QGIS select by location and use field calculator python

Question

I am trying to automate the repetative task of selecting by location and updating a field according to the selection. The issue is that field calculator uses all features instead of the selected. Is there a simple way to do this with qgis python? Here is my code:

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()

A similar thread I've found is here QGIS selecting features and using field calculator to update existing field using python, though I don't think it works for my situation. Would graphical modeler be a better way to do this?

Answer 1

Obviously, there are several misunderstanding. First addfieldtoattributestable might not be necessary cause the fieldcalculator could create a new field if you need (set the proper parameter to True). Selectbylocation is done on filepath.shp not on the active layer. Then, in qgis:fieldcalculator, your are trying to set a string as a float (). Finaly your output will be populate with selected features, If there is no selection, all the feature are saved. Be sure your selection is done on the input layer of fieldcalculator. Please send a the simplest code as possible to be more helpfull.