How to open GPKG layer fo processing in QGIS

Question

I am making a model with QGIS 3.4 graphical modeler that should open and process a layer from the GPKG located in the project folder.

Since the folder path may be changed, and QGIS does not allow using relative paths in the processing tools, I decided to write a script that will open a specific GPKG layer from the project folder without any requests to the user for the GPKG path .

Summarize, in the tool, I want to specify the name of the layer and the script opens it from the GPKG in the project folder and outputs it for further processing.

I am new to Pyqgis and Python in general, so the script ends with an unknown error. Here is my code:

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


class OpenGpkgLayer(QgsProcessingAlgorithm):

    LAYERNAME = 'LAYERNAME'
    OUTPUT = 'OUTPUT'


    def tr(self, string):

        return QCoreApplication.translate('Processing', string)

    def createInstance(self):
        return OpenGpkgLayer()

    def name(self):

        return 'OpenGpkgLayer'

    def displayName(self):

        return self.tr('Open GPKG Layer')

    def group(self):

        return self.tr('GPZU')

    def groupId(self):

        return 'GPZU'

    def shortHelpString(self):

        return self.tr("Open provided layer from GPZU gpkg")

    def initAlgorithm(self, config=None):

        self.addParameter(
            QgsProcessingParameterString(
                self.LAYERNAME,
                self.tr('Input layer name')
            )
        )

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

    def processAlgorithm(self, parameters, context, feedback):

        #a temporary layer name
        lnameTranslated='test_test'
        #open  promted gpkg layer
        layer = QgsVectorLayer(QgsProject.instance().readPath("./") +'/GPZU.gpkg|layername='+self.LAYERNAME,lnameTranslated,"ogr")

        source = layer


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


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

        total = 100.0 / source.featureCount() if source.featureCount() else 0

        features = source.getFeatures()

        for current, feature in enumerate(features):
            # Stop the algorithm if cancel button has been clicked
            if feedback.isCanceled():
                break

            # Add a feature in the sink
            sink.addFeature(feature, QgsFeatureSink.FastInsert)

            # Update the progress bar
            feedback.setProgress(int(current * total))


        return {self.OUTPUT: dest_id}

I'm pretty sure I am doing it wrong. I tried a lot of variants (mainly random :-/) But script still fails with unknown error

Answer 1

I find out that instead of 'self.LAYERNAME' I should use an explicit string variable initialized as

layer_name = self.parameterAsString(parameters, self.LAYERNAME, context)

So, the final result should be look like

layer_name = self.parameterAsString(parameters, self.LAYERNAME, context)

layer = QgsVectorLayer(QgsProject.instance().readPath("./") +'/GPZU.gpkg|layername='+layer_name ,lnameTranslated,"ogr")