Make processing.runalg use only selected features

Question

I have a feature layer and a buffer of that feature layer and am trying to cut out the features from the buffers. This would be trivial, except that the buffers overlay multiple features, so I need to select each feature from the two layers individually, use the difference tool to export them to individual layers, and finally merge them all at the end.

I've written a Python script to automate the selection/difference tool part of this but it seems to be using all features from the layers, and not only the selected ones. I have tested that the code for the selection is working so I believe it is the call of the difference tool that is not working as expected. I understood from the documentation that it should only use the selected features, but perhaps either my understanding or my code is wrong.

I am using QGIS 2.18.3.

import processing
buffer_layer = QgsVectorLayer("buffer.shp", "buffer_layer", "ogr")
patch_layer = QgsVectorLayer("patches.shp", "patch_layer", "ogr")
for feature in buffer_layer.getFeatures():
 buffer_layer.setSelectedFeatures([feature.id()]) 
 site_name = feature.attributes()[0]
 patch_list = []
 for patch in patch_layer.getFeatures():
 if patch.attributes()[1] == site_name:
 patch_list.append(patch.id())
 patch_layer.setSelectedFeatures(patch_list)
processing.runalg("qgis:difference",buffer_layer,patch_layer,0,"output_%s.shp" % site_name)

• DIV: Make sure again that your script selects features, cuz it looks like it doesn't. Add to your code some prints, like buffer_layer.selectedFeatures() and patch_layer.selectedFeatures() to be sure.
• Meghann Mears: Thanks, @DIVAD. I did add code like that to make sure it is selecting, and it is.
• DIV: @MeghannMears Have you tried doing this manually? Do you have the same problem?

Answer 1

You will need to load the layers in QGIS for processing algorithms to work on selected features.

Try replacing -

buffer_layer = QgsVectorLayer("buffer.shp", "buffer_layer", "ogr")
patch_layer = QgsVectorLayer("patches.shp", "patch_layer", "ogr")

with -

buffer_layer = iface.addVectorLayer("buffer.shp", "buffer_layer", "ogr")
patch_layer = iface.addVectorLayer("patches.shp", "patch_layer", "ogr")

in your code.