[WIP] make This inherit from Instance and add ThisType

traitlets/tests/test_traitlets.py

@@ -22,7 +22,7 @@
     Union, All, Undefined, Type, This, Instance, TCPAddress, List, Tuple,
     ObjectName, DottedObjectName, CRegExp, link, directional_link,
     ForwardDeclaredType, ForwardDeclaredInstance, validate, observe, default,
-    observe_compat, BaseDescriptor, HasDescriptors,
+    observe_compat, BaseDescriptor, HasDescriptors, ThisType
 )
 
 import six
@@ -1050,6 +1050,61 @@ class Tree(HasTraits):
         with self.assertRaises(TraitError):
             tree.leaves = [1, 2]
 
+class TestThisType(TestCase):
+
+    def test_this_class(self):
+        class Foo(HasTraits):
+            this_type = ThisType()
+
+        f = Foo()
+        self.assertEqual(f.this_type, Foo)
+        f.this_type = Foo
+        self.assertEqual(f.this_type, Foo)
+        self.assertRaises(TraitError, setattr, f, 'this_type', 10)
+
+    def test_this_inst(self):
+        class Foo(HasTraits):
+            this_type = ThisType()
+
+        f = Foo()
+        f.this_type = Foo
+        self.assertTrue(issubclass(f.this_type, Foo))
+
+    def test_subclass(self):
+        class Foo(HasTraits):
+            t = ThisType()
+        class Bar(Foo):
+            pass
+        f = Foo()
+        b = Bar()
+        f.t = Bar
+        b.t = Foo
+        self.assertEqual(f.t, Bar)
+        self.assertEqual(b.t, Foo)
+
+    def test_subclass_override(self):
+        class Foo(HasTraits):
+            t = ThisType()
+        class Bar(Foo):
+            t = ThisType()
+        f = Foo()
+        b = Bar()
+        f.t = Bar
+        self.assertEqual(f.t, Bar)
+        self.assertRaises(TraitError, setattr, b, 't', Foo)
+
+    def test_this_in_container(self):
+
+        class A(HasTraits):
+            types = Dict(ThisType())
+        class B(A): pass
+        class C(B): pass
+        a = A(
+            types={'b': B, 'c': C}
+        )
+        with self.assertRaises(TraitError):
+            a.types = {'b': 1, 'c': 2}
+
 class TraitTestBase(TestCase):
     """A best testing class for basic trait types."""
 

traitlets/traitlets.py

@@ -1727,27 +1727,44 @@ class ForwardDeclaredInstance(ForwardDeclaredMixin, Instance):
     pass
 
 
-class This(ClassBasedTraitType):
-    """A trait for instances of the class containing this trait.
+class ThisClassMixin(object):
 
-    Because how how and when class bodies are executed, the ``This``
-    trait can only have a default value of None.  This, and because we
-    always validate default values, ``allow_none`` is *always* true.
+    # A temporary value until class_init is called
+    klass = type('UndefinedClass', (object,), {})
+
+    def class_init(self, cls, name):
+        super(ThisClassMixin, self).class_init(cls, name)
+        self.klass = self.this_class
+
+
+class This(ThisClassMixin, Instance):
+    """A trait for instances of the class owning this trait.
+
+    Because of how and when class bodies are executed, the ``This``
+    trait holds a temporary class type until the owner class has been
+    setup by :class:`MetaHasDescriptors`. At which point, the final
+    instance type is assigned by :meth:`class_init`.
     """
 
     info_text = 'an instance of the same type as the receiver or None'
+    allow_none = True
 
-    def __init__(self, **metadata):
-        super(This, self).__init__(None, **metadata)
 
-    def validate(self, obj, value):
-        # What if value is a superclass of obj.__class__?  This is
-        # complicated if it was the superclass that defined the This
-        # trait.
-        if isinstance(value, self.this_class) or (value is None):
-            return value
-        else:
-            self.error(obj, value)
+class ThisType(ThisClassMixin, Type):
+    """A trait for subclasses of the class owning this trait.
+
+    Because of how and when class bodies are executed, the ``This``
+    trait holds a temporary class type until the owner class has been
+    setup by :class:`MetaHasDescriptors`. At which point, the final
+    instance type is assigned by :meth:`class_init`.
+    """
+
+    def __init__(self, default_value=Undefined, **metadata):
+        super(ThisType, self).__init__(default_value, self.klass, **metadata)
+
+    def class_init(self, cls, name):
+        super(ThisType, self).class_init(cls, name)
+        self.default_value = self.klass
 
 
 class Union(TraitType):