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:

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,
 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:

enter image description here

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')))