Clean up compute_interfaces_info.py

Source/bindings/scripts/generate_event_interfaces.py

 #!/usr/bin/python
 #
 # Copyright (C) 2013 Google Inc. All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 # notice, this list of conditions and the following disclaimer.
@@ skipping 64 matching lines @@
         parser.error('No arguments allowed, but %d given.' % len(args))
     return options
 
 
 def write_event_interfaces_file(interfaces_info, destination_filename, only_if_changed):
     # Event interfaces are interfaces that inherit from Event, and Event itself
     event_interfaces = set(
         interface_name
         for interface_name, interface_info in interfaces_info.iteritems()
         if (interface_name == 'Event' or
-            ('ancestors' in interface_info and
+            (interface_info['ancestors'] and
              interface_info['ancestors'][-1] == 'Event')))
 
     def extended_attribute_string(name, value):
         if name == 'RuntimeEnabled':
             value += 'Enabled'
         return name + '=' + value
 
     def interface_line(interface_name):
         full_path = interfaces_info[interface_name]['full_path']
 
@@ skipping 25 matching lines @@
     options = parse_options()
     with open(options.interfaces_info_file) as interfaces_info_file:
         interfaces_info = pickle.load(interfaces_info_file)
     write_event_interfaces_file(interfaces_info,
                                 options.event_interfaces_file,
                                 options.write_file_only_if_changed)
 
 
 if __name__ == '__main__':
     sys.exit(main())