Merge pull request #39 from takluyver/type-default-sentinel

Use NoDefaultSpecified as sentinel for Type default

traitlets/tests/test_traitlets.py

@@ -453,6 +453,41 @@ class A(HasTraits):
         self.assertEqual(a.klass, B)
         self.assertRaises(TraitError, setattr, a, 'klass', 10)
 
+    def test_default_options(self):
+
+        class B(object): pass
+        class C(B): pass
+        class A(HasTraits):
+            # Different possible combinations of options for default_value
+            # and klass. default_value=None is only valid with allow_none=True.
+            k1 = Type()
+            k2 = Type(None, allow_none=True)
+            k3 = Type(B)
+            k4 = Type(klass=B)
+            k5 = Type(default_value=None, klass=B, allow_none=True)
+            k6 = Type(default_value=C, klass=B)
+
+        self.assertIs(A.k1.default_value, object)
+        self.assertIs(A.k1.klass, object)
+        self.assertIs(A.k2.default_value, None)
+        self.assertIs(A.k2.klass, object)
+        self.assertIs(A.k3.default_value, B)
+        self.assertIs(A.k3.klass, B)
+        self.assertIs(A.k4.default_value, B)
+        self.assertIs(A.k4.klass, B)
+        self.assertIs(A.k5.default_value, None)
+        self.assertIs(A.k5.klass, B)
+        self.assertIs(A.k6.default_value, C)
+        self.assertIs(A.k6.klass, B)
+
+        a = A()
+        self.assertIs(a.k1, object)
+        self.assertIs(a.k2, None)
+        self.assertIs(a.k3, B)
+        self.assertIs(a.k4, B)
+        self.assertIs(a.k5, None)
+        self.assertIs(a.k6, C)
+
     def test_value(self):
 
         class B(object): pass

traitlets/traitlets.py

@@ -903,7 +903,7 @@ def error(self, obj, value):
 class Type(ClassBasedTraitType):
     """A trait whose value must be a subclass of a specified class."""
 
-    def __init__ (self, default_value=None, klass=None, **metadata):
+    def __init__ (self, default_value=NoDefaultSpecified, klass=None, **metadata):
         """Construct a Type trait
 
         A Type trait specifies that its values must be subclasses of
@@ -919,26 +919,31 @@ def __init__ (self, default_value=None, klass=None, **metadata):
             the str must be a fully specified class name, like 'foo.bar.Bah'.
             The string is resolved into real class, when the parent
             :class:`HasTraits` class is instantiated.
-        klass : class, str, None
+        klass : class, str [ default object ]
             Values of this trait must be a subclass of klass.  The klass
             may be specified in a string like: 'foo.bar.MyClass'.
             The string is resolved into real class, when the parent
             :class:`HasTraits` class is instantiated.
         allow_none : bool [ default False ]
             Indicates whether None is allowed as an assignable value.
         """
-        if default_value is None:
-            if klass is None:
-                default_value = klass = object
-        elif klass is None:
-            klass = default_value
+        if default_value is NoDefaultSpecified:
+            new_default_value = object if (klass is None) else klass
+        else:
+            new_default_value = default_value
+
+        if klass is None:
+            if (default_value is None) or (default_value is NoDefaultSpecified):
+                klass = object
+            else:
+                klass = default_value
 
         if not (inspect.isclass(klass) or isinstance(klass, py3compat.string_types)):
             raise TraitError("A Type trait must specify a class.")
 
         self.klass       = klass
 
-        super(Type, self).__init__(default_value, **metadata)
+        super(Type, self).__init__(new_default_value, **metadata)
 
     def validate(self, obj, value):
         """Validates that the value is a valid object instance."""