'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)