def depth_first_search(self, node_key, nodes_visited, nodes_in_cycle=[]):
        """
        The nodes_visited is a set of nodes which indicates it has been visited.
        The node_in_cycle is a list of nodes in the potential cycle.
        Returns the list of nodes in the cycle or None.
        """
        nodes_visited.add(node_key)
        nodes_in_cycle.append(node_key)
        for node in self.nodes[node_key]['next_nodes']:
            if node in nodes_in_cycle:
                # The graph cycle starts at the index of node in nodes_in_cycle.
                return nodes_in_cycle[nodes_in_cycle.index(node):]
            if node in nodes_visited:  <<<<------------ Should be "node not in nodes_visited"
                dfs_nodes = self.depth_first_search(node, nodes_visited, nodes_in_cycle)
                if dfs_nodes:
                    return dfs_nodes

        # This node_key is not part of the graph cycle.
        nodes_in_cycle.pop()
        return None