QGIS Processing Script - Add field to output

Question

My objective is to add a new calculated field to the output layer from a processing script.

This layer is created as follows:-

source = self.parameterAsSource(
            parameters,
            self.INPUT,
            context
        )

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

I am then adding a feature to the sink layer from source in an iteration loop

 for current, feature in enumerate(source.getFeatures()):
      value = 1
      #I want to add value to feature as an attribute, how??
      sink.addFeature(feature, QgsFeatureSink.FastInsert)

How do I add the value attribute to the sink layer?

Should it be done when declaring sink?

Answer 1

Ok I managed it, bit hacky and I need to copy in other fields from feature

Declare the sink with new fields

new_fields = source.fields()
new_fields.append(QgsField('overlap', QVariant.Double))
(sink, dest_id) = self.parameterAsSink(
            parameters,
            self.OUTPUT,
            context,
            new_fields,
            source.wkbType(),
            source.sourceCrs()
)

Create a new feature and output

 new_feat = QgsFeature()
 new_feat.setGeometry(feature.geometry())
 new_feat.setFields(new_fields)
 new_feat['overlap'] = overlap
 sink.addFeature(new_feat, QgsFeatureSink.FastInsert)

Answer 2

I have been looking for this answer for WAY too long.

My task was keeping some old features as well, ended up needing to copy them over.

Here is my solution

        #adding a field for a sampled DEM 
        new_fields = source.fields()
        new_fields.append(QgsField('DEM', QVariant.Double))

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

        #we do not add the original features, we are adding a copy of the old features
             # construct a new feature as unfortunately features form a source object can't be manipulated 
            new_feat = QgsFeature()

            #new fields is old fields plus one new, see up there
            new_feat.setFields(new_fields)

            # iterate over the old features attributes to fill up its copy
            for current2,att in enumerate(feature.attributes()):
                 new_feat.setAttribute(new_fields.field(current2).name(),att)

            
            #add the sampled dem here
            new_feat.setAttribute('z',sampled_values[current])
            
            #feedback.pushInfo(new_feat.attributes)
            sink.addFeature(new_feat, QgsFeatureSink.FastInsert)
            
            # Update the progress bar
            feedback.setProgress(int(current * total))