I try to run algorithms on layers but only on selected features with QgsProcessingFeatureSourceDefinition in a loop and then write the result with QgsVectorFileWriter :

layer1 = QgsVectorLayer(path1, 'layer 1', 'ogr')
layer2 = QgsVectorLayer(path2, 'layer 2', 'ogr')

idx = layer2.dataProvider().fieldNameIndex('colL2')
col2values = layer2.uniqueValues(idx)

fields = layer1.dataProvider().fields()
writer = QgsVectorFileWriter(pathOutput, "CP1250", fields, QgsWkbTypes.LineString, layer1.crs(), "ESRI shapefile")

for value in col2values:
    # Layer 1 and Layer 2 have the same column name 'colL2' but Layer 1 values are longer
    selectLayer2 = QgsExpression(" \"colL2\" LIKE '" + str(value) + "' ")
    selectLayer1 = QgsExpression(" \"colL2\" LIKE '" + str(value) + "%' ")
    selectedLayer2 = layer2.getFeatures(QgsFeatureRequest(selectLayer2))
    idsLayer2 = [f.id() for f in selectedLayer2]

    selectedLayer1 = layer1.getFeatures(QgsFeatureRequest(selectLayer1))
    idsLayer1 = [f.id() for f in selectedLayer1]

    parameters = {'INPUT': QgsProcessingFeatureSourceDefinition(layer1.id(), True),
                  'OUTPUT': 'memory:'}
    layer1_line = processing.run('qgis:polygonstolines', parameters)

    parameters = {'INPUT': QgsProcessingFeatureSourceDefinition(layer2.id(), True),
                  'OUTPUT': 'memory:'}
    layer2_line = processing.run('qgis:polygonstolines', parameters)

    parameters = {'INPUT': layer1_line['OUTPUT'],
                  'OVERLAY': layer2_line['OUTPUT'],
                  'OUTPUT': 'memory:'}
    differenced = processing.run('qgis:difference', parameters)



del writer

But I get following error : raise QgsProcessingException(msg) _core.QgsProcessingException: Unable to execute algorithm Could not load source layer for INPUT: layer_1_XXX not found


you need add the layer to project. For add without showing it use:

QgsProject.instance().addMapLayer(layer1, False)

Example using only one layer:

layer1 = QgsVectorLayer(r"C:\test\grassland.shp", 'layer 1', 'ogr')
QgsProject.instance().addMapLayer(layer1, False)

parameters = {'INPUT': QgsProcessingFeatureSourceDefinition(layer1.id(), True),
              'OUTPUT': 'memory:'}
layer1_line = processing.run('qgis:polygonstolines', parameters)
  • I try to use it with a temporary layer like following : 'OVERLAY': QgsProcessingFeatureSourceDefinition(layer['OUTPUT'].id(), True), but it returns Could not load source layer for OVERLAY : output_.... not found. Can't the layer be temporary ? – dmjf08 Apr 10 at 12:33
  • yes, but you have to add it to the project QgsProject.instance().addMapLayer(layer['OUTPUT'], False) try that – Fran Raga Apr 10 at 13:25
  • It works but the output of the algorithm is not a QgsVectorLayer as usual, it's a string "output_...". How can I get the QgsVectorLayer ? – dmjf08 Apr 24 at 16:53
  • If is a string,you need create a qgsvectorlayer using this string then – Fran Raga Apr 24 at 16:57
  • Okay thanks. Just to be sure : is that normal that I get QgsVectorLayer with for example qgis:intersection and with the same principle I get a string in the output of qgis:difference ? – dmjf08 Apr 24 at 20:04

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