[graph-tool] Accessing the integer attribute in Vertex/Edge object directly/faster

Hi, Given a `graph_tool.Edge` instance, is it possible to directly access the integer value (or its index value) of the source vertex and target vertex? My current solution is ``` s = int(e.source()) t = int(e.target()) ``` I have some function that calls the above routine many times and it is not fast enough. I am wondering if I can do it faster. Similarly, can I access the index/integer value of a Vertex object directly? -- Best Han