In QGIS, I try to write a script for a custom toolbox. I'm using the processing framework logic and following official documentation. I add an input vector layer, but I can't handle it has a QgsVectorLayer.

For example when I wan't to apply a filter to the selected layer I have an error message:

AttributeError: 'QgsProcessingFeatureSource' object has no attribute 'setSubsetString'

Here is an extract of my script:

class DissolveProcessingAlgorithm(QgsProcessingAlgorithm):

def initAlgorithm(self, config=None):
                self.tr('Input layer'),

def processAlgorithm(self, parameters, context, feedback):
        source = self.parameterAsSource(
        # Filter
        filter_value = '1'
        source.setSubsetString('"my_field"=\'%s\'' % filter_value)

How to handle QgsProcessingParameterFeatureSource as a vector layer has defined in initAlgorithm method ?

  • 2
    Use the materialize() method to return an in-memory vector layer from the QgsProcessingFeatureSource object.
    – Ben W
    Sep 20, 2023 at 23:49
  • That's the good way. I used new_layer = source.materialize(QgsFeatureRequest().setFilterFids(source.allFeatureIds())). After this I can do new_layer.setSubsetString('"my_field"=\'%s\'' % filter_value)
    – GeoGyro
    Sep 22, 2023 at 13:40

1 Answer 1


Create a new vector layer and copy what you need to it. There's no point in modifying the input.

I recommend to check the plugin builder template https://github.com/g-sherman/Qgis-Plugin-Builder/blob/master/plugin_templates/processing_provider/template/module_name_algorithm.tmpl

(install 'plugin builder' plugin)

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    Sep 20, 2023 at 20:25

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