Add the names of all neighboring polygons as a new attribute to every polygon in layer (QGIS 3)

neighbors.py

from qgis.utils import iface
from PyQt5.QtCore import QVariant

NAME_COLUMN = 'NAME'
NEIGHBORS_NAME_COLUMN = 'NEIGHBORS'

layer = iface.activeLayer()
layer.startEditing()
layer.dataProvider().addAttributes([QgsField(NEIGHBORS_NAME_COLUMN, QVariant.String)])
layer.updateFields()

feature_dict = {f.id(): f for f in layer.getFeatures()}

index = QgsSpatialIndex()

for f in feature_dict.values():
	index.insertFeature(f)

for f in feature_dict.values():
	print('Working on %s' % f[NAME_COLUMN])
	geom = f.geometry()
	intersecting_ids = index.intersects(geom.boundingBox())
	neighbors = []
	for intersecting_id in intersecting_ids:
		intersecting_f = feature_dict[intersecting_id]
		if (f != intersecting_f and
			not intersecting_f.geometry().disjoint(geom)):
			neighbors.append(intersecting_f[NAME_COLUMN])
	f[NEIGHBORS_NAME_COLUMN] = ','.join(neighbors)
	layer.updateFeature(f)

layer.commitChanges()

print('Processing complete.')