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

Lib/genericpath.py

@@ -8,7 +8,8 @@ import stat
 
 __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()."""