networkx subgraph as node

Question

I want to replace a subgraph S of a networkx graph G by a single node N that again contains the entire subgraph S. I need to do that because I need an edge from N to other nodes of my graph.

Because I did not get the subgraph method of network x to work, I wrote my own code to do it. But I am confused with the results.

This is a small example script:

import networkx as nx
from copy import deepcopy
from collections import deque


class XGraph(nx.MultiDiGraph):

    def dographthings(self, graph_edges, graph_nodes, subgraph_nodes):
    
        self.add_edges_from(graph_edges)
        
        subgraph = deepcopy(self)
        
        # remove all nodes and their transitive children from subgraph,that are
        # not in subgraph_nodes
        remove_subtree(deque((set(graph_nodes) - set(subgraph_nodes))), subgraph)

        # remove all nodes from self that are now in subgraph
        self.remove_nodes_from(subgraph)


        print "subgraph:"
        print type(subgraph)
        for node in subgraph.nodes_iter():
            print node
        
        print "self:"   
        print type(self)
        for node in self.nodes_iter():
            print node

        self.add_node(subgraph)
        
        print self.node[subgraph]
        
        
        
        
def remove_subtree(nodes, graph):
    """
    Removes all nodes that are successors of the nodes in ``nodes``.
    Is robust for cyclic graphs.

    Parameters
    ----------
    graph : referance to networkx.graph
        graph to remove nodes from
    nodes : deque of nodes-ids
        the nodes the successors of which to remove from graph
    """
    to_remove = set()
    to_add = list()
    for node in nodes:
        to_remove.add(node)
        if node in graph:
            to_add.extend(graph.successors(node))
            graph.remove_node(node)
    for node in to_remove:
        nodes.remove(node)
    for node in to_add:
        nodes.append(node)
    if len(nodes) > 0:
        graph = remove_subtree(nodes, graph)
        
    
    
g = XGraph()
g.dographthings([(1,2),(2,3),(2,4),(1,5)], [1,2,3,4,5], [3,2,1])

The class XGraph has a method that add edges to the graph and also build a subgraph as described above. When I then iterate over the nodes of the graph and the subgraph, everything appears to be correct. Then when I add the subgraph as a node, and access it via the get_item-method, it seems to have become an empty dictionary rather than a MultiDiGraph, as it was before adding it as a node.

The output of the script is this:

subgraph:
<class '__main__.XGraph'>
1
2
3
self:
<class '__main__.XGraph'>
4
5
{}

Why does my subgraph become a dictionary upon being added as a node and where does all its data go?

EDIT

I accessed the node incorrectly. Doing it like this works:

for node in self.nodes_iter(data=True):
    if isinstance(node[0], nx.MultiDiGraph):
        print "this is the subgraph-node:"
        print node
        print "these are its internal nodes:"
        for x in node[0].nodes_iter():
            print x
    else:
        print "this is an atomic node:"
        print node

The output:

this is the subgraph-node:
(<__main__.XGraph object at 0xb5ec21ac>, {})
these are its internal nodes:
1
2
3
this is an atomic node:
(4, {})
this is an atomic node:
(5, {})

Aric Aric · Accepted Answer · 2016-01-31T01:41:06

I can't quite see why your code isn't working. Here is a small example that might help

import networkx as nx

G = nx.Graph()
G.add_path([1,2,3,4])
S = G.subgraph([2,3]) # S is the graph 2-3
# add the subgraph as a node in the original graph
G.add_node(S)
# connect S to the neighbors of 2 and 3 and remove 2,3
for n in S:
    nbrs = set(G.neighbors(n))
    for nbr in nbrs - set([S]):
        G.add_edge(S,nbr)
    G.remove_node(n)
print(G.nodes()) # 1,4, <graph id>
print(G.edges()) # [(1, <graph id>), (<graph id>,4)]

networkx subgraph as node

EDIT

3 Answers