code: def key_value(self, *args, **kwargs):
          
          new_df = copy_df(self)
          fields = _render_field_set(args)
          self._assert_ml_fields_valid(*fields)
          new_df._perform_operation(
              op.FieldKVConfigOperation(dict((_get_field_name(f), KVConfig(**kwargs)) for f in fields)))
          return new_df
docs: Set fields to be key-value represented.
  
          :rtype: DataFrame
  
          :Example:
  
          >>> new_ds = df.key_value('f1 f2', kv=':', item=',')