Allow Type trait to be set with an importable string

IPython/utils/tests/test_traitlets.py

@@ -531,6 +531,15 b' class TestType(TestCase):'
 
         self.assertRaises(TraitError, setattr, a, 'klass', 10)
 
+    def test_set_str_klass(self):
+
+        class A(HasTraits):
+            klass = Type()
+
+        a = A(klass='IPython.utils.ipstruct.Struct')
+        from IPython.utils.ipstruct import Struct
+        self.assertEqual(a.klass, Struct)
+
 class TestInstance(TestCase):
 
     def test_basic(self):

IPython/utils/traitlets.py

@@ -755,6 +755,12 b' class Type(ClassBasedTraitType):'
 
     def validate(self, obj, value):
         """Validates that the value is a valid object instance."""
+        if isinstance(value, py3compat.string_types):
+            try:
+                value = import_item(value)
+            except ImportError:
+                raise TraitError("The '%s' trait of %s instance must be a type, but "
+                                "%r could not be imported" % (self.name, obj, value))
         try:
             if issubclass(value, self.klass):
                 return value