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