Building a Graph with Repeated Non-Unique Nodes

I'm trying to read data into a graph using Graph-Tool. My code is further
below. I'm having the issue of invalid vertex descriptors.

Each row in has a start, end, and transition weight. My goal is to
iterate through each row, adding the edges (start,end) that have that edge
weight of trans.

Suppose on the first iteration, has (1,1,2) = (start, trans, end)
which is to describe an edge between vertices labeled 1 and 2, with the
weight of '1'.

Then on the second iteration, suppose has (1,5,3) = (start, trans,
end), which is to describe an edge between vertices labeled 1 and 3, with
the weight of '5'.

Then I'd want a graph with three vertices, 1, 2, 3 and has two edges (1,2)
and (1,3).

My strategy is to check if a vertex of each label (start,end) exists, and
then to add those vertices if they do not exist. The try-catch statements
seem to work properly. But when we try to access the added vertices, I get
the invalid vertex descriptor (value error) exception.

    def gtBuildGraph(my):
        """build a graph using 'Graph-Tool'

        my.graph = gt.Graph()
        my.weights = my.graph.new_edge_property("int")

        for row in
            start = int(row['start_state'])
            end = int(row['end_state'])
            trans = int(row['transition'])

            #sometimes, start and end nodes are non-unique

            start = my.graph.vertex(start)
            end = my.graph.vertex(end)
            newEdge = my.graph.add_edge(start, end)
            my.weights[newEdge] = trans

Vertices are indexed from 0 to N - 1. When you call g.add_vertex(n) it
adds n vertices to the graph. If you had zero vertices before, the
vertex with index n does not exist yet in the graph (the last one has
index n-1), and hence g.vertex(n) is invalid.