gh-88569: add `ntpath.isreserved()` (#95486)

Add `ntpath.isreserved()`, which identifies reserved pathnames such as "NUL", "AUX" and "CON".

Deprecate `pathlib.PurePath.is_reserved()`.

---------

Co-authored-by: Eryk Sun <eryksun@gmail.com>
Co-authored-by: Brett Cannon <brett@python.org>
Co-authored-by: Steve Dower <steve.dower@microsoft.com>

1 files changed, 7 insertions, 21 deletions

diff --git a/Lib/pathlib/__init__.py b/Lib/pathlib/__init__.py
index eee82ef..cc159ed 100644
--- a/Lib/pathlib/__init__.py
+++ b/Lib/pathlib/__init__.py
@@ -33,15 +33,6 @@ __all__ = [
     ]
 
 
-# Reference for Windows paths can be found at
-# https://learn.microsoft.com/en-gb/windows/win32/fileio/naming-a-file .
-_WIN_RESERVED_NAMES = frozenset(
-    {'CON', 'PRN', 'AUX', 'NUL', 'CONIN$', 'CONOUT$'} |
-    {f'COM{c}' for c in '123456789\xb9\xb2\xb3'} |
-    {f'LPT{c}' for c in '123456789\xb9\xb2\xb3'}
-)
-
-
 class _PathParents(Sequence):
     """This object provides sequence-like access to the logical ancestors
     of a path.  Don't try to construct it yourself."""
@@ -433,18 +424,13 @@ class PurePath(_abc.PurePathBase):
     def is_reserved(self):
         """Return True if the path contains one of the special names reserved
         by the system, if any."""
-        if self.pathmod is not ntpath or not self.name:
-            return False
-
-        # NOTE: the rules for reserved names seem somewhat complicated
-        # (e.g. r"..\NUL" is reserved but not r"foo\NUL" if "foo" does not
-        # exist). We err on the side of caution and return True for paths
-        # which are not considered reserved by Windows.
-        if self.drive.startswith('\\\\'):
-            # UNC paths are never reserved.
-            return False
-        name = self.name.partition('.')[0].partition(':')[0].rstrip(' ')
-        return name.upper() in _WIN_RESERVED_NAMES
+        msg = ("pathlib.PurePath.is_reserved() is deprecated and scheduled "
+               "for removal in Python 3.15. Use os.path.isreserved() to "
+               "detect reserved paths on Windows.")
+        warnings.warn(msg, DeprecationWarning, stacklevel=2)
+        if self.pathmod is ntpath:
+            return self.pathmod.isreserved(self)
+        return False
 
     def as_uri(self):
         """Return the path as a URI."""