Making Stacktrace immutable, so it can be hashable

appengine/findit/crash/crash_report.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 namedtuple
 
 from crash.stacktrace import Stacktrace
 
 class CrashReport(namedtuple('CrashReport',
@@ skipping 17 matching lines @@
       we do not store the string itself.
     regression_range : a pair of the last-good and first-bad
       versions. N.B., because this is an input, it is up to clients
       to call DetectRegressionRange (or whatever else) in order to
       provide this information.
   """
   __slots__ = ()
 
   def __new__(cls, crashed_version, signature, platform, stacktrace,
       regression_range):
-    # TODO: should raise a TypeError rather than an AssertionError
-    assert isinstance(stacktrace, Stacktrace), (
+    assert isinstance(stacktrace, Stacktrace), TypeError(
         'In the fourth argument to CrashReport constructor, '
         'expected Stacktrace object, but got %s object instead.'
         % stacktrace.__class__.__name__)
 
     return super(cls, CrashReport).__new__(cls,
         crashed_version, signature, platform, stacktrace, regression_range)