Difference between features in the same vector layer using PyQGIS

Question

I have a vector layer containing 3 rows (see image2). Each row is a polygon. The blue polygon is number3 and the magenta polygon is number2 (see image1)

I need to subtract the polygon that has the attribute "number"=3 to polygon that has the attribute "number"=2.

How can I do it in PyQGIS?

I have tried something like this:

import processing

layer = QgsProject.instance().mapLayersByName("Langham_SPZs_v1_diss")[0]

outfn2 = "Langham_SPZs_final.shp"

layer2=layer.selectByExpression('"number"=2')

layer3=layer.selectByExpression('"number"=3')

processing.run("native:difference", {layer3, layer2, 'OUTPUT':outfn2})

Answer 1

Field number is a string in your data. So you have to use "number='2'" as expression. And you should extract the selected polygon with number=2 since both polygons are in the same layer. Use this script:

import processing

layer = QgsProject.instance().mapLayersByName("Langham_SPZs_v1_diss")[0]

outfn2 = "Langham_SPZs_final.shp"    

layer.selectByExpression("number='2'")

layer2 = processing.run("native:saveselectedfeatures",
     {'INPUT':layer ,
      'OUTPUT':'TEMPORARY_OUTPUT'})["OUTPUT"]

layer.selectByExpression("number='3'")

processing.run("native:difference",
    {'INPUT':QgsProcessingFeatureSourceDefinition(layer.id(),
             selectedFeaturesOnly=True),
     'OVERLAY':layer2,
     'OUTPUT':outfn2})

Answer 2

Another option is to use the materialize() method.

from qgis import processing

layer = QgsProject.instance().mapLayersByName('Langham_SPZs_v1_diss')[0]

outfn2 = "Langham_SPZs_final.shp"

layer.selectByExpression("number='2'")
layer2 = layer.materialize(QgsFeatureRequest().setFilterFids(layer.selectedFeatureIds()))

layer.selectByExpression("number='3'")
layer3 = layer.materialize(QgsFeatureRequest().setFilterFids(layer.selectedFeatureIds()))

processing.run("native:difference", {'INPUT' : layer3, 'OVERLAY' : layer2, 'OUTPUT': outfn2})

Note: this method is available since QGIS 3.0.

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References:

• PyQGIS Developer Cookbook | 21. Cheat sheet for PyQGIS