Making a weighted graph with graph-tool

Toy-graph.py

def make_toy_graph():
    T = gt.Graph(directed = False)
    edge_weights = T.new_edge_property('double')
    T.properties[("e","weight")] = edge_weights

    T.add_vertex(n=4)

    e_1 = T.add_edge(0,1)
    e_2 = T.add_edge(1,2)
    e_3 = T.add_edge(0,0)
    e_4 = T.add_edge(1,3)

    edge_weights[e_1]= 8
    edge_weights[e_2]= 15
    edge_weights[e_3]= 7
    edge_weights[e_4]= 20
    return T