A Graph in Python - and pythonic surprises

I started implementing a Graph in python for a project and I encountered an unexpected behavior. See if you can spot the problem.

Code for the graph is here:



However this is buggy. Each time an edge is added to one node, it gets added to all the nodes. Adding an edge from 'bellevue' to 'lynwood' added the edge to both vertices 'bellevue' and 'lynwood'.

Code/Output:

g.add_node(GraphNode('seattle', [Edge('seattle', 'bellevue', 'dist', 10), Edge('seattle', 'lynwood', 'dist', 20)]))

g.add_edge(('bellevue', 'lynwood', 'dist', 5))

print (g)

bellevue -> bellevue:lynwood:dist:5
lynwood -> bellevue:lynwood:dist:5
seattle -> seattle:bellevue:dist:10 seattle:lynwood:dist:20

After a lengthy debugging stint, the issue was identified to be the way Python evaluates default argument values to functions.