I run a processing algorithm in QGIS in an iteration with a stepwise parameter increment from numeric model input A (minimum) to numeric model input B (maximum) by a fixed step size.

Here's what the processing script for a buffer example currently looks like (exported from a graphical model and modified for iteration):

from qgis.core import (
import processing

class ProcessingParameterIteration(QgsProcessingAlgorithm):

    def initAlgorithm(self, config=None):
        self.addParameter(QgsProcessingParameterVectorLayer('polygon_layer', 'Polygon layer', types=[QgsProcessing.TypeVectorPolygon], defaultValue=None))
        self.addParameter(QgsProcessingParameterNumber('minimum_buffer', 'Minimum Buffer', type=QgsProcessingParameterNumber.Integer, minValue=25, maxValue=5000, defaultValue=25))
        self.addParameter(QgsProcessingParameterNumber('maximum_buffer', 'Maximum Buffer', type=QgsProcessingParameterNumber.Integer, minValue=25, maxValue=7500, defaultValue=50))
        self.addParameter(QgsProcessingParameterFeatureSink('Buffered', 'buffered', type=QgsProcessing.TypeVectorPolygon, createByDefault=True, supportsAppend=True, defaultValue=None))

    def processAlgorithm(self, parameters, context, model_feedback):
        # Use a multi-step feedback, so that individual child algorithm progress reports are adjusted for the
        # overall progress through the model
        feedback = QgsProcessingMultiStepFeedback(1, model_feedback)
        results = {}
        outputs = {}
        current_buffer_size = parameters['minimum_buffer']
        global renamer

        while current_buffer_size <= parameters['maximum_buffer']:
            # Buffer
            alg_params = {
                'DISSOLVE': False,
                'DISTANCE': current_buffer_size,
                'END_CAP_STYLE': 0,  # Round
                'INPUT': parameters['polygon_layer'],
                'JOIN_STYLE': 0,  # Round
                'MITER_LIMIT': 2,
                'SEGMENTS': 5,
                'SEPARATE_DISJOINT': False,
                'OUTPUT': parameters['Buffered']
            outputs['Buffer'] = processing.run('native:buffer', alg_params, context=context, feedback=feedback, is_child_algorithm=True)
            buffer_layer_name = QgsProcessingUtils.mapLayerFromString(parameters['polygon_layer'], context).name()
            renamer = LayerRenamer(buffer_layer_name + '_buffered'  + str(current_buffer_size) + 'm')
            current_buffer_size += 25
        results['Buffered'] = outputs['Buffer']['OUTPUT']
        return results

    def name(self):
        return 'processing_parameter_iteration'

    def displayName(self):
        return 'Processing Parameter Iteration'

    def group(self):
        return ''

    def groupId(self):
        return ''

    def createInstance(self):
        return ProcessingParameterIteration()
class LayerRenamer(QgsProcessingLayerPostProcessorInterface):
    def __init__(self, layer_name):
        self.name = layer_name
    def postProcessLayer(self, layer, context, feedback):

The LayerRenamer() class has been added based on this answer.

So far, renaming only works for the last iteration. How do I have to modify the script to set the modified name for all iterations?

  • 2
    This post may help. Nov 13, 2023 at 7:05
  • Thank you, Kadir. Indeed, it looks very helpful and will hopefully solve my issue. I will test it and if my case adds something still missing in the linked posts, I will post an answer here.
    – winnewoerp
    Nov 13, 2023 at 17:33
  • Hi @KadirŞahbaz (and all): See my edited answer. With this edit, it's definitely not a duplicate, from my point of view. The renaming unfortunately only works for the last iteration so far.
    – winnewoerp
    Nov 15, 2023 at 5:17


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