Handle the case where there are no skipped files properly....

webkit/tools/layout_tests/run_webkit_tests.py

 #!/usr/bin/env python
 # Copyright (c) 2006-2009 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Run layout tests using the test_shell.
 
 This is a port of the existing webkit test script run-webkit-tests.
 
 The TestRunner class runs a series of tests (TestType interface) against a set
@@ skipping 183 matching lines @@
       else:
         raise err
 
   def PrepareListsAndPrintOutput(self):
     """Create appropriate subsets of test lists and returns a ResultSummary
     object. Also prints expected test counts."""
 
     # Remove skipped - both fixable and ignored - files from the
     # top-level list of files to test.
     num_all_test_files = len(self._test_files)
-    skipped = ()
+    skipped = set()
     if num_all_test_files > 1 and not self._options.force:
       skipped = self._expectations.GetTestsWithResultType(
                      test_expectations.SKIP)
       self._test_files -= skipped
 
     # Create a sorted list of test files so the subset chunk, if used, contains
     # alphabetically consecutive tests.
     self._test_files_list = list(self._test_files)
     if self._options.randomize_order:
       random.shuffle(self._test_files_list)
@@ skipping 1242 matching lines @@
                                  "this script."))
   option_parser.add_option("", "--find-baselines", action="store_true",
                            default=False,
                            help="Prints a table mapping tests to their "
                                 "expected results")
   option_parser.add_option("", "--experimental-fully-parallel",
                            action="store_true", default=False,
                            help="run all tests in parallel")
   options, args = option_parser.parse_args()
   main(options, args)