Merge pull request #200 from robbievanleeuwen/sub_fix

Author: connorferster

sectionproperties/pre/geometry.py

@@ -1058,25 +1058,56 @@ def __sub__(self, other):
         """
         Perform difference on Geometry objects with the - operator
         """
-        material = self.material or other.material
-        try:
-            new_polygon = filter_non_polygons(self.geom - other.geom)
-            if isinstance(new_polygon, MultiPolygon):
-                return CompoundGeometry(
-                    [Geometry(polygon, material) for polygon in new_polygon.geoms]
+        material = pre.DEFAULT_MATERIAL
+        if isinstance(self, CompoundGeometry):
+            subs_geom_acc = []
+            for geom in self.geoms:
+                new_geom = geom - other
+                subs_geom_acc.append(new_geom)
+            return CompoundGeometry(subs_geom_acc)
+        else:
+            material = self.material or pre.DEFAULT_MATERIAL
+            try:
+                new_polygon = filter_non_polygons(self.geom - other.geom)
+                if isinstance(new_polygon, GeometryCollection):  # Non-polygon results
+                    return None
+                elif isinstance(new_polygon, MultiPolygon):
+                    return CompoundGeometry(
+                        [Geometry(polygon, material) for polygon in new_polygon.geoms]
+                    )
+                elif isinstance(new_polygon, Polygon):
+                    if self.assigned_control_point and new_polygon.contains(
+                        self.assigned_control_point
+                    ):
+                        return Geometry(
+                            new_polygon, material, self.assigned_control_point
+                        )
+                    else:
+                        return Geometry(new_polygon, material)
+            except:
+                raise ValueError(
+                    f"Cannot perform 'difference' on these two objects: {self} - {other}"
                 )
-            if isinstance(new_polygon, GeometryCollection):
-                return None
-            # Check to see if assigned_control_point is still valid
-            if self.assigned_control_point and new_polygon.contains(
-                self.assigned_control_point
-            ):
-                return Geometry(new_polygon, material, self.control_points[0])
-            return Geometry(new_polygon, material)
-        except:
-            raise ValueError(
-                f"Cannot perform 'difference' on these two objects: {self} - {other}"
-            )
+
+        # material = self.material or other.material
+        # try:
+        #     new_polygon = filter_non_polygons(self.geom - other.geom)
+        #     if isinstance(new_polygon, MultiPolygon):
+        #         return CompoundGeometry(
+        #             [Geometry(polygon, material) for polygon in new_polygon.geoms]
+        #         )
+        #     if isinstance(new_polygon, GeometryCollection):
+        #         return None
+        #     # Check to see if assigned_control_point is still valid
+        #     if self.assigned_control_point and new_polygon.contains(
+        #         self.assigned_control_point
+        #     ):
+        #         return Geometry(new_polygon, material, self.control_points[0])
+        #     return Geometry(new_polygon, material)
+        # except:
+        #     raise ValueError(
+        #         f"Cannot perform 'difference' on these two objects: {self} - {other}"
+        #     )
 
     def __add__(self, other):
         """

sectionproperties/tests/test_sections.py

@@ -6,7 +6,7 @@
 from sectionproperties.pre.library.steel_sections import *
 from sectionproperties.pre.library.nastran_sections import *
 from sectionproperties.analysis.section import Section
-from sectionproperties.pre.pre import Material
+from sectionproperties.pre.pre import DEFAULT_MATERIAL, Material
 from sectionproperties.pre.rhino import load_3dm, load_brep_encoding
 from shapely.geometry import (
     Polygon,
@@ -44,14 +44,15 @@
 overlay_geom.create_mesh([50])
 overlay_sec = Section(overlay_geom)
 
+steel = Material("steel", 200e3, 0.3, 7.85e-6, 400, "grey")
+
 
 def test_material_persistence():
     # Test ensures that the material attribute gets transformed
     # through all of the Geometry transformation methods, each which
     # returns a new Geometry object.
     # The material assignment should persist through all of the
     # transformations
-    steel = Material("steel", 200e3, 0.3, 7.85e-6, 400, "grey")
     big_sq.material = steel
     new_geom = (
         big_sq.align_to(small_sq, on="left", inner=False)
@@ -72,6 +73,25 @@ def test_for_incidental_holes():
     assert len(nested_geom.holes) == 0
 
 
+def test__sub__():
+    small_hole.material = steel
+    top_left = (
+        small_hole.align_to(big_sq, on="left")
+        .align_to(big_sq, on="top")
+        .shift_section(20, -20)
+    )
+    top_right = top_left.shift_section(x_offset=200)
+
+    compound = big_sq - top_left
+    compound = compound + top_left
+    compound = compound - top_right
+    compound = compound + top_right
+
+    assert len(compound.control_points) == 3
+    # Incomplete test to validate that the iterative __sub__ produces
+    # three distinct regions with proper material assignments
+
+
 def test_geometry_from_points():
     # Geometry.from_points() tests a shape with exactly one exterior
     # and an arbitrary number of interiors being built from the legacy