python: avoid an assertion in trade.MeshData.index_count.

Emit a Python exception instead.

doc/python/magnum.trade.rst

@@ -66,6 +66,9 @@
 .. py:property:: magnum.trade.ImageData3D.pixels
     :raise AttributeError: If :ref:`is_compressed` is :py:`True`
 
+.. py:property:: magnum.trade.MeshData.index_count
+    :raise AttributeError: If :ref:`is_indexed` is :py:`False`
+
 .. py:class:: magnum.trade.ImporterManager
     :summary: Manager for :ref:`AbstractImporter` plugin instances
 

src/python/magnum/test/test_trade.py

@@ -107,7 +107,11 @@ class MeshData(unittest.TestCase):
 
         mesh = importer.mesh(1)
         mesh_refcount = sys.getrefcount(mesh)
+        self.assertTrue(mesh.is_indexed)
         self.assertEqual(mesh.primitive, MeshPrimitive.TRIANGLES)
+        self.assertEqual(mesh.vertex_count, 3)
+        self.assertEqual(mesh.index_count, 3)
+        self.assertEqual(mesh.attribute_count, 2)
         # TODO: test more, once it's exposed
 
         index_data = mesh.index_data
@@ -126,6 +130,21 @@ class MeshData(unittest.TestCase):
         del vertex_data
         self.assertEqual(sys.getrefcount(mesh), mesh_refcount)
 
+    def test_nonindexed(self):
+        # The only way to get a mesh instance is through a manager
+        importer = trade.ImporterManager().load_and_instantiate('GltfImporter')
+        importer.open_file(os.path.join(os.path.dirname(__file__), 'mesh.glb'))
+
+        mesh = importer.mesh(0)
+        self.assertFalse(mesh.is_indexed)
+
+        # Accessing the index data should be possible, they're just empty
+        self.assertEqual(len(mesh.index_data), 0)
+
+        # Accessing any other index-related info should cause an exception
+        with self.assertRaisesRegex(AttributeError, "mesh is not indexed"):
+            mesh.index_count
+
 class Importer(unittest.TestCase):
     def test(self):
         manager = trade.ImporterManager()

src/python/magnum/trade.cpp

@@ -314,7 +314,14 @@ void trade(py::module_& m) {
             return Containers::pyArrayViewHolder(self.vertexData(), py::cast(self));
         }, "Raw vertex data")
         .def_property_readonly("is_indexed", &Trade::MeshData::isIndexed, "Whether the mesh is indexed")
-        .def_property_readonly("index_count", &Trade::MeshData::indexCount)
+        .def_property_readonly("index_count", [](Trade::MeshData& self) {
+            if(!self.isIndexed()) {
+                PyErr_SetString(PyExc_AttributeError, "mesh is not indexed");
+                throw py::error_already_set{};
+            }
+
+            return self.indexCount();
+        })
         .def_property_readonly("vertex_count", &Trade::MeshData::vertexCount)
         .def_property_readonly("attribute_count", static_cast<UnsignedInt(Trade::MeshData::*)() const>(&Trade::MeshData::attributeCount));