Checks for the presence of a ``result_key``, which defines what data
        should make up an instance.

        Returns ``None`` if there is no ``result_key``.

        :param op_name: The operation name to look for the ``result_key`` in.
        :type op_name: string

        :returns: The expected key to look for data within
        :rtype: string or None

def result_key_for(self, op_name):
        """
        Checks for the presence of a ``result_key``, which defines what data
        should make up an instance.

        Returns ``None`` if there is no ``result_key``.

        :param op_name: The operation name to look for the ``result_key`` in.
        :type op_name: string

        :returns: The expected key to look for data within
        :rtype: string or None
        """
        ops = self.resource_data.get('operations', {})
        op = ops.get(op_name, {})
        key = op.get('result_key', None)
        return key