BUG: raise informative error for MultiPolygon in corner-based shape metrics (#739) (#763)

corners, squareness and centroid_corner_distance use GeoSeries.exterior when
include_interiors=False, but exterior is undefined (None) for MultiPolygons.
The dropped rows make the grouped result shorter than the input index, so
corners/squareness raise a cryptic 'Length mismatch' and
centroid_corner_distance silently returns all-NaN. Add a shared
_check_no_multipolygons guard that raises an actionable ValueError pointing to
explode() or include_interiors=True.

Adds a parametrized test over the three metrics.

Per the discussion on #739 this takes the informative-error path; happy to
extend to full MultiPolygon support instead if preferred.

Author: gaoflow

momepy/shape.py

@@ -455,6 +455,24 @@ def shape_index(
     return np.sqrt(geometry.area / np.pi) / (0.5 * longest_axis_length)
 
 
+def _check_no_multipolygons(geometry: GeoDataFrame | GeoSeries, func_name: str) -> None:
+    """Ensure no MultiPolygons are present when relying on polygon exteriors.
+
+    Corner-based metrics use ``GeoSeries.exterior``, which is undefined for
+    MultiPolygons. Without this check the metrics either fail with a cryptic
+    length-mismatch error or silently return ``NaN``. Raising here gives users an
+    actionable message instead.
+    """
+    if (geometry.geom_type == "MultiPolygon").any():
+        raise ValueError(
+            f"momepy.{func_name} does not support MultiPolygon geometries when "
+            "include_interiors=False, as it relies on polygon exteriors. Explode "
+            "MultiPolygons into single-part Polygons first (e.g. "
+            "``geometry.explode(ignore_index=True)``) or pass "
+            "include_interiors=True."
+        )
+
+
 def corners(
     geometry: GeoDataFrame | GeoSeries,
     eps: float = 10,
@@ -508,6 +526,7 @@ def _count_corners(points: DataFrame, eps: float) -> np.integer:
     if include_interiors:
         coords = geometry.reset_index(drop=True).get_coordinates(index_parts=False)
     else:
+        _check_no_multipolygons(geometry, "corners")
         coords = geometry.reset_index(drop=True).exterior.get_coordinates(
             index_parts=False
         )
@@ -571,6 +590,7 @@ def _squareness(points: DataFrame, eps: float):
     if include_interiors:
         coords = geometry.reset_index(drop=True).get_coordinates(index_parts=False)
     else:
+        _check_no_multipolygons(geometry, "squareness")
         coords = geometry.reset_index(drop=True).exterior.get_coordinates(
             index_parts=False
         )
@@ -729,6 +749,7 @@ def _ccd(points: DataFrame, eps: float) -> Series:
     if include_interiors:
         coords = geometry.get_coordinates(index_parts=False)
     else:
+        _check_no_multipolygons(geometry, "centroid_corner_distance")
         coords = geometry.exterior.get_coordinates(index_parts=False)
     coords[["cent_x", "cent_y"]] = geometry.centroid.get_coordinates(index_parts=False)
     ccd = coords.groupby(level=0).apply(_ccd, eps=eps)

momepy/tests/test_shape.py

@@ -1,6 +1,8 @@
 import geopandas as gpd
 import numpy as np
 import pandas as pd
+import pytest
+import shapely
 from libpysal.graph import Graph
 from pandas.testing import assert_frame_equal, assert_series_equal
 
@@ -272,6 +274,32 @@ def test_centroid_corner_distance(self):
         r = mm.centroid_corner_distance(self.df_buildings, include_interiors=True)
         assert_frame_equal(r.describe(), expected)
 
+    @pytest.mark.parametrize(
+        "func", ["corners", "squareness", "centroid_corner_distance"]
+    )
+    def test_corner_metrics_multipolygon(self, func):
+        # corner-based metrics rely on polygon exteriors, which are undefined for
+        # MultiPolygons. With include_interiors=False this used to raise a cryptic
+        # length-mismatch error (corners, squareness) or silently return NaN
+        # (centroid_corner_distance). See GH#739.
+        polygons = self.df_buildings.geometry.iloc[:3]
+        multi = gpd.GeoSeries(
+            [shapely.MultiPolygon([poly]) for poly in polygons],
+            crs=self.df_buildings.crs,
+        )
+
+        with pytest.raises(
+            ValueError, match="does not support MultiPolygon geometries"
+        ):
+            getattr(mm, func)(multi)
+
+        # include_interiors=True handles MultiPolygons and must keep working
+        getattr(mm, func)(multi, include_interiors=True)
+
+        # exploding into single-part Polygons is the documented workaround
+        exploded = multi.explode(ignore_index=True)
+        getattr(mm, func)(exploded)
+
     def test_linearity(self):
         expected = {
             "mean": 0.9976310491404173,