How to union a list of geometries in GeoDjango

unions

from django.contrib.gis.geos import GEOSGeometry

# sample data
geom_1 = GEOSGeometry('POLYGON((-71.8 42.1,-70.6 42.1,-70.5 41.2,-71.8 41.2,-71.8 42.1))')
geom_2 = GEOSGeometry('POLYGON((-71.12 42.23,-71.48 42.34,-71.52 42.55,-71.12 42.23))')
geom_3 = GEOSGeometry('POLYGON((-73.12 42.23,-71.48 42.34,-71.52 42.55,-73.12 42.23))')
polygons = [geom_1, geom_2, geom_3]

# get first polygon
polygon_union = polygons[0]

# update list
polygons = polygons[1:]

# loop through list of polygons
for poly in polygons:
    polygon_union = polygon_union.union(poly)