Improve ALL_BUT_LAST.

Author: serhiy-storchaka

Lib/genericpath.py

@@ -8,7 +8,7 @@
 
 __all__ = ['commonprefix', 'exists', 'getatime', 'getctime', 'getmtime',
            'getsize', 'isdevdrive', 'isdir', 'isfile', 'isjunction', 'islink',
-           'lexists', 'samefile', 'sameopenfile', 'samestat']
+           'lexists', 'samefile', 'sameopenfile', 'samestat', 'ALL_BUT_LAST']
 
 
 # Does a path exist?
@@ -189,3 +189,13 @@ def _check_arg_types(funcname, *args):
                             f'os.PathLike object, not {s.__class__.__name__!r}') from None
     if hasstr and hasbytes:
         raise TypeError("Can't mix strings and bytes in path components") from None
+
+
+# A singleton with a true boolean value.
+@object.__new__
+class ALL_BUT_LAST:
+    """Special value for use in realpath()."""
+    def __repr__(self):
+        return self.__class__.__name__
+    def __reduce__(self):
+        return self.__class__.__name__

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"]
+           "isdevdrive", 'ALL_BUT_LAST']
 
 def _get_bothseps(path):
     if isinstance(path, bytes):
@@ -597,9 +597,6 @@ def abspath(path):
                 path = join(getcwd(), path)
         return normpath(path)
 
-# A singleton with true boolean value
-ALL_BUT_LAST = ['ALL_BUT_LAST']
-
 try:
     from nt import _findfirstfile, _getfinalpathname, readlink as _nt_readlink
 except ImportError:

Lib/posixpath.py

@@ -36,7 +36,7 @@
            "samefile","sameopenfile","samestat",
            "curdir","pardir","sep","pathsep","defpath","altsep","extsep",
            "devnull","realpath","supports_unicode_filenames","relpath",
-           "commonpath", "isjunction","isdevdrive"]
+           "commonpath", "isjunction","isdevdrive", 'ALL_BUT_LAST']
 
 
 def _get_sep(path):
@@ -388,9 +388,6 @@ def abspath(path):
 # Return a canonical path (i.e. the absolute location of a file on the
 # filesystem).
 
-# A singleton with true boolean value
-ALL_BUT_LAST = ['ALL_BUT_LAST']
-
 def realpath(filename, *, strict=False):
     """Return the canonical path of the specified filename, eliminating any
 symbolic links encountered in the path."""

Lib/test/test_genericpath.py

@@ -320,6 +320,20 @@ def test_sameopenfile(self):
                 fd2 = fp2.fileno()
                 self.assertTrue(self.pathmodule.sameopenfile(fd1, fd2))
 
+    def test_all_but_last(self):
+        ALL_BUT_LAST = self.pathmodule.ALL_BUT_LAST
+        self.assertEqual(repr(ALL_BUT_LAST), 'ALL_BUT_LAST')
+        self.assertTrue(ALL_BUT_LAST)
+        import copy
+        self.assertIs(copy.copy(ALL_BUT_LAST), ALL_BUT_LAST)
+        self.assertIs(copy.deepcopy(ALL_BUT_LAST), ALL_BUT_LAST)
+        import pickle
+        for proto in range(pickle.HIGHEST_PROTOCOL+1):
+            with self.subTest(protocol=proto):
+                pickled = pickle.dumps(ALL_BUT_LAST, proto)
+                unpickled = pickle.loads(pickled)
+                self.assertIs(unpickled, ALL_BUT_LAST)
+
 
 class TestGenericTest(GenericTest, unittest.TestCase):
     # Issue 16852: GenericTest can't inherit from unittest.TestCase