BUG: fix a case when there's only a single building to be passed to voronoi_frames

momepy/functional/_elements.py

@@ -307,10 +307,16 @@ def _tess(ix, poly, blg, threshold, shrink, segment, enclosure_id):
         tess[enclosure_id] = ix
         return tess
 
+    ## in case a single building is left in blg
+    if len(blg) == 1:
+        assigned_ix = blg.index[0]
+    else:
+        assigned_ix = -1
+
     return GeoDataFrame(
         {enclosure_id: ix},
         geometry=[poly],
-        index=[-1],
+        index=[assigned_ix],
         crs=blg.crs,
     )
 

momepy/functional/tests/test_elements.py

@@ -332,6 +332,29 @@ def test_blocks_inner(self):
         else:
             assert len(blocks.sindex.query_bulk(blocks.geometry, "overlaps")[0]) == 0
 
+    def test_tess_single_building_edge_case(self):
+        tessellations = mm.enclosed_tessellation(
+            self.df_buildings, self.enclosures.geometry, n_jobs=-1
+        )
+        orig_grouper = tessellations.groupby("enclosure_index")
+        idxs = ~self.df_buildings.index.isin(orig_grouper.get_group(8).index)
+        idxs[1] = True
+        idxs[21] = False
+        idxs[23] = False
+
+        new_blg = self.df_buildings[idxs]
+        new_blg.loc[22, "geometry"] = new_blg.loc[22, "geometry"].buffer(20)
+        new_tess = mm.enclosed_tessellation(new_blg, self.enclosures.geometry, n_jobs=1)
+
+        # assert that buildings 1 and 22 intersect the same enclosure
+        inp, res = self.enclosures.sindex.query(
+            new_blg.geometry, predicate="intersects"
+        )
+        assert np.isclose(new_blg.iloc[inp[res == 8]].index.values, [1, 22]).all()
+
+        # assert that there is a tessellation for building 1
+        assert 1 in new_tess.index
+
 
 class TestElementsEquivalence:
     def setup_method(self):