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