[graph-tool] About the usage of weights in shortest_path

Hello, I don't get how the weight parameter is used in the shortest_path function when the pred_map is provided. More precisely the code: https://github.com/count0/graph-tool/blob/master/src/graph_tool/topology/__i... seems to examine each and any edge when the weight parameter is provided. What is the point of this behavior ? what do I miss ? Best, f.