Updating Feature Attributes in QGIS

qgis_add_features.py

# To avoid 'QVariant' is not defined error
from PyQt4.QtCore import *
import json

# Get all layers on the map's canvas
canvas = qgis.utils.iface.mapCanvas()
all_layers = canvas.layers()

# Make dictionary of election district features (individual
# districts) for inserting into spatial index
election_district = [i for i in all_layers if i.name() == 'election_district'][0]
all_ed_features = election_district.getFeatures()

# Make election district spatial index
ed_index = QgsSpatialIndex()
for f in all_ed_features:
    ed_index.insertFeature(f)

# Now, create the parent layers, and set up geography for the
# intersection analysis
parent_layers = [i for i in all_layers if i.name() != 'election_district']

for layer in parent_layers:
    # add the child ids as a new attribute
    layer.dataProvider().addAttributes([
        QgsField('ElectDist', QVariant.String)])
    layer.updateFields()

    # layer editing
    layer.startEditing()

    all_parent_features = layer.getFeatures()

    for feature in all_parent_features:
        # Get only the ids that intersect the proper election_district
        ids = ed_index.intersects(feature.geometry().boundingBox())
        str_ids = json.dumps(ids)
        # Modify the election_districts attribute
        feature['ElectDist'] = str_ids
        layer.updateFeature(feature)

    # commit to save the changes
    layer.commitChanges()

    # write the layer to an output geojson
    QgsVectorFileWriter.writeAsVectorFormat(layer, '/Users/smithgall/Desktop/test_json.geojson', 'utf-8', None, 'GeoJSON')