Question

In a multigraph each call to *add_edge(a,b,weight=1)* will add a new edge between nodes a and b. When building the graph, is it possible to modify this weight when a and b are found again. Right now I make a check to find whether (a, b) or (b, a) are connected, then have to delete the edge, and add a new one. It seems to me that I should simply be able to update the weight.

Note: I do need multigraphs because I use different types of edges between nodes (differentiated using key)

Answer 1

The Multigraph.add_edge documentation indicates that you should use the key argument to uniquely identify edges in a multigraph. Here’s an example:

>>> import networkx as nx
>>> G = nx.MultiGraph()
>>> G.add_edge(1, 2, key='xyz', weight=2)
>>> G.add_edge(1, 2, key='abc', weight=1)
>>> G.edges(data=True)
[(1, 2, {'weight': 2}), (1, 2, {'weight': 1})]

Now, to update the edge keyed by xyz, just pass that parameter in again:

>>> G.add_edge(1, 2, key='xyz', weight=7)
>>> G.edges(data=True)
[(1, 2, {'weight': 7}), (1, 2, {'weight': 1})]

To read the previous value, you can use get_edge_data like this:

>>> G.get_edge_data(1, 2, key='xyz')
{'weight': 7}