DEV: Forward this_class in container traits.

Makes it possible to do things like List(This()) and have This()
correctly refer to the parent class.

IPython/utils/tests/test_traitlets.py

@@ -684,6 +684,20 b' class TestThis(TestCase):'
         self.assertEqual(f.t, b)
         self.assertRaises(TraitError, setattr, b, 't', f)
 
+    def test_this_in_container(self):
+
+        class Tree(HasTraits):
+            value = Unicode()
+            leaves = List(This())
+
+        tree = Tree(
+            value='foo',
+            leaves=[Tree('bar'), Tree('buzz')]
+        )
+
+        with self.assertRaises(TraitError):
+            tree.leaves = [1, 2]
+
 class TraitTestBase(TestCase):
     """A best testing class for basic trait types."""
 

IPython/utils/traitlets.py

@@ -1354,6 +1354,8 b' class Container(Instance):'
         return self.klass(validated)
 
     def instance_init(self, obj):
+        if isinstance(self._trait, TraitType):
+            self._trait.this_class = self.this_class
         if isinstance(self._trait, Instance):
             self._trait._resolve_classes()
         super(Container, self).instance_init(obj)