pyqgis - Computing distance between two points following network and doing it for 100.000 points?

My starting data :

• a collection of points which are buildings


• a collection of points which are electrical transformation plant


• a network of vectors which is the public electric network from an operator


• buildings are not connected to the electric network

I use QGIS to find which are the buildings which are located less than 250 meters from an electrical transformation plant by following the network. I need this information more 100.000 buildings.

To do it by hand :

• trace a line between the building and the nearest electrical network segment,


• compute the distance by the network between this entry point and the nearest tranformation plant. I have found the RoadGraph plugin (http://gis-lab.info/qa/road-graph-eng.html) which works for that


• add the two distances and check if < 250m.

Now, my question is: how to automatize it for the 99.999 other buildings ? I know Python a lot but I am a beginner with QGIS. I did not find a way to automatically interact with the RoadGraph plugin and source code seems not to be available anymore.

Maybe I am doing it wrong ?