Added ability to index Scenes to reference child primitives

plato/draw/Scene.py

@@ -28,6 +28,12 @@ class Scene:
       for prim in scene:
           # (do something with prim)
 
+    Primitives can also be accessed in the order they were added to
+    the scene using list-like syntax::
+
+      first_prim = scene[:3]
+      last_prim = scene[-1]
+
     Optional rendering arguments are enabled as *features*, which are
     name-value pairs identifying a feature by name and any
     configuration of the feature in the value.
@@ -83,6 +89,10 @@ def __init__(self, primitives=[], features={}, size=(40, 30),
         for name in kwargs:
             setattr(self, name, kwargs[name])
 
+    def __getitem__(self, key):
+        """Returns the primitive(s) given an integer index or slice."""
+        return self._primitives[key]
+
     def __iter__(self):
         for prim in self._primitives:
             yield prim

test/test_Scene.py

@@ -83,5 +83,18 @@ def test_features(self):
             scene.get_feature_config('test3'),
             {'value': 'auto_value', 'another_value': None})
 
+    def test_indexing(self):
+        prim0 = draw.Spheres()
+        prim1 = draw.ConvexPolyhedra()
+
+        scene = draw.Scene([prim0, prim1])
+
+        self.assertIs(scene[0], prim0)
+        self.assertIs(scene[-1], prim1)
+        self.assertEqual(scene[:3], [prim0, prim1])
+
+        with self.assertRaises(IndexError):
+            scene[5]
+
 if __name__ == '__main__':
     unittest.main()