Access string output of QGIS 3 processing script in graphical modeler

Question

In the good ould days of QGIS 2 I created a processing script, that takes as input a table and a table field (table has only one record) and returns the string attribute value of the table field:

##mygroup=group
##myname=name
##input=table
##a=field input
##text=output string

formeln=processing.getObject(input)
for fo in formeln.getFeatures():
    text = fo[a]

Embedding this script in graphical modeler in QGIS 2 and using the output e.g. in a Field calculator algorithm lets me chose its output string as input for (here) the formula:

So far so good, now on to QGIS 3. At least I succeeded (?) in rewriting this script, my result:

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

class ExampleProcessingAlgorithm(QgsProcessingAlgorithm):
    FORMULA_COLLECTION = 'FORMULA_COLLECTION'
    FORMULA_ID = 'FORMULA_ID'
    OUTPUT_STRING = 'OUTPUT'

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

    def createInstance(self):
        return ExampleProcessingAlgorithm()

    def name(self):
        return 'output_table_field_value_as_string'

    def displayName(self):
        return self.tr('Table field value as string')

    def group(self):
        return self.tr('giswg')

    def groupId(self):
        return 'giswg'

    def shortHelpString(self):
        return self.tr("This returns the value of the specified field of a table. Implies, that the table has exacly one record.")

    def initAlgorithm(self, config=None):
        #input parameter
        self.addParameter(QgsProcessingParameterFeatureSource(self.FORMULA_COLLECTION, self.tr('The formula collection'), [QgsProcessing.TypeVector]))
        self.addParameter(QgsProcessingParameterField(self.FORMULA_ID, None, self.tr('The formula identificator'), self.FORMULA_COLLECTION, QgsProcessingParameterField.String))
        #output parameter
        self.addParameter(QgsProcessingParameterString(self.OUTPUT_STRING, self.tr('Output string'), ''))

    def processAlgorithm(self, parameters, context, feedback):
        formula_collection = self.parameterAsSource(parameters, self.FORMULA_COLLECTION, context)
        formula_id = self.parameterAsString(parameters, self.FORMULA_ID, context)

        out_string = self.parameterAsString(parameters, self.OUTPUT_STRING, context)

        #this portion is taken from the old QGIS 2 script
        for fo in formula_collection.getFeatures():
            out_string = fo[formula_id]

        return {self.OUTPUT_STRING: out_string}

Now I would like to do the same in QGIS 3, but the 'Formula' field refuses to let me chose the output of my script as input:

Any suggestions what might be wrong with my script or what else I'm probably missing?

Answer 1

You're adding your string output as an input parameter. It should be:

self.addOutput(QgsProcessingOutputString(self.OUTPUT_STRING, self.tr('Output string')))