teach rebaseline_server to generate diffs of rendered SKPs

gm/rebaseline_server/results.py

 #!/usr/bin/python
 
 """
 Copyright 2013 Google Inc.
 
 Use of this source code is governed by a BSD-style license that can be
 found in the LICENSE file.
 
 Repackage expected/actual GM results as needed by our HTML rebaseline viewer.
 """
@@ skipping 285 matching lines @@
     """
     output_dict = {}
     for subdict in input_dict.values():
       for subdict_key, subdict_value in subdict.iteritems():
         if subdict_key in output_dict:
           raise Exception('duplicate key %s in combine_subdicts' % subdict_key)
         output_dict[subdict_key] = subdict_value
     return output_dict
 
   @staticmethod
-  def get_multilevel(input_dict, *keys):
-    """ Returns input_dict[key1][key2][...], or None if any key is not found.
+  def get_default(input_dict, default_value, *keys):
+    """Returns input_dict[key1][key2][...], or default_value.
+
+    If input_dict is None, or any key is missing along the way, this returns
+    default_value.
+
+    Args:
+      input_dict: dictionary to look within
+      key: key indicating which value to return from input_dict
+      default_value: value to return if input_dict is None or any key cannot
+          be found along the way
     """
+    if input_dict == None:
+      return default_value
     for key in keys:
+      input_dict = input_dict.get(key, None)
       if input_dict == None:
-        return None
-      input_dict = input_dict.get(key, None)
+        return default_value
     return input_dict