I can comment it well enough.

     # Change {body: {'key': 'val'}} to {'val': 'key'}.
     REVERSE_DESC_MAPPING = reduce(lambda a, b: dict(a, **b),  # Merge list of dicts into one dict.
         [dict((unicode(v), unicode(k)) for k, v in body_mapping.iteritems())  # Reverse the nested dict.
          for body, body_mapping in DESC_MAPPING.iteritems()])  # Loop over the ratings bodies.