No need to use a geoseries

geopandas/tools/overlay.py

@@ -166,11 +166,6 @@ def overlay(df1, df2, how, use_sindex=True):
 
         # Create a geoseries and add it to the collection
         out_series['geometry'] = newpoly
-
-        # FIXME is there a more idomatic way to append a geometry to a Series 
-        # and get a GeoSeries back? 
-        out_series.__class__ = GeoSeries
-        out_series._geometry_column_name = 'geometry'
         collection.append(out_series)
 
     # Return geodataframe with new indicies

tests/test_overlay.py

@@ -38,6 +38,7 @@ class TestDataFrame(unittest.TestCase):
 
     def test_union(self):
         df = overlay(self.polydf, self.polydf2, how="union")
+        self.assertTrue(type(df) is GeoDataFrame)
         self.assertEquals(df.shape, self.union_shape)
         self.assertTrue('value1' in df.columns and 'Shape_Area' in df.columns)