Finding separate graphs within a graph object in networkx

Question

I have an enormous graph dataset - let's say it is like this, but on a much bigger level:

1 -> 2 3 -> 4 

1,2,3,4 are nodes and the arrows are directed edges. Let's say that they are all in a single graph object:

import networkx as nx G = nx.DiGraph() G.add_nodes_from([1,2,3,4]) G.add_edge(1,2) G.add_edge(3,4) 

Given an object like this, which has two mini graphs within a graph, how can we pull out each mini graph? I feel like there must be some word for this? My end result would look like:

for mini_graph in G:     print mini_graph.nodes()  ... [1,2] [3,4] 

Answer 1

If the parts of the graph are truly disjoint (as per your small example), then consider extracting the subgraphs with connected_component_subgraphs().

This only works on an undirected graph, so if you are using a directed graph then you'll need to convert to undirected first.

import networkx as nx G = nx.DiGraph() G.add_nodes_from([1,2,3,4]) G.add_edge(1,2) G.add_edge(3,4)  # make an undirected copy of the digraph UG = G.to_undirected()  # extract subgraphs sub_graphs = nx.connected_component_subgraphs(UG)  for i, sg in enumerate(sub_graphs):     print "subgraph {} has {} nodes".format(i, sg.number_of_nodes())     print "\tNodes:", sg.nodes(data=True)     print "\tEdges:", sg.edges() 

which yields:

subgraph 1 has 2 nodes     Nodes: [(1, {}), (2, {})]     Edges: [(1, 2)] subgraph 1 has 2 nodes     Nodes: [(3, {}), (4, {})]     Edges: [(3, 4)] 

and you could use the subgraph node labels to operate on your data in the initial graph,

sg.nodes()[0] in G >>>  True 

Reading the answer linked by EdChum, it appears that weakly_connected_component_subgraphs() operates on a directed graph but treats it as undirected, so saving the copy might be crucial. However, the docs on this and the related function weakly_connected_components() are a bit thin at present.

Answer 2

As of 2018 the above answer is deprecated (link to docs). You are advised to use:

(G.subgraph(c) for c in connected_components(G)) 

or

(G.subgraph(c).copy() for c in connected_components(G))