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