So far our Vertex class has stored edges inside of a dictionary with keys of the connected vertex’s name and the value simply set to True.

We can make our implementation support edge weights with a few small changes. To keep this class as flexible as possible, we’ll introduce a default weight argument to .add_edge() in the Graph and Vertex classes. With no explicit weight argument, it will default to 0. We’ll then set the appropriate value in the dictionary to that weight.

Weighted edges allow us to make graphs that represent rail systems with a travel-time between stations.

railway = Graph()

callan = Vertex('callan')
peel = Vertex('peel')
harwick = Vertex('harwick')

railway.add_vertex(callan)
railway.add_vertex(peel)
railway.add_vertex(harwick)

# Travel-time between callan and peel: 12
railway.add_edge(callan, peel, 12)
# Travel-time between harwick and callan: 7
railway.add_edge(harwick, callan, 7)

print(callan.edges)
# { 'peel': 12 }
print(harwick.edges)
# { 'callan': 7 }