Making Stacktrace immutable, so it can be hashable

appengine/findit/crash/test/changelist_classifier_test.py

 # Copyright 2016 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.
 
 from collections import defaultdict
 import copy
 
 from common.dependency import Dependency
 from common.dependency import DependencyRoll
 from common import chrome_dependency_fetcher
@@ skipping 89 matching lines @@
 # revision_range (even if the versions therein are None), because
 # ChangelistClassifier.__call__ will take it apart in order to call
 # GetDEPSRollsDict; if it can't then it will immediately return the
 # empty list of results, breaking many of the tests here. Of course,
 # taking revision_range apart isn't actually required for the tests,
 # since we mock GetDEPSRollsDict. So, really what we ought to do in the
 # long run is redesign things so that GetDEPSRollsDict takes the
 # CrashReport directly and pulls out the revision_range and platform
 # itself; that way ChangelistClassifier.__call__ needn't worry about it,
 # allowing us to clean up the tests here.
+# TODO(wrengr): this empty stacktrace causes the unittests to emit
+# logging warnings about the fact that we can't get the crash_stack. Would
+# be nice to avoid that.
 DUMMY_REPORT = CrashReport(None, None, None, Stacktrace(), (None, None))
 
 class ChangelistClassifierTest(CrashTestSuite):
 
   def setUp(self):
     super(ChangelistClassifierTest, self).setUp()
     self.changelist_classifier = changelist_classifier.ChangelistClassifier(
         GitilesRepository(self.GetMockHttpClient()), 7)
 
   def testSkipAddedAndDeletedRegressionRolls(self):
@@ skipping 345 matching lines @@
       return [match_result1, match_result2]
 
     self.mock(changelist_classifier, 'FindMatchResults', _MockFindMatchResults)
     self.mock(chrome_dependency_fetcher.ChromeDependencyFetcher,
         'GetDependencyRollsDict',
         lambda *_: {'src/': DependencyRoll('src/', 'https://repo', '1', '2')})
     self.mock(chrome_dependency_fetcher.ChromeDependencyFetcher,
         'GetDependency', lambda *_: {})
 
     self.assertListEqual(self.changelist_classifier(DUMMY_REPORT), [])