Copy selected attributes from one field to another using PyQGIS

Question

QGIS version 3.4

I need to copy all selected attributes from the field 'HomePostcode' into the field 'Postcode' through PyQGIS. Using the field calculator I would simply use the expression "HomePostcode", ensuring I'm updating field 'Postcode's selected features only.

I've searched high and low for relatable code and the following is the best I can come up with:

#adds layer to map
uri = 'C:/Users/xxxx/Cleaned.csv'
join_layer = iface.addVectorLayer(uri, 'Cleaned', 'ogr')
target_field = 'Postcode'

def calculate_attributes():
    """Calculates values for 'Postcode' by copying attributes from HomePostcode column and adds them to 'Postcode' column"""

with edit(join_layer):
    for feature in join_layer.getFeatures():
        feature.setAttribute(feature.fieldNameIndex('Postcode'), feature['HomePostcode'])
        join_layer.updateFeature(feature)
print(f"Attribute calculated for {target_field} field")

calculate_attributes()

The Python console reads the script but I get a critical error "OGR error syncing to disk: Cannot rename files".

I know the above (if it worked) would just copy all values from HomePostcode to Postcode but I cannot find any clear documentation to target selected features only for QGIS 3.0.

Answer 1

Like this to address only the selected features:

fids = join_layer.selectedFeatureIds()
request = QgsFeatureRequest()
request.setFilterFids(fids)
with edit(join_layer):
    for feature in join_layer.getFeatures(request):
        feature.setAttribute(feature.fieldNameIndex('Postcode'), feature['HomePostcode'])
        join_layer.updateFeature(feature)

Since you cannot edit the .csv directly, you can make a copy to memory, alter that and then export again to .csv. Somewhere in between, there must be a step, where you actually select the features, anyways.