Return a JSON string representation of this instance

        :param indent: specify an indent level or a string used to indent each
                       level
        :param sort_keys: the output is sorted by key

def to_json(self, indent=None, sort_keys=True):
        """Return a JSON string representation of this instance

        :param indent: specify an indent level or a string used to indent each
                       level
        :param sort_keys: the output is sorted by key
        """
        return json.dumps(self.to_dict(), indent=indent, sort_keys=sort_keys)