DEV: Add ForwardDeclaredType and ForwardDeclaredInstance trait types.

These traits take advantage of the this_class attribute to resolve a
supplied name relative to the module in which the containing class was
defined.  You can hack this already by leveraging `__name__`, but this
implementation makes it a bit clearer what you're actually trying to do.

IPython/utils/tests/test_traitlets.py

@@ -20,7 +20,7 b' from IPython.utils.traitlets import ('
     Int, Long, Integer, Float, Complex, Bytes, Unicode, TraitError,
     Undefined, Type, This, Instance, TCPAddress, List, Tuple,
     ObjectName, DottedObjectName, CRegExp, link, directional_link,
-    EventfulList, EventfulDict
+    EventfulList, EventfulDict, ForwardDeclaredType, ForwardDeclaredInstance,
 )
 from IPython.utils import py3compat
 from IPython.testing.decorators import skipif
@@ -969,6 +969,23 b' class TestInstanceList(TraitTestBase):'
     _good_values = [[Foo(), Foo(), None], None]
     _bad_values = [['1', 2,], '1', [Foo]]
 
+class ForwardDeclaredInstanceListTrait(HasTraits):
+
+    value = List(ForwardDeclaredInstance('Foo'))
+
+class TestForwardDeclaredInstanceList(TraitTestBase):
+
+    obj = ForwardDeclaredInstanceListTrait()
+
+    def test_klass(self):
+        """Test that the instance klass is properly assigned."""
+        self.assertIs(self.obj.traits()['value']._trait.klass, Foo)
+
+    _default_value = []
+    _good_values = [[Foo(), Foo(), None], None]
+    _bad_values = [['1', 2,], '1', [Foo]]
+
+
 class LenListTrait(HasTraits):
 
     value = List(Int, [0], minlen=1, maxlen=2)
@@ -1320,3 +1337,33 b' class TestEventful(TestCase):'
 
         # Is the output correct?
         self.assertEqual(a.x, {c: c for c in 'bz'})
+
+
+class ForwardDeclaredBar(object):
+    pass
+
+class ForwardDeclaredBarSub(ForwardDeclaredBar):
+    pass
+
+class ForwardDeclaredInstanceTrait(HasTraits):
+
+    value = ForwardDeclaredInstance(klass='ForwardDeclaredBar')
+
+class TestForwardDeclaredInstanceTrait(TraitTestBase):
+
+    obj = ForwardDeclaredInstanceTrait()
+    _default_value = None
+    _good_values = [None, ForwardDeclaredBar(), ForwardDeclaredBarSub()]
+    _bad_values = ['foo', 3, ForwardDeclaredBar, ForwardDeclaredBarSub]
+
+
+class ForwardDeclaredTypeTrait(HasTraits):
+
+    value = ForwardDeclaredType(klass='ForwardDeclaredBar')
+
+class TestForwardDeclaredInstanceTrait(TraitTestBase):
+
+    obj = ForwardDeclaredTypeTrait()
+    _default_value = None
+    _good_values = [None, ForwardDeclaredBar, ForwardDeclaredBarSub]
+    _bad_values = ['foo', 3, ForwardDeclaredBar(), ForwardDeclaredBarSub()]

IPython/utils/traitlets.py

@@ -967,6 +967,35 b' class Instance(ClassBasedTraitType):'
             return dv
 
 
+class ForwardDeclaredMixin(object):
+    """
+    Mixin for forward-declared versions of Instance and Type.
+    """
+    def _resolve_classes(self):
+        """
+        Find the specified class name by looking for it in the module in which
+        our this_class attribute was defined.
+        """
+        try:
+            modname = self.this_class.__module__
+            self.klass = import_item('.'.join([modname, self.klass]))
+        except AttributeError:
+            raise ImportError(
+                "Module {} has no attribute {}".format(modname, self.klass)
+            )
+
+class ForwardDeclaredType(ForwardDeclaredMixin, Type):
+    """
+    Forward-declared version of Type.
+    """
+    pass
+
+class ForwardDeclaredInstance(ForwardDeclaredMixin, Instance):
+    """
+    Forward-declared version of Instance.
+    """
+    pass
+
 class This(ClassBasedTraitType):
     """A trait for instances of the class containing this trait.