.assertRaises(TypeError):
-            p < q
-        with self.assertRaises(TypeError):
-            p <= q
-        with self.assertRaises(TypeError):
-            p > q
-        with self.assertRaises(TypeError):
-            p >= q
-
     def _check_str_subclass(self, *args):
         # Issue #21127: it should be possible to construct a PurePath object
         # from a str subclass instance, and it then gets converted to
@@ -1420,7 +1392,7 @@ def test_resolve_common(self):
         self._check_resolve_relative(p, P(BASE, 'dirB', 'fileB', 'foo', 'in',
                                           'spam'), False)
         p = P(BASE, 'dirA', 'linkC', '..', 'foo', 'in', 'spam')
-        if os.name == 'nt' and isinstance(p, pathlib.Path):
+        if self.cls.pathmod is not posixpath:
             # In Windows, if linkY points to dirB, 'dirA\linkY\..'
             # resolves to 'dirA' without resolving linkY first.
             self._check_resolve_relative(p, P(BASE, 'dirA', 'foo', 'in',
@@ -1440,7 +1412,7 @@ def test_resolve_common(self):
         self._check_resolve_relative(p, P(BASE, 'dirB', 'foo', 'in', 'spam'),
                                      False)
         p = P(BASE, 'dirA', 'linkX', 'linkY', '..', 'foo', 'in', 'spam')
-        if os.name == 'nt' and isinstance(p, pathlib.Path):
+        if self.cls.pathmod is not posixpath:
             # In Windows, if linkY points to dirB, 'dirA\linkY\..'
             # resolves to 'dirA' without resolving linkY first.
             self._check_resolve_relative(p, P(d, 'foo', 'in', 'spam'), False)
@@ -1473,7 +1445,7 @@ def _check_symlink_loop(self, *args):
     def test_resolve_loop(self):
         if not self.can_symlink:
             self.skipTest("symlinks required")
-        if os.name == 'nt' and issubclass(self.cls, pathlib.Path):
+        if self.cls.pathmod is not posixpath:
             self.skipTest("symlink loops work differently with concrete Windows paths")
         # Loops with relative symlinks.
         self.cls(BASE, 'linkX').symlink_to('linkX/inside')