Try to be a bit more consistent on all platforms:

python .
  python < .

both print a message, return non-zero and do not core dump.

Lib/test/test_cmd_line.py

@@ -16,19 +16,8 @@ class CmdLineTest(unittest.TestCase):
         return subprocess.call([sys.executable, cmd_line], stderr=subprocess.PIPE)
 
     def test_directories(self):
-        if sys.platform == 'win32':
-            # Exit code for "python .", Error 13: permission denied = 2
-            expected_exit_code = 2
-        elif sys.platform.startswith('freebsd'):
-            # On FreeBSD, it more likely raise SyntaxError for binary
-            # directory data.
-            expected_exit_code = 1
-        else:
-            # Linux has no problem with "python .", Exit code = 0
-            expected_exit_code = 0
-        self.assertEqual(self.exit_code('.'), expected_exit_code)
-
-        self.assertTrue(self.exit_code('< .') != 0)
+        self.assertNotEqual(self.exit_code('.'), 0)
+        self.assertNotEqual(self.exit_code('< .'), 0)
 
     def verify_valid_flag(self, cmd_line):
         data = self.start_python(cmd_line)

Modules/main.c

@@ -364,7 +364,8 @@ Py_Main(int argc, char **argv)
 				struct stat sb;
 				if (fstat(fileno(fp), &sb) == 0 &&
 				    S_ISDIR(sb.st_mode)) {
-					fprintf(stderr, "%s: warning '%s' is a directory\n", argv[0], filename);
+					fprintf(stderr, "%s: '%s' is a directory, cannot continue\n", argv[0], filename);
+					return 1;
 				}
 			}
 		}

Python/sysmodule.c

@@ -1037,7 +1037,10 @@ _PySys_Init(void)
 		struct stat sb;
 		if (fstat(fileno(stdin), &sb) == 0 &&
 		    S_ISDIR(sb.st_mode)) {
-			Py_FatalError("<stdin> is a directory");
+			/* There's nothing more we can do. */
+			/* Py_FatalError() will core dump, so just exit. */
+			PySys_WriteStderr("Python error: <stdin> is a directory, cannot continue\n");
+			exit(EXIT_FAILURE);
 		}
 	}