Adding new attribute field to processing output algorithm in PyQGIS?

Question

I have a processing script that will carry out a selection by expression of the input layer, then do a join attributes by location with a join layer.

The next stage will be to add a new field and calculate the field, I got a python script that runs this but I am struggling to adapt it into the processing tool.

The code is below:

def initAlgorithm(self, config=None):
 """
 Here we define the inputs and output of the algorithm, along
 with some other properties.
 """
 # We add the input vector features source. It can have any kind of
 # geometry.
 self.addParameter(
 QgsProcessingParameterFeatureSource(
 self.INPUT,
 self.tr('NRD Layer'),
 [QgsProcessing.TypeVectorPoint]
 )
 )
 self.addParameter(
 QgsProcessingParameterFeatureSource(
 self.JOIN,
 self.tr('Parliamentary Constituency Layer'),
 [QgsProcessing.TypeVectorPolygon]
 )
 )
 self.addParameter(
 QgsProcessingParameterField(
 self.JOIN_FIELDS,
 self.tr('Choose Join Field'),
 parentLayerParameterName = self.JOIN,
 allowMultiple=False,
 optional=False
 )
 )
 self.addParameter(
 QgsProcessingParameterVectorDestination(
 self.OUTPUT,
 self.tr('Output layer')
 )
 )
def processAlgorithm(self, parameters, context, feedback):
 selectData = processing.run(
 'qgis:selectbyexpression',
 {
 "INPUT":parameters['INPUT'],
 "EXPRESSION":'\"mcmcode\" !=\'1\' AND \"mcmcode\" !=\'9\' AND \"mcmcode\" !=\'960\' AND \"mcmcode\" !=\'999\'',
 "METHOD":0,
 'OUTPUT':parameters['OUTPUT']
 },
 context=context,
 feedback=feedback 
 )
 joinoutput = processing.run(
 'qgis:joinattributesbylocation', 
 {
 'INPUT':QgsProcessingFeatureSourceDefinition(selectData['OUTPUT'], True),
 'JOIN':parameters['JOIN'] ,
 'PREDICATE':[0],
 'JOIN_FIELDS':parameters['join_field'],
 'METHOD':1,
 'DISCARD_NONMATCHING':True,
 'PREFIX':'',
 'OUTPUT':parameters['OUTPUT']
 }, 
 context = context, 
 feedback = feedback
 )
 NRDLayer = joinoutput['OUTPUT']
 F_Onset = QgsField('F_Onset',QVariant.Double)
 NRDLayer.dataProvider().addAttributes([F_Onset])
 NRDLayer.updateFields()
 return {'OUTPUT':joinoutput['OUTPUT']}

The error suggests you can't use dataprovider on joinoutput['OUTPUT'].

I thought I might need to use ParameterasSink to get my output in order to be able to use things like .featureCount(), .dataProvider() with it etc.

However not understanding this very well I would rather avoid that. The tool works up to that point.

Any ideas on what to do in QGIS 3?

• Fran Raga: check this solution gis.stackexchange.com/a/304791/49538
• Carlos GC: Thanks, I am trying to avoid using parametersAsSink, firstly because my current method (QgsProcessingParameterVectorDestination) works up to this point, and secondly because i dont quite understand it or know how to apply it (it seems far more complicated? What is the main difference?) Do you know when one has to use parametersAsSink? If I am adding and calculating a field do I need to?

Answer 1

Okay so it was actually quite simple, you can't use the output of the algorithm straight away, you first need to do this:

layer = self.parameterAsVectorLayer(parameters, self.OUTPUT, context)

Then you can run functions like layer.getFeatures() etc. on that.

Don't know if there are better ways to do what I am trying to do but this works! Anyone who has better ways feel welcome to post a comment.