code: def produces(*content_types):
      
      def inner(o):
          if not all(isinstance(content_type, _compat.string_types) for content_type in content_types):
              raise ValueError("In parameter not a valid value.")
          try:
              getattr(o, 'produces').update(content_types)
          except AttributeError:
              setattr(o, 'produces', set(content_types))
          return o
      return inner
docs: Define content types produced by an endpoint.