from qgis.core import *
from qgis.gui import *
from qgis.utils import iface

@qgsfunction(args="auto", group='Custom')
def getAttributeIntersectFeature(intersectLayername, intersectFieldname, geom, feature, parent):
# If geom is empty, return empty string
if (geom is None):
    return ''

# Get layer reference from layername
intersectLayer = QgsProject.instance().mapLayersByName(intersectLayername)[0]

# Raise if layer not found
if intersectLayer is None:
    raise Exception("Layer not found: " + intersectLayername)

# Count point within current Atlas feature
attributename = ''
for ft in intersectLayer.getFeatures():
    intersectGeom = ft.geometry()
    if (intersectGeom is None):
        continue
    if intersectGeom.intersects(geom):
        attributename += ft[intersectFieldname] + ','

return attributename[:-1]