gh-73045 Fixed behaviour of os.path.ismount(), for Windows, for non-existent drives and paths

Doc/library/os.path.rst

@@ -299,8 +299,8 @@ the :mod:`glob` module.)
    i-node on the same device --- this should detect mount points for all Unix
    and POSIX variants.  It is not able to reliably detect bind mounts on the
    same filesystem. On Linux systems, it will always return ``True`` for btrfs
-   subvolumes, even if they aren't mount points. On Windows, a drive letter root
-   and a share UNC are always mount points, and for any other path
+   subvolumes, even if they aren't mount points. On Windows, a existent drive
+   letter root and a share UNC are always mount points, and for any other path
    ``GetVolumePathName`` is called to see if it is different from the input path.
 
    .. versionchanged:: 3.4

Lib/ntpath.py

@@ -285,14 +285,19 @@ def ismount(path):
     path = abspath(path)
     drive, root, rest = splitroot(path)
     if drive and drive[0] in seps:
+        # Share path is a mount point if it conforms to UNC.
         return not rest
     if root and not rest:
-        return True
-
+        # Drive root is a mount point if it exists.
+        return exists(path)
     if _getvolumepathname:
-        x = path.rstrip(seps)
-        y =_getvolumepathname(path).rstrip(seps)
-        return x.casefold() == y.casefold()
+        try:
+            # The path is a mount point if it's a mounted volume.
+            x = path.rstrip(seps)
+            y = _getvolumepathname(path).rstrip(seps)
+            return x.casefold() == y.casefold()
+        except (OSError, FileNotFoundError):
+            return False
     else:
         return False
 

Lib/test/test_ntpath.py

@@ -1354,19 +1354,31 @@ def test_sameopenfile(self):
                     ntpath.sameopenfile(-1, -1)
 
     def test_ismount(self):
-        self.assertTrue(ntpath.ismount("c:\\"))
-        self.assertTrue(ntpath.ismount("C:\\"))
-        self.assertTrue(ntpath.ismount("c:/"))
-        self.assertTrue(ntpath.ismount("C:/"))
-        self.assertTrue(ntpath.ismount("\\\\.\\c:\\"))
-        self.assertTrue(ntpath.ismount("\\\\.\\C:\\"))
-
-        self.assertTrue(ntpath.ismount(b"c:\\"))
-        self.assertTrue(ntpath.ismount(b"C:\\"))
-        self.assertTrue(ntpath.ismount(b"c:/"))
-        self.assertTrue(ntpath.ismount(b"C:/"))
-        self.assertTrue(ntpath.ismount(b"\\\\.\\c:\\"))
-        self.assertTrue(ntpath.ismount(b"\\\\.\\C:\\"))
+        if sys.platform == "win32":
+            self.assertTrue(ntpath.ismount("c:\\"))
+            self.assertTrue(ntpath.ismount("C:\\"))
+            self.assertTrue(ntpath.ismount("c:/"))
+            self.assertTrue(ntpath.ismount("C:/"))
+            self.assertTrue(ntpath.ismount("\\\\.\\c:\\"))
+            self.assertTrue(ntpath.ismount("\\\\.\\C:\\"))
+
+            self.assertTrue(ntpath.ismount(b"c:\\"))
+            self.assertTrue(ntpath.ismount(b"C:\\"))
+            self.assertTrue(ntpath.ismount(b"c:/"))
+            self.assertTrue(ntpath.ismount(b"C:/"))
+            self.assertTrue(ntpath.ismount(b"\\\\.\\c:\\"))
+            self.assertTrue(ntpath.ismount(b"\\\\.\\C:\\"))
+
+            # Look for a non-existent drive letter that can be used to test
+            # behaviour of ismount().
+            for drive in "DEFGHIJKLMNOPQRSTUVWXYZ":
+                if not ntpath.exists(drive + ":\\"):
+                    self.assertFalse(ntpath.ismount(drive + ":\\"))
+                    self.assertFalse(ntpath.ismount(drive + ":\\NotExist"))
+                    break
+                else:
+                    if support.verbose:
+                        print("No missing drive found to test 'ismount'")
 
         with os_helper.temp_dir() as d:
             self.assertFalse(ntpath.ismount(d))

Misc/ACKS

@@ -498,6 +498,7 @@ Eugene Dvurechenski
 Karmen Dykstra
 Josip Dzolonga
 Maxim Dzumanenko
+Thomas Earp
 Hans Eckardt
 Rodolpho Eckhardt
 Ulrich Eckhardt

Misc/NEWS.d/next/Library/2025-10-10-16-13-00.gh-issue-73045.4ks8g7.rst

@@ -0,0 +1 @@
+For Windows, os.path.ismount() now returns False for non-existent drive letter roots.