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))