code: def _description_columns_json(self, cols=None):
          
          ret = {}
          cols = cols or []
          d = {k: v for (k, v) in self.description_columns.items() if k in cols}
          for key, value in d.items():
              ret[key] = as_unicode(_(value).encode("UTF-8"))
          return ret
docs: Prepares dict with col descriptions to be JSON serializable