working_with_graph.py

g = { "a" : {"d":2},
      "b" : {"c":2},
      "c" : {"b":5,  "d":3, "e":5}
    }
graph = Graph(g)
print("Vertices of graph:")
print(graph.vertices())
print("Edges of graph:")
print(graph.edges())
print("Add vertex:")
graph.add_vertex("z")
print("Vertices of graph:")
print(graph.vertices()) 
print("Add an edge:")
graph.add_edge({"a","z"})    
print("Vertices of graph:")
print(graph.vertices())
print("Edges of graph:")
print(graph.edges())
print('Adding an edge {"x","y"} with new vertices:')
graph.add_edge({"x","y"})
print("Vertices of graph:")
print(graph.vertices())
print("Edges of graph:")
print(graph.edges())

graph = Graph (g)

in this line I get that it is not defined and I do not execute
I am new at this