diff -r 57649a9a683c Lib/test/support.py
--- a/Lib/test/support.py	Wed Nov 07 23:20:10 2012 +0100
+++ b/Lib/test/support.py	Thu Nov 08 23:48:31 2012 +0100
@@ -692,7 +692,10 @@ elif sys.platform != 'darwin':
 
 # TESTFN_UNDECODABLE is a filename (bytes type) that should *not* be able to be
 # decoded from the filesystem encoding (in strict mode). It can be None if we
-# cannot generate such filename.
+# cannot generate such filename (ex: the latin1 encoding can decode any byte
+# sequence). On UNIX, TESTFN_UNDECODABLE can be decoded by os.fsdecode() thanks
+# to the surrogateescape error handler (PEP 383), but not from the filesystem
+# encoding in strict mode.
 TESTFN_UNDECODABLE = None
 # b'\xff' is not decodable by os.fsdecode() with code page 932. Windows
 # accepts it to create a file or a directory, or don't accept to enter to
@@ -700,7 +703,7 @@ TESTFN_UNDECODABLE = None
 # not decodable from cp932.
 for name in (b'\xe7w\xf0', b'abc\xff'):
     try:
-        os.fsdecode(name)
+        name.decode(TESTFN_ENCODING)
     except UnicodeDecodeError:
         TESTFN_UNDECODABLE = name
         break
@@ -710,6 +713,9 @@ if FS_NONASCII:
 else:
     TESTFN_NONASCII = None
 
+print("TESTFN_UNDECODABLE = %a" % TESTFN_UNDECODABLE)
+print("TESTFN_NONASCII = %a" % TESTFN_NONASCII)
+
 # Save the initial cwd
 SAVEDCWD = os.getcwd()
 
diff -r 57649a9a683c Lib/test/test_cmd_line_script.py
--- a/Lib/test/test_cmd_line_script.py	Wed Nov 07 23:20:10 2012 +0100
+++ b/Lib/test/test_cmd_line_script.py	Thu Nov 08 23:48:31 2012 +0100
@@ -363,11 +363,22 @@ class CmdLineTest(unittest.TestCase):
             self.assertTrue(text[1].startswith('  File '))
             self.assertTrue(text[3].startswith('NameError'))
 
-    @unittest.skipUnless(support.TESTFN_NONASCII, 'need support.TESTFN_NONASCII')
     def test_non_ascii(self):
+        if (support.TESTFN_UNDECODABLE
+        # Mac OS X denies the creation of a directory with an invalid
+        # UTF-8 name. Windows allows to create a directory with an
+        # arbitrary bytes name, but fails to enter this directory
+        # (when the bytes name is used).
+        and sys.platform not in ('win32', 'darwin')):
+            name = os.fsdecode(support.TESTFN_UNDECODABLE)
+        elif support.TESTFN_NONASCII:
+            name = support.TESTFN_NONASCII
+        else:
+            self.skipTest("need support.TESTFN_NONASCII")
+
         # Issue #16218
         source = 'print(ascii(__file__))\n'
-        script_name = _make_test_script(os.curdir, support.TESTFN_NONASCII, source)
+        script_name = _make_test_script(os.curdir, name, source)
         self.addCleanup(support.unlink, script_name)
         rc, stdout, stderr = assert_python_ok(script_name)
         self.assertEqual(
diff -r 57649a9a683c Lib/test/test_genericpath.py
--- a/Lib/test/test_genericpath.py	Wed Nov 07 23:20:10 2012 +0100
+++ b/Lib/test/test_genericpath.py	Thu Nov 08 23:48:31 2012 +0100
@@ -309,26 +309,17 @@ class CommonTest(GenericTest):
                     self.assertIsInstance(abspath(path), str)
 
     def test_nonascii_abspath(self):
-        # Test non-ASCII in the path
-        if sys.platform in ('win32', 'darwin'):
-            if support.TESTFN_NONASCII:
-                name = support.TESTFN_NONASCII
-            else:
-                # Mac OS X denies the creation of a directory with an invalid
-                # UTF-8 name. Windows allows to create a directory with an
-                # arbitrary bytes name, but fails to enter this directory
-                # (when the bytes name is used).
-                self.skipTest("need support.TESTFN_NONASCII")
+        if (support.TESTFN_UNDECODABLE
+        # Mac OS X denies the creation of a directory with an invalid
+        # UTF-8 name. Windows allows to create a directory with an
+        # arbitrary bytes name, but fails to enter this directory
+        # (when the bytes name is used).
+        and sys.platform not in ('win32', 'darwin')):
+            name = support.TESTFN_UNDECODABLE
+        elif support.TESTFN_NONASCII:
+            name = support.TESTFN_NONASCII
         else:
-            if support.TESTFN_UNDECODABLE:
-                name = support.TESTFN_UNDECODABLE
-            elif support.TESTFN_NONASCII:
-                name = support.TESTFN_NONASCII
-            else:
-                # On UNIX, the surrogateescape error handler is used to
-                # decode paths, so any byte is allowed, it does not depend
-                # on the locale
-                name = b'a\xffb\xe7w\xf0'
+            self.skipTest("need support.TESTFN_NONASCII")
 
         with warnings.catch_warnings():
             warnings.simplefilter("ignore", DeprecationWarning)