Making CallStack immutable, so it can be hashable

appengine/findit/crash/predator.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 crash.culprit import Culprit
 
 # TODO(http://crbug.com/659346): write coverage tests.
 class Predator(object): # pragma: no cover
   """The Main entry point into the Predator library."""
 
   def __init__(self, cl_classifier, component_classifier, project_classifier):
     self.cl_classifier = cl_classifier
     self.component_classifier = component_classifier
     self.project_classifier = project_classifier
 
   def FindCulprit(self, report):
     """Given a CrashReport, return a Culprit."""
     suspected_cls = self.cl_classifier(report)
+    assert suspected_cls is not None
 
     suspected_project = self.project_classifier.Classify(
         suspected_cls, report.stacktrace.crash_stack)
 
     suspected_components = self.component_classifier.Classify(
         suspected_cls, report.stacktrace.crash_stack)
 
     return Culprit(
         project = suspected_project,
         components = suspected_components,
         cls = suspected_cls,
         regression_range = report.regression_range,
         algorithm = 'core_algorithm')