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:

##a=field input
##text=output string

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:

enter image description here

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,
import processing

class ExampleProcessingAlgorithm(QgsProcessingAlgorithm):

    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:

enter image description here

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


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

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

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