code: def to_json(self, indent=None, separators=None, sort_keys=False):
      def remove_callables(x):
        if isinstance(x, dict):
          return {k: remove_callables(v) for k, v in six.iteritems(x)
                  if not callable(v)}
        elif isinstance(x, list):
          return [remove_callables(i) for i in x if not callable(i)]
        return x
      return json.dumps(
          remove_callables(self.values()),
          indent=indent,
          separators=separators,
          sort_keys=sort_keys)