pythonGH-44626, pythonGH-105476: Fix ntpath.isabs() handling of part-absolute paths.

On Windows, `os.path.isabs()` now returns `False` when given a path that
starts with exactly one (back)slash. This is more compatible with other
functions in `os.path`, and with Microsoft's own documentation.

Also adjust `pathlib.PureWindowsPath.is_absolute()` to call
`ntpath.isabs()`, which corrects its handling of partial UNC/device paths
like `//foo`.

Co-authored-by: Jon Foster <jon@jon-foster.co.uk>

Author: barneygale

Doc/library/os.path.rst

@@ -239,12 +239,16 @@ the :mod:`glob` module.)
 .. function:: isabs(path)
 
    Return ``True`` if *path* is an absolute pathname.  On Unix, that means it
-   begins with a slash, on Windows that it begins with a (back)slash after chopping
-   off a potential drive letter.
+   begins with a slash, on Windows that it begins with two (back)slashes, or a
+   drive letter, colon, and (back)slash together.
 
    .. versionchanged:: 3.6
       Accepts a :term:`path-like object`.
 
+   .. versionchanged:: 3.13
+      On Windows, returns ``False`` if the given path starts with exactly one
+      (back)slash.
+
 
 .. function:: isfile(path)
 

Doc/whatsnew/3.13.rst

@@ -307,6 +307,13 @@ os
   :c:func:`!posix_spawn_file_actions_addclosefrom_np`.
   (Contributed by Jakub Kulik in :gh:`113117`.)
 
+os.path
+-------
+
+* On Windows, :func:`os.path.isabs` no longer considers paths starting with
+  exactly one (back)slash to be absolute.
+  (Contributed by Barney Gale and Jon Foster in :gh:`44626`.)
+
 pathlib
 -------
 

Lib/ntpath.py

@@ -77,29 +77,22 @@ def normcase(s):
         return s.replace('/', '\\').lower()
 
 
-# Return whether a path is absolute.
-# Trivial in Posix, harder on Windows.
-# For Windows it is absolute if it starts with a slash or backslash (current
-# volume), or if a pathname after the volume-letter-and-colon or UNC-resource
-# starts with a slash or backslash.
-
 def isabs(s):
     """Test whether a path is absolute"""
     s = os.fspath(s)
     if isinstance(s, bytes):
         sep = b'\\'
         altsep = b'/'
         colon_sep = b':\\'
+        double_sep = b'\\\\'
     else:
         sep = '\\'
         altsep = '/'
         colon_sep = ':\\'
+        double_sep = '\\\\'
     s = s[:3].replace(altsep, sep)
     # Absolute: UNC, device, and paths with a drive and root.
-    # LEGACY BUG: isabs("/x") should be false since the path has no drive.
-    if s.startswith(sep) or s.startswith(colon_sep, 1):
-        return True
-    return False
+    return s.startswith(colon_sep, 1) or s.startswith(double_sep)
 
 
 # Join two (or more) paths.

Lib/pathlib/_abc.py

@@ -1,5 +1,4 @@
 import functools
-import ntpath
 import posixpath
 import sys
 from errno import ENOENT, ENOTDIR, EBADF, ELOOP, EINVAL
@@ -433,10 +432,7 @@ def parents(self):
     def is_absolute(self):
         """True if the path is absolute (has both a root and, if applicable,
         a drive)."""
-        if self.pathmod is ntpath:
-            # ntpath.isabs() is defective - see GH-44626.
-            return bool(self.drive and self.root)
-        elif self.pathmod is posixpath:
+        if self.pathmod is posixpath:
             # Optimization: work with raw paths on POSIX.
             for path in self._raw_paths:
                 if path.startswith('/'):

Lib/test/test_ntpath.py

@@ -227,10 +227,18 @@ def test_split(self):
         tester('ntpath.split("//conky/mountpoint/")', ('//conky/mountpoint/', ''))
 
     def test_isabs(self):
+        tester('ntpath.isabs("foo\\bar")', 0)
+        tester('ntpath.isabs("foo/bar")', 0)
         tester('ntpath.isabs("c:\\")', 1)
+        tester('ntpath.isabs("c:\\foo\\bar")', 1)
+        tester('ntpath.isabs("c:/foo/bar")', 1)
         tester('ntpath.isabs("\\\\conky\\mountpoint\\")', 1)
-        tester('ntpath.isabs("\\foo")', 1)
-        tester('ntpath.isabs("\\foo\\bar")', 1)
+
+        # gh-44626: paths with only a drive or root are not absolute.
+        tester('ntpath.isabs("\\foo\\bar")', 0)
+        tester('ntpath.isabs("/foo/bar")', 0)
+        tester('ntpath.isabs("c:foo\\bar")', 0)
+        tester('ntpath.isabs("c:foo/bar")', 0)
 
         # gh-96290: normal UNC paths and device paths without trailing backslashes
         tester('ntpath.isabs("\\\\conky\\mountpoint")', 1)

Lib/test/test_pathlib/test_pathlib.py

@@ -970,10 +970,14 @@ def test_is_absolute(self):
         self.assertTrue(P('c:/a').is_absolute())
         self.assertTrue(P('c:/a/b/').is_absolute())
         # UNC paths are absolute by definition.
+        self.assertTrue(P('//').is_absolute())
+        self.assertTrue(P('//a').is_absolute())
         self.assertTrue(P('//a/b').is_absolute())
         self.assertTrue(P('//a/b/').is_absolute())
         self.assertTrue(P('//a/b/c').is_absolute())
         self.assertTrue(P('//a/b/c/d').is_absolute())
+        self.assertTrue(P('//?/UNC/').is_absolute())
+        self.assertTrue(P('//?/UNC/spam').is_absolute())
 
     def test_join(self):
         P = self.cls

Misc/NEWS.d/next/Tests/2024-01-08-21-15-48.gh-issue-44626.DRq-PR.rst

@@ -0,0 +1,5 @@
+Fix :func:`os.path.isabs` incorrectly returning ``True`` when given a path
+that starts with exactly one (back)slash on Windows.
+
+Fix :meth:`pathlib.PureWindowsPath.is_absolute` incorrectly returning
+``False`` for some paths beginning with two (back)slashes.