BUG: Traitlets now accepts instantiated sub-traits in List, Tuple, etc.

For example, List(Int) worked as expected, but List(Instance(MyClass)) treated
the argument as a default value.

IPython/utils/traitlets.py

@@ -117,6 +117,13 b' def repr_type(obj):'
     return msg
 
 
+def is_trait(t):
+    """ Returns whether the given value is an instance or subclass of TraitType.
+    """
+    return (isinstance(t, TraitType) or
+            (isinstance(t, type) and issubclass(t, TraitType)))
+
+
 def parse_notifier_name(name):
     """Convert the name argument to a list of names.
 
@@ -1165,10 +1172,8 b' class Container(Instance):'
             further keys for extensions to the Trait (e.g. config)
 
         """
-        istrait = lambda t: isinstance(t, type) and issubclass(t, TraitType)
-
         # allow List([values]):
-        if default_value is None and not istrait(trait):
+        if default_value is None and not is_trait(trait):
             default_value = trait
             trait = None
 
@@ -1179,8 +1184,8 b' class Container(Instance):'
         else:
             raise TypeError('default value of %s was %s' %(self.__class__.__name__, default_value))
 
-        if istrait(trait):
-            self._trait = trait()
+        if is_trait(trait):
+            self._trait = trait() if isinstance(trait, type) else trait
             self._trait.name = 'element'
         elif trait is not None:
             raise TypeError("`trait` must be a Trait or None, got %s"%repr_type(trait))
@@ -1327,10 +1332,8 b' class Tuple(Container):'
         default_value = metadata.pop('default_value', None)
         allow_none = metadata.pop('allow_none', True)
 
-        istrait = lambda t: isinstance(t, type) and issubclass(t, TraitType)
-
         # allow Tuple((values,)):
-        if len(traits) == 1 and default_value is None and not istrait(traits[0]):
+        if len(traits) == 1 and default_value is None and not is_trait(traits[0]):
             default_value = traits[0]
             traits = ()
 
@@ -1343,7 +1346,7 b' class Tuple(Container):'
 
         self._traits = []
         for trait in traits:
-            t = trait()
+            t = trait() if isinstance(trait, type) else trait
             t.name = 'element'
             self._traits.append(t)