code: def result_key_for(self, op_name):
          
          ops = self.resource_data.get('operations', {})
          op = ops.get(op_name, {})
          key = op.get('result_key', None)
          return key
docs: 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