Making CallStack immutable, so it can be hashable

appengine/findit/crash/changelist_classifier.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.
 
 import logging
 from collections import defaultdict
 from collections import namedtuple
 
 from common import chrome_dependency_fetcher
 from crash import crash_util
@@ skipping 40 matching lines @@
     """
     if not report.regression_range:
       logging.warning('ChangelistClassifier.__call__: Missing regression range '
           'for report: %s', str(report))
       return []
     last_good_version, first_bad_version = report.regression_range
     logging.info('ChangelistClassifier.__call__: Regression range %s:%s',
         last_good_version, first_bad_version)
 
     # Restrict analysis to just the top n frames in each callstack.
-    # TODO(wrengr): move this to be a Stacktrace method?
     stacktrace = Stacktrace([
-        CallStack(stack.priority,
-                  format_type=stack.format_type,
-                  language_type=stack.language_type,
-                  frame_list=stack[:self.top_n_frames])
+        stack.SliceFrames(None, self.top_n_frames)
         for stack in report.stacktrace])
 
     # We are only interested in the deps in crash stack (the callstack that
     # caused the crash).
     # TODO(wrengr): we may want to receive the crash deps as an argument,
     # so that when this method is called via Findit.FindCulprit, we avoid
     # doing redundant work creating it.
     stack_deps = GetDepsInCrashStack(
         report.stacktrace.crash_stack,
         chrome_dependency_fetcher.ChromeDependencyFetcher(
@@ skipping 226 matching lines @@
         repository.repo_url = stack_deps[dep].repo_url
         blame = repository.GetBlame(touched_file_path,
                                     stack_deps[dep].revision)
 
         # Generate/update each result(changelog) in changelogs, blame is used
         # to calculate distance between touched lines and crashed lines in file.
         match_results.GenerateMatchResults(
             touched_file_path, dep, stack_infos, changelogs, blame)
 
   return match_results.values()