code: def add_node(self, node):
          
          if self.controllers.get(node.controller_type, None):
              raise RuntimeError("Cannot add node {} to the node group. A node for {} group is already assigned".format(
                  node,
                  node.controller_type
              ))
          self.nodes.append(node)
          if node.controller:
              self.controllers[node.controller_type] = node.controller
              setattr(self, node.controller_type, node.controller)
docs: A a Node object to the group. Only one node per cgroup is supported