import processing
moment_all = iface.activeLayer()
moment_all.startEditing()
processing.runalg("qgis:addfieldtoattributestable","filepath.shp","blast",2,2,0,"/Users/pgcseismolab/Desktop/qgis/moment_all.shp")
processing.runalg("qgis:selectbylocation","filepath.shp","filepath2.shp",['within'],0,0)
processing.runalg("qgis:fieldcalculator","filepath.shp","blast",0,10,3,False,"'b'","filepath.shp")
moment_all.commitChanges()