The writing of processing scripts in QGIS3 changed substantially. On the basis of many tutorials/blog posts and answers on this site I got a running processing script in QGIS3. But it was, for a lousy programmer like me, a hard piece of work, because of many errors in logic and code I produced.

Here is what I tried:

  • To avoid painfull testing inside QGIS, I decided to import modules into my processing script, that I can test outside of QGIS (I am using PyScripter). To get this working I had to manipulate sys.path, because local imports (in the same (sub-)directory) seem to do not work in processing scripts.
  • When changing something in an imported module, QGIS3 doesn't notice the change in the code, it seems not to re-import when starting a processing script. OK, I maybe can avoid that, by using reload - but I didn't test it
  • I also tried to use a standalone pyqgis script to have the chance to debug it. It works, but it is very confusing, because the stack gets very "long" and while stepping through function calls, I easily loose orientation and can't find out, why an error occured.

So I'm looking for a method or procedure to ease the development of processing scripts.

My processing script is quite straightforward, based on the template by underdark and the foss4g workshop materials and at last a tutorial by Ujaval Gandhi!

Here are my imports and the head of that script

from PyQt5.QtCore import QCoreApplication, QVariant
from qgis.core import (QgsApplication,
from qgis.core import (QgsProcessingParameterString,
import processing, sys

# use this to import local modules
sys.path.append(QgsApplication.qgisSettingsDirPath() + '\\processing\\scripts')
from pomodules.poqgsstations import PoQgsStations
from pomodules.poqgscurrentw import PoQgsCurrentW

class PegelOnlineLoader(QgsProcessingAlgorithm):
    INPUT = 'Query'
    OUTPUT = 'Result'
    QUERY_OPTIONS = ['Stations', 'Waterlevels']

as you can see, i have a directory, called pomodules with modules doing the "real" work (download json data from a rest-api and turn them into a list of QgsFeatures, po stands for PegelOnline, a german [website] (https://www.pegelonline.wsv.de/gast/start) providing the REST-Service)

The here is the main part of my processing script:

    def initAlgorithm(self, config=None):
        Here we define the inputs and output of the algorithm, along
        with some other properties.
                                              self.tr('Output layer'),

    def processAlgorithm(self, parameters, context, feedback):
        Here is where the processing itself takes place.
        qx = self.parameterAsInt(parameters, self.INPUT, context)
        query = self.QUERY_OPTIONS[qx]

        # we're loading from PegelOnline
        if query == 'Stations':
            stat = PoQgsStations()
        elif query == 'Waterlevels':
            stat = PoQgsCurrentW()

        features = stat.getFeatures()
        # field definitions for stations
        fields = stat.fields
        crs = stat.crs

        # our output to qgis
        sink, dest_id = self.parameterAsSink(parameters, self.OUTPUT, context, fields, QgsWkbTypes.Point, crs)

        feedback.pushInfo(str('Total Length {}'.format(len(features))))

        for f in features:
            sink.addFeature(f, QgsFeatureSink.FastInsert)

        return {self.OUTPUT: dest_id}

As i said the script is working, but I'm unsure if this is good practise. As described above, I also wrote a standalone script for debugging purposes using the answer by Germán Carrillo here. I run this in PyScripter using the debug mode:

import sys
import os
from qgis.core import QgsApplication, QgsProcessingFeedback

pre = os.environ['QGIS_PREFIX_PATH']

QgsApplication.setPrefixPath(pre, True)

print (QgsApplication.qgisSettingsDirPath())

# Append the path where processing plugin can be found
sys.path.append(pre + '\\python\\plugins')

import processing
from processing.core.Processing import Processing

# parameters needed by the algorithm
params = {
    'Query' : 0,
    'Result' : 'c:/temp/stations.shp'
feedback = QgsProcessingFeedback()

res = processing.run('script:PegelOnlineLoader', params, feedback=feedback)
res['Result'] # Access your output layer

You find the complete example on github. I hope you don't mind my long text (the topic is quite new and needs some explanation, I think).

How can I ease the development of processing scripts and get a better way of debugging?

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