Minor cleanup of some recipe framework code.

scripts/slave/recipe_modules/step_history/api.py

 # Copyright 2013 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 collections
 
 from slave import recipe_api
 
 class StepHistoryApi(recipe_api.RecipeApi, collections.Mapping):
   """
   Provide an OrderedDict-like view into the steps that have run, and what
   data they've returned.
 
   Each entry in step_history is an object() which has the attributes:
     * retcode
     * <module_name>.<module_specific_data>
   """
 
   def __init__(self, step_history, **kwargs):
     super(StepHistoryApi, self).__init__(**kwargs)
+    # step_history is instantiated in annontated_run.py. Recipe engine is
+    # responsible for updating it.
+    assert isinstance(step_history, collections.OrderedDict)
     self._step_history = step_history
 
   def __getitem__(self, key):
     # NOTE: Ideally, we could constify the values here. However, APIs are
     # allowed to add any arbitrary data to step history items, so this would
     # be essentially impossible without significant additional code.
     return self._step_history[key]
 
   def __iter__(self):  # pragma: no cover
     return iter(self._step_history)
 
   def __len__(self):  # pragma: no cover
     return len(self._step_history)
 
   @property
   def failed(self):
     """Return status of the build so far, as a bool."""
     return self._step_history.failed
 
   def last_step(self):
     """Return the last StepData object, or None if no steps have run."""
     key = next(reversed(self._step_history), None)
     return self[key] if key else None