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

enter image description here

• Fran Raga: you can use QgsProcessingFeatureSourceDefinition and ecapsulate selected id

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()

• Jose: Thanks @Francisco Raga. Just one thing, I had to add QgsProcessingFeatureSourceDefinition(intersect_vlayer.id(), True) to get it work. Thanks.
• Fran Raga: Ooops!yes,your are correct!add this to my answer
• dmjf08: When I add QgsProcessingFeatureSourceDefinition to the INPUT parameter it says me : Could not load source layer for INPUT: layerXXX not found
• Fran Raga: you can use this parameter in the INPUT flag!Put your code in other question,because probably the INPUT layer is not correct
• dmjf08: I posted my question with my code : gis.stackexchange.com/questions/311336/…