'Could not load source layer for INPUT: layer not found' error when running QGIS algorithm on selected features using PyQGIS

Question

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) + "' ")
 selectedLayer2 = layer2.getFeatures(QgsFeatureRequest(selectLayer2))
 idsLayer2 = [f.id() for f in selectedLayer2]
 layer2.selectByIds(idsLayer2)
 selectLayer1 = QgsExpression(" \"colL2\" LIKE '" + str(value) + "%' ")
 selectedLayer1 = layer1.getFeatures(QgsFeatureRequest(selectLayer1))
 idsLayer1 = [f.id() for f in selectedLayer1]
 layer1.selectByIds(idsLayer1)
 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)
 writer.addFeatures(differenced['OUTPUT'].getFeatures())
 layer1.removeSelection()
 layer2.removeSelection()
del writer

But I get the following error:

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

Answer 1

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)

• dmjf08: 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 ?
• Fran Raga: yes, but you have to add it to the project QgsProject.instance().addMapLayer(layer['OUTPUT'], False) try that
• dmjf08: 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 ?
• Fran Raga: If is a string,you need create a qgsvectorlayer using this string then
• dmjf08: 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 ?