Running QGIS algorithm with selected features from layer using PyQGIS

Question

In the QGIS GUI, the select-by-location algorithm has a choice to only use the selected features from the INTERSECT layer. How can I do this (or use any other algorithm) in a python script for QGIS 3?

I do not want to save temporal layer as mentioned in Run QGIS Model on selected features only?

I have been running this script but it does not use the selected features.

import processing

input_vlayer = 'Some path'
intersect_vlayer = 'Some path'

input_vlayer = iface.addVectorLayer(input_vlayer, 'input_vlayer', 'ogr')
intersect_vlayer = iface.addVectorLayer(intersect_vlayer, 'intersect_vlayer', 'ogr')

# Pass the first 5 features
for i in range(5):
    
    # Select the features
    intersect_vlayer.select(i)
    # Set input params
    params = {'INPUT':input_vlayer,
              'PREDICATE':0,
              'INTERSECT':intersect_vlayer, # USE HERE THE SELECTED FEATURES
              'METHOD':0}
    result = processing.run("qgis:selectbylocation", params)

    # Remove the current selection and then pass to the next one
    intersect_vlayer.removeSelection()

Also, from QGIS select by location doesn't use selected features only they mentioned to enable [Processing] > [Options] > [General] > [Use only selected features]. However I can't see this option in my qgis version. I'm using QGIS Madeira 3.4.3

Answer 1

For only selected feature use the QgsProcessingFeatureSourceDefinition class.

Code below with the input of selected features only:

import processing

input_vlayer = 'Some path'
intersect_vlayer = 'Some path'

input_vlayer = iface.addVectorLayer(input_vlayer, 'input_vlayer', 'ogr')
intersect_vlayer = iface.addVectorLayer(intersect_vlayer, 'intersect_vlayer', 'ogr')


for i in range(5):

    # Select the features
    intersect_vlayer.select(i)
    # Set input params
    params = {'INPUT':input_vlayer,
              'PREDICATE':0,
              'INTERSECT':QgsProcessingFeatureSourceDefinition(intersect_vlayer.id(), True), # USE HERE THE SELECTED FEATURES
              'METHOD':0}
    result = processing.run("qgis:selectbylocation", params)

    # Remove the current selection and then pass to the next one
    intersect_vlayer.removeSelection()