Trails and Graph Theory 3: NetworkX

Let us create a small imaginary trail graph in Python with NetworkX, and play around with plotting and paths. Suppose we have a graph with nodes from 0 to 15:

import networkx as nx
import matplotlib.pyplot as plt



The tuple (8,11,2) means an edge goes from node 8 to node 11, with weight 2.

Now we specify where the nodes are located.

#pos=nx.spring_layout(G) does not give satisfactory results.
# Draw node positions the hard way.


We are almost ready to draw the network. Since we want to draw weights for the edges, we have an extra step.

options = {
    "font_size": 16,
    "node_size": 1000,
    "node_color": "white",
    "edgecolors": "black",
    "linewidths": 5,
    "width": 5,
nx.draw_networkx(G, pos,**options)

labels = {e: G.edges[e]['weight'] for e in G.edges}
nx.draw_networkx_edge_labels(G, pos, edge_labels=labels, font_color="red")

And here is our result.

Think how we can convert this into a Euler graph, so all nodes have an even number of edges. We can try a Networkx library function, but should not expect it will do what we want, because it will create a multigraph, with more than one edge between nodes.

H = nx.eulerize(G)

Our command to plot the resulting graph has to be a bit more complicated, to allow for showing more than one edge between pairs of nodes. [hat-tip]

nx.draw_networkx_nodes(H, pos, node_color="white", node_size=1000)
ax = plt.gca()
for e in H.edges:
                xy=pos[e[0]], xycoords='data',
                xytext=pos[e[1]], textcoords='data',
                    shrinkA=5, shrinkB=5,
                    patchA=None, patchB=None,

Well, we expected to eliminate nodes 0-2, as a dead-end on our Euler path. After that, there are 4 pairs of nodes with multiple edges. If we remove every double-edge, we hope to still have an Euler graph. Let’s write code to remove all double-edges. Python does not like when we modify a graph during a for-loop, so a better practice is to create a new graph, copying all edges that are not double-edges.

J = nx.Graph()
for node in H.nodes(): 
    # We look for adjacent nodes
    neighbors = H.neighbors(node) #Find all neighbors of node n
    for adj_node in neighbors:
        if len(H[node][adj_node]) == 1: 
            # get weight of the edge       
            w = H.edges[node,adj_node,0]['weight'] 
            J.add_edge(node, adj_node, weight = w)

A subtlety here for newbie NetworkX users like myself is to use
>> H.edges[node,adj_node,0][‘weight’]
instead of
>> H.edges[node,adj_node][‘weight’]
because H is a multigraph, and could possibly have more than one edge between each pair of nodes. Since J is a simple graph, it should not matter if we add an edge twice, since the edge would just get overwritten. Plotting the result:

The output is not ideal, because our naive algorithm disconnected the graph.

Alternatively, manually choose better edges to delete , and defer the max-path algorithm to another post.

print("Is G an  Eulerian graph? : ",nx.is_eulerian(G))
Is G an  Eulerian graph? :  True
[(3, 4), (4, 9), (9, 5), (5, 6), (6, 7), (7, 10), (10, 12), (12, 13), (13, 15), (15, 14), (14, 11), (11, 8), (8, 3)]
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Author: Jim, Sagebrush

Jim (trail-name Sagebrush) codes audio software for Windows, Linux, Android, and embedded systems. When not working at, he enjoys backpacking, which this blog is about.

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