gh-71189: Support all-but-last mode in os.path.realpath() (GH-117562)

Author: serhiy-storchaka

Doc/library/os.path.rst

@@ -424,6 +424,8 @@ the :mod:`glob` module.)
    re-raised.
    In particular, :exc:`FileNotFoundError` is raised if *path* does not exist,
    or another :exc:`OSError` if it is otherwise inaccessible.
+   If *strict* is :data:`ALL_BUT_LAST`, the last component of the path
+   is allowed to be missing, but all other errors are raised.
 
    If *strict* is :py:data:`os.path.ALLOW_MISSING`, errors other than
    :exc:`FileNotFoundError` are re-raised (as with ``strict=True``).
@@ -448,15 +450,22 @@ the :mod:`glob` module.)
       The *strict* parameter was added.
 
    .. versionchanged:: next
-      The :py:data:`~os.path.ALLOW_MISSING` value for the *strict* parameter
-      was added.
+      The :data:`ALL_BUT_LAST` and :data:`ALLOW_MISSING` values for
+      the *strict* parameter was added.
+
+.. data:: ALL_BUT_LAST
+
+   Special value used for the *strict* argument in :func:`realpath`.
+
+   .. versionadded:: next
 
 .. data:: ALLOW_MISSING
 
    Special value used for the *strict* argument in :func:`realpath`.
 
    .. versionadded:: next
 
+
 .. function:: relpath(path, start=os.curdir)
 
    Return a relative filepath to *path* either from the current directory or

Doc/whatsnew/3.15.rst

@@ -264,13 +264,15 @@ math
 os.path
 -------
 
+* Add support of the all-but-last mode in :func:`~os.path.realpath`.
+  (Contributed by Serhiy Storchaka in :gh:`71189`.)
+
 * The *strict* parameter to :func:`os.path.realpath` accepts a new value,
   :data:`os.path.ALLOW_MISSING`.
   If used, errors other than :exc:`FileNotFoundError` will be re-raised;
   the resulting path can be missing but it will be free of symlinks.
   (Contributed by Petr Viktorin for :cve:`2025-4517`.)
 
-
 shelve
 ------
 

Lib/genericpath.py

@@ -8,7 +8,8 @@
 
 __all__ = ['commonprefix', 'exists', 'getatime', 'getctime', 'getmtime',
            'getsize', 'isdevdrive', 'isdir', 'isfile', 'isjunction', 'islink',
-           'lexists', 'samefile', 'sameopenfile', 'samestat', 'ALLOW_MISSING']
+           'lexists', 'samefile', 'sameopenfile', 'samestat',
+           'ALL_BUT_LAST', 'ALLOW_MISSING']
 
 
 # Does a path exist?
@@ -190,7 +191,17 @@ def _check_arg_types(funcname, *args):
     if hasstr and hasbytes:
         raise TypeError("Can't mix strings and bytes in path components") from None
 
-# A singleton with a true boolean value.
+
+# Singletons with a true boolean value.
+
+@object.__new__
+class ALL_BUT_LAST:
+    """Special value for use in realpath()."""
+    def __repr__(self):
+        return 'os.path.ALL_BUT_LAST'
+    def __reduce__(self):
+        return self.__class__.__name__
+
 @object.__new__
 class ALLOW_MISSING:
     """Special value for use in realpath()."""

Lib/ntpath.py

@@ -29,7 +29,7 @@
            "abspath","curdir","pardir","sep","pathsep","defpath","altsep",
            "extsep","devnull","realpath","supports_unicode_filenames","relpath",
            "samefile", "sameopenfile", "samestat", "commonpath", "isjunction",
-           "isdevdrive", "ALLOW_MISSING"]
+           "isdevdrive", "ALL_BUT_LAST", "ALLOW_MISSING"]
 
 def _get_bothseps(path):
     if isinstance(path, bytes):
@@ -726,7 +726,8 @@ def realpath(path, /, *, strict=False):
 
         if strict is ALLOW_MISSING:
             ignored_error = FileNotFoundError
-            strict = True
+        elif strict is ALL_BUT_LAST:
+            ignored_error = FileNotFoundError
         elif strict:
             ignored_error = ()
         else:
@@ -746,6 +747,12 @@ def realpath(path, /, *, strict=False):
                 raise OSError(str(ex)) from None
             path = normpath(path)
         except ignored_error as ex:
+            if strict is ALL_BUT_LAST:
+                dirname, basename = split(path)
+                if not basename:
+                    dirname, basename = split(path)
+                if not isdir(dirname):
+                    raise
             initial_winerror = ex.winerror
             path = _getfinalpathname_nonstrict(path,
                                                ignored_error=ignored_error)

Lib/posixpath.py

@@ -36,7 +36,8 @@
            "samefile","sameopenfile","samestat",
            "curdir","pardir","sep","pathsep","defpath","altsep","extsep",
            "devnull","realpath","supports_unicode_filenames","relpath",
-           "commonpath", "isjunction","isdevdrive","ALLOW_MISSING"]
+           "commonpath","isjunction","isdevdrive",
+           "ALL_BUT_LAST","ALLOW_MISSING"]
 
 
 def _get_sep(path):
@@ -404,7 +405,8 @@ def realpath(filename, /, *, strict=False):
         getcwd = os.getcwd
     if strict is ALLOW_MISSING:
         ignored_error = FileNotFoundError
-        strict = True
+    elif strict is ALL_BUT_LAST:
+        ignored_error = FileNotFoundError
     elif strict:
         ignored_error = ()
     else:
@@ -418,7 +420,7 @@ def realpath(filename, /, *, strict=False):
     # indicates that a symlink target has been resolved, and that the original
     # symlink path can be retrieved by popping again. The [::-1] slice is a
     # very fast way of spelling list(reversed(...)).
-    rest = filename.split(sep)[::-1]
+    rest = filename.rstrip(sep).split(sep)[::-1]
 
     # Number of unprocessed parts in 'rest'. This can differ from len(rest)
     # later, because 'rest' might contain markers for unresolved symlinks.
@@ -427,6 +429,7 @@ def realpath(filename, /, *, strict=False):
     # The resolved path, which is absolute throughout this function.
     # Note: getcwd() returns a normalized and symlink-free path.
     path = sep if filename.startswith(sep) else getcwd()
+    trailing_sep = filename.endswith(sep)
 
     # Mapping from symlink paths to *fully resolved* symlink targets. If a
     # symlink is encountered but not yet resolved, the value is None. This is
@@ -459,7 +462,8 @@ def realpath(filename, /, *, strict=False):
         try:
             st_mode = lstat(newpath).st_mode
             if not stat.S_ISLNK(st_mode):
-                if strict and part_count and not stat.S_ISDIR(st_mode):
+                if (strict and (part_count or trailing_sep)
+                    and not stat.S_ISDIR(st_mode)):
                     raise OSError(errno.ENOTDIR, os.strerror(errno.ENOTDIR),
                                   newpath)
                 path = newpath
@@ -486,7 +490,8 @@ def realpath(filename, /, *, strict=False):
                 continue
             target = readlink(newpath)
         except ignored_error:
-            pass
+            if strict is ALL_BUT_LAST and part_count:
+                raise
         else:
             # Resolve the symbolic link
             if target.startswith(sep):

Lib/test/test_genericpath.py

@@ -2,8 +2,10 @@
 Tests common to genericpath, ntpath and posixpath
 """
 
+import copy
 import genericpath
 import os
+import pickle
 import sys
 import unittest
 import warnings
@@ -320,6 +322,21 @@ def test_sameopenfile(self):
                 fd2 = fp2.fileno()
                 self.assertTrue(self.pathmodule.sameopenfile(fd1, fd2))
 
+    def test_realpath_mode_values(self):
+        for name in 'ALL_BUT_LAST', 'ALLOW_MISSING':
+            with self.subTest(name):
+                mode = getattr(self.pathmodule, name)
+                self.assertEqual(repr(mode), 'os.path.' + name)
+                self.assertEqual(str(mode), 'os.path.' + name)
+                self.assertTrue(mode)
+                self.assertIs(copy.copy(mode), mode)
+                self.assertIs(copy.deepcopy(mode), mode)
+                for proto in range(pickle.HIGHEST_PROTOCOL+1):
+                    with self.subTest(protocol=proto):
+                        pickled = pickle.dumps(mode, proto)
+                        unpickled = pickle.loads(pickled)
+                        self.assertIs(unpickled, mode)
+
 
 class TestGenericTest(GenericTest, unittest.TestCase):
     # Issue 16852: GenericTest can't inherit from unittest.TestCase