[GIS] Output layer from PyQGIS 3 processing script is empty

Question

I've created a processing script in QGIS 3, and although my output layer has shows a feature count greater than zero when using the feedback command, the resulting feature layer that appears in QGIS has no features. What am I doing wrong?

# -*- coding: utf-8 -*-
##source_layer=vector
##output_layer=output vector

from PyQt5.QtCore import QCoreApplication
from qgis.core import (QgsProcessing,
                       QgsFeatureSink,
                       QgsProcessingException,
                       QgsProcessingAlgorithm,
                       QgsProcessingParameterFeatureSource,
                       QgsProcessingParameterFeatureSink)
import processing


class ExampleProcessingAlgorithm(QgsProcessingAlgorithm):

    INPUT = 'INPUT'
    OUTPUT = 'OUTPUT'

    def tr(self, string):
        return QCoreApplication.translate('Processing', string)

    def createInstance(self):
        return ExampleProcessingAlgorithm()

    def name(self):
        return 'myscript'

    def displayName(self):
        return self.tr('My Script')

    def group(self):
        return self.tr('Example scripts')

    def groupId(self):
        return 'examplescripts'

    def shortHelpString(self):
        return self.tr("Example algorithm short description")

    def initAlgorithm(self, config=None):
        self.addParameter(
            QgsProcessingParameterFeatureSource(
                self.INPUT,
                self.tr('Input layer'),
                [QgsProcessing.TypeVectorAnyGeometry]
            )
        )

        self.addParameter(
            QgsProcessingParameterFeatureSink(
                self.OUTPUT,
                self.tr('Output layer')
            )
        )

    def processAlgorithm(self, parameters, context, feedback):
        source = self.parameterAsVectorLayer(
            parameters,
            self.INPUT,
            context
        )

        if source is None:
            raise QgsProcessingException(self.invalidSourceError(parameters, self.INPUT))

        (sink, dest_id) = self.parameterAsSink(
            parameters,
            self.OUTPUT,
            context,
            source.fields(),
            source.wkbType(),
            source.sourceCrs()
        )

        feedback.pushInfo('CRS is {}'.format(source.sourceCrs().authid()))

        if sink is None:
            raise QgsProcessingException(self.invalidSinkError(parameters, self.OUTPUT))


        sink=processing.run("native:buffer", {
            'INPUT': source,
            'DISTANCE': -4.0,
            'SEGMENTS': 5,
            'END_CAP_STYLE': 0,
            'JOIN_STYLE': 0,
            'MITER_LIMIT': 2,
            'DISSOLVE': False,
            'OUTPUT': 'memory:'
        }, context=context, feedback=feedback)['OUTPUT']

        feedback.pushInfo('sink output number of features is {}'.format(sink.featureCount()))

        results = {}
        results[self.OUTPUT] = sink
        return results

Subsequently, I've tried changing the last section to:

    bufferedLayer=processing.run("native:buffer", {
        'INPUT': source,
        'DISTANCE': -4.0,
        'SEGMENTS': 5,
        'END_CAP_STYLE': 0,
        'JOIN_STYLE': 0,
        'MITER_LIMIT': 2,
        'DISSOLVE': False,
        'OUTPUT': parameters['OUTPUT']
    }, context=context, feedback=feedback)['OUTPUT']
    feedback.pushInfo('sink output number of features is {}'.format(bufferedLayer.featureCount()))

    return {self.OUTPUT: bufferedLayer}

The function down gives the gives an error when it runs. Here is the output log for the function:

Processing algorithm…
Algorithm 'My Script' starting…
Input parameters:
{'INPUT':'C:/GIS/VBS/map_nsc_geotypical_ne_europe/map_nsc_geotypical_ne_europe_shared/source/multimap/tiles_3x3/mapframes/gce_3x3_001_001.shp', 'OUTPUT' : 'memory:' }
CRS is EPSG:32634
Results: {'OUTPUT': 'output_7cd8f3b7_6e2b_457b_be6b_da9c71439756'}
sink output number of features is 1
Execution completed in 0.03 seconds
Results:
{'OUTPUT': }

Loading resulting layers
The following layers were not correctly generated.<ul><li>output_7cd8f3b7_6e2b_457b_be6b_da9c71439756</li></ul>You can check the 'Log Messages Panel' in QGIS main window to find more information about the execution of the algorithm.

I've tried following Jochen's suggestion of using an output rather than a parameter. By doing this the script window changes from this:

to this, it no longer has the option to specify the output layer:

Running the script also causes QGIS to crash :/

Answer 1

After some reasearching I found that the key problem seems to be that you first defined sink

(sink, dest_id) = self.parameterAsSink( ...

and later on overwrite it with the result of processing.run...

sink=processing.run("native:buffer", { ...

I tried the following which worked for me:

#create a temporary layer as a processing result
lyr = processing.run("native:buffer", {
        'INPUT': source,
        'DISTANCE': 40,
        'SEGMENTS': 5,
        'END_CAP_STYLE': 0,
        'JOIN_STYLE': 0,
        'MITER_LIMIT': 2,
        'DISSOLVE': False,
        'OUTPUT': 'memory:'
    }, context=context, feedback=feedback)['OUTPUT']

    feedback.pushInfo('sink output number of features is {}'.format(lyr.featureCount()))

# and then copy the content of the temporary processing result into the output sink    
for f in lyr.getFeatures():
    sink.addFeature(f)

Note that when you define the output sink as follows

(sink, dest_id) = self.parameterAsSink(
        parameters,
        self.OUTPUT,
        context,
        source.fields(),
        source.wkbType(),
        source.sourceCrs()
    )

you are restricted to the geometry type of the source layer (source.wkbType()), which may cause problems (crash) when you try to buffer e.g. a point layer.

But with the mentioned slight alterations your script works: