diff --git a/IPython/core/shellapp.py b/IPython/core/shellapp.py
index adb9b40..669febf 100644
--- a/IPython/core/shellapp.py
+++ b/IPython/core/shellapp.py
@@ -173,15 +173,15 @@ class InteractiveShellApp(Configurable):
     module_to_run = Unicode('', config=True,
         help="Run the module as a script."
     )
-    gui = CaselessStrEnum(gui_keys, config=True,
+    gui = CaselessStrEnum(gui_keys, config=True, allow_none=True,
         help="Enable GUI event loop integration with any of {0}.".format(gui_keys)
     )
-    matplotlib = CaselessStrEnum(backend_keys,
+    matplotlib = CaselessStrEnum(backend_keys, allow_none=True,
         config=True,
         help="""Configure matplotlib for interactive use with
         the default matplotlib backend."""
     )
-    pylab = CaselessStrEnum(backend_keys,
+    pylab = CaselessStrEnum(backend_keys, allow_none=True,
         config=True,
         help="""Pre-load matplotlib and numpy for interactive use,
         selecting a particular matplotlib backend and loop integration.
diff --git a/IPython/html/services/contents/manager.py b/IPython/html/services/contents/manager.py
index 446aed6..3f2e2a2 100644
--- a/IPython/html/services/contents/manager.py
+++ b/IPython/html/services/contents/manager.py
@@ -109,7 +109,7 @@ class ContentsManager(LoggingConfigurable):
 
     checkpoints_class = Type(Checkpoints, config=True)
     checkpoints = Instance(Checkpoints, config=True)
-    checkpoints_kwargs = Dict(allow_none=False, config=True)
+    checkpoints_kwargs = Dict(config=True)
 
     def _checkpoints_default(self):
         return self.checkpoints_class(**self.checkpoints_kwargs)
diff --git a/IPython/html/widgets/widget.py b/IPython/html/widgets/widget.py
index 062b671..50d9b90 100644
--- a/IPython/html/widgets/widget.py
+++ b/IPython/html/widgets/widget.py
@@ -140,7 +140,7 @@ class Widget(LoggingConfigurable):
     
     _property_lock = Tuple((None, None))
     _send_state_lock = Int(0)
-    _states_to_send = Set(allow_none=False)
+    _states_to_send = Set()
     _display_callbacks = Instance(CallbackDispatcher, ())
     _msg_callbacks = Instance(CallbackDispatcher, ())
     
diff --git a/IPython/html/widgets/widget_box.py b/IPython/html/widgets/widget_box.py
index 555db22..ca73e0d 100644
--- a/IPython/html/widgets/widget_box.py
+++ b/IPython/html/widgets/widget_box.py
@@ -18,16 +18,16 @@ class Box(DOMWidget):
     # Child widgets in the container.
     # Using a tuple here to force reassignment to update the list.
     # When a proper notifying-list trait exists, that is what should be used here.
-    children = Tuple(sync=True, allow_none=False)
+    children = Tuple(sync=True)
     
     _overflow_values = ['visible', 'hidden', 'scroll', 'auto', 'initial', 'inherit', '']
     overflow_x = CaselessStrEnum(
         values=_overflow_values, 
-        default_value='', allow_none=False, sync=True, help="""Specifies what
+        default_value='', sync=True, help="""Specifies what
         happens to content that is too large for the rendered region.""")
     overflow_y = CaselessStrEnum(
         values=_overflow_values, 
-        default_value='', allow_none=False, sync=True, help="""Specifies what
+        default_value='', sync=True, help="""Specifies what
         happens to content that is too large for the rendered region.""")
 
     box_style = CaselessStrEnum(
@@ -59,10 +59,10 @@ class FlexBox(Box):
     _locations = ['start', 'center', 'end', 'baseline', 'stretch']
     pack = CaselessStrEnum(
         values=_locations, 
-        default_value='start', allow_none=False, sync=True)
+        default_value='start', sync=True)
     align = CaselessStrEnum(
         values=_locations, 
-        default_value='start', allow_none=False, sync=True)
+        default_value='start', sync=True)
 
 
 def VBox(*pargs, **kwargs):
diff --git a/IPython/html/widgets/widget_float.py b/IPython/html/widgets/widget_float.py
index a25f568..47f6951 100644
--- a/IPython/html/widgets/widget_float.py
+++ b/IPython/html/widgets/widget_float.py
@@ -130,8 +130,7 @@ class FloatSlider(_BoundedFloat):
     """
     _view_name = Unicode('FloatSliderView', sync=True)
     orientation = CaselessStrEnum(values=['horizontal', 'vertical'], 
-        default_value='horizontal', 
-        help="Vertical or horizontal.", allow_none=False, sync=True)
+        default_value='horizontal', help="Vertical or horizontal.", sync=True)
     _range = Bool(False, help="Display a range selector", sync=True)
     readout = Bool(True, help="Display the current value of the slider next to it.", sync=True)
     slider_color = Unicode(sync=True)
@@ -285,8 +284,7 @@ class FloatRangeSlider(_BoundedFloatRange):
     """
     _view_name = Unicode('FloatSliderView', sync=True)
     orientation = CaselessStrEnum(values=['horizontal', 'vertical'], 
-        default_value='horizontal', allow_none=False, 
-        help="Vertical or horizontal.", sync=True)
+        default_value='horizontal', help="Vertical or horizontal.", sync=True)
     _range = Bool(True, help="Display a range selector", sync=True)
     readout = Bool(True, help="Display the current value of the slider next to it.", sync=True)
     slider_color = Unicode(sync=True)
diff --git a/IPython/html/widgets/widget_int.py b/IPython/html/widgets/widget_int.py
index 4913ac1..c455615 100644
--- a/IPython/html/widgets/widget_int.py
+++ b/IPython/html/widgets/widget_int.py
@@ -84,8 +84,7 @@ class IntSlider(_BoundedInt):
     """Slider widget that represents a int bounded by a minimum and maximum value."""
     _view_name = Unicode('IntSliderView', sync=True)
     orientation = CaselessStrEnum(values=['horizontal', 'vertical'], 
-        default_value='horizontal', allow_none=False, 
-        help="Vertical or horizontal.", sync=True)
+        default_value='horizontal', help="Vertical or horizontal.", sync=True)
     _range = Bool(False, help="Display a range selector", sync=True)
     readout = Bool(True, help="Display the current value of the slider next to it.", sync=True)
     slider_color = Unicode(sync=True)
@@ -196,8 +195,7 @@ class IntRangeSlider(_BoundedIntRange):
     """Slider widget that represents a pair of ints between a minimum and maximum value."""
     _view_name = Unicode('IntSliderView', sync=True)
     orientation = CaselessStrEnum(values=['horizontal', 'vertical'], 
-        default_value='horizontal', allow_none=False, 
-        help="Vertical or horizontal.", sync=True)
+        default_value='horizontal', help="Vertical or horizontal.", sync=True)
     _range = Bool(True, help="Display a range selector", sync=True)
     readout = Bool(True, help="Display the current value of the slider next to it.", sync=True)
     slider_color = Unicode(sync=True)
diff --git a/IPython/kernel/tests/test_message_spec.py b/IPython/kernel/tests/test_message_spec.py
index 3dabcc4..9e8f21b 100644
--- a/IPython/kernel/tests/test_message_spec.py
+++ b/IPython/kernel/tests/test_message_spec.py
@@ -106,7 +106,7 @@ class MimeBundle(Reference):
 
 class ExecuteReply(Reference):
     execution_count = Integer()
-    status = Enum((u'ok', u'error'))
+    status = Enum((u'ok', u'error'), default_value=u'ok')
     
     def check(self, d):
         Reference.check(self, d)
@@ -139,7 +139,7 @@ class ArgSpec(Reference):
 
 
 class Status(Reference):
-    execution_state = Enum((u'busy', u'idle', u'starting'))
+    execution_state = Enum((u'busy', u'idle', u'starting'), default_value=u'busy')
 
 
 class CompleteReply(Reference):
@@ -165,7 +165,7 @@ class KernelInfoReply(Reference):
 
 
 class IsCompleteReply(Reference):
-    status = Enum((u'complete', u'incomplete', u'invalid', u'unknown'))
+    status = Enum((u'complete', u'incomplete', u'invalid', u'unknown'), default_value=u'complete')
     
     def check(self, d):
         Reference.check(self, d)
@@ -187,7 +187,7 @@ Error = ExecuteReplyError
 
 
 class Stream(Reference):
-    name = Enum((u'stdout', u'stderr'))
+    name = Enum((u'stdout', u'stderr'), default_value=u'stdout')
     text = Unicode()
 
 
diff --git a/IPython/parallel/controller/scheduler.py b/IPython/parallel/controller/scheduler.py
index 4f13a1b..55cc37f 100644
--- a/IPython/parallel/controller/scheduler.py
+++ b/IPython/parallel/controller/scheduler.py
@@ -171,8 +171,8 @@ class TaskScheduler(SessionFactory):
         """
     )
     scheme_name = Enum(('leastload', 'pure', 'lru', 'plainrandom', 'weighted', 'twobin'),
-        'leastload', config=True, allow_none=False,
-        help="""select the task scheduler scheme  [default: Python LRU]
+        'leastload', config=True,
+help="""select the task scheduler scheme  [default: Python LRU]
         Options are: 'pure', 'lru', 'plainrandom', 'weighted', 'twobin','leastload'"""
     )
     def _scheme_name_changed(self, old, new):
diff --git a/IPython/terminal/console/interactiveshell.py b/IPython/terminal/console/interactiveshell.py
index ef97c87..cef8630 100644
--- a/IPython/terminal/console/interactiveshell.py
+++ b/IPython/terminal/console/interactiveshell.py
@@ -50,7 +50,7 @@ class ZMQTerminalInteractiveShell(TerminalInteractiveShell):
     )
 
     image_handler = Enum(('PIL', 'stream', 'tempfile', 'callable'),
-                         config=True, help=
+                         config=True, allow_none=True, help=
         """
         Handler for image type output.  This is useful, for example,
         when connecting to the kernel in which pylab inline backend is
@@ -99,7 +99,7 @@ class ZMQTerminalInteractiveShell(TerminalInteractiveShell):
 
     mime_preference = List(
         default_value=['image/png', 'image/jpeg', 'image/svg+xml'],
-        config=True, allow_none=False, help=
+        config=True, help=
         """
         Preferred object representation MIME type in order.  First
         matched MIME type will be used.
diff --git a/IPython/utils/tests/test_traitlets.py b/IPython/utils/tests/test_traitlets.py
index 2a3ba48..8344a3d 100644
--- a/IPython/utils/tests/test_traitlets.py
+++ b/IPython/utils/tests/test_traitlets.py
@@ -976,8 +976,8 @@ class TestInstanceList(TraitTestBase):
         self.assertIs(self.obj.traits()['value']._trait.klass, Foo)
 
     _default_value = []
-    _good_values = [[Foo(), Foo(), None], None]
-    _bad_values = [['1', 2,], '1', [Foo]]
+    _good_values = [[Foo(), Foo(), None], []]
+    _bad_values = [['1', 2,], '1', [Foo], None]
 
 class LenListTrait(HasTraits):
 
@@ -1431,14 +1431,14 @@ class TestForwardDeclaredInstanceList(TraitTestBase):
         [ForwardDeclaredBar(), ForwardDeclaredBarSub(), None],
         [None],
         [],
-        None,
     ]
     _bad_values = [
         ForwardDeclaredBar(),
         [ForwardDeclaredBar(), 3],
         '1',
         # Note that this is the type, not an instance.
-        [ForwardDeclaredBar]
+        [ForwardDeclaredBar],
+        None,
     ]
 
 class TestForwardDeclaredTypeList(TraitTestBase):
@@ -1454,14 +1454,14 @@ class TestForwardDeclaredTypeList(TraitTestBase):
         [ForwardDeclaredBar, ForwardDeclaredBarSub, None],
         [],
         [None],
-        None,
     ]
     _bad_values = [
         ForwardDeclaredBar,
         [ForwardDeclaredBar, 3],
         '1',
         # Note that this is an instance, not the type.
-        [ForwardDeclaredBar()]
+        [ForwardDeclaredBar()],
+        None,
     ]
 ###
 # End Forward Declaration Tests
diff --git a/IPython/utils/traitlets.py b/IPython/utils/traitlets.py
index 4c26e36..c69aeeb 100644
--- a/IPython/utils/traitlets.py
+++ b/IPython/utils/traitlets.py
@@ -815,7 +815,7 @@ class Type(ClassBasedTraitType):
             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 : boolean
+        allow_none : bool [ default True ]
             Indicates whether None is allowed as an assignable value. Even if
             ``False``, the default value may be ``None``.
         """
@@ -912,7 +912,7 @@ class Instance(ClassBasedTraitType):
             Positional arguments for generating the default value.
         kw : dict
             Keyword arguments for generating the default value.
-        allow_none : bool
+        allow_none : bool [default True]
             Indicates whether None is allowed as a value.
 
         Notes
@@ -1334,9 +1334,9 @@ class CBool(Bool):
 class Enum(TraitType):
     """An enum that whose value must be in a given sequence."""
 
-    def __init__(self, values, default_value=None, allow_none=True, **metadata):
+    def __init__(self, values, default_value=None, **metadata):
         self.values = values
-        super(Enum, self).__init__(default_value, allow_none=allow_none, **metadata)
+        super(Enum, self).__init__(default_value, **metadata)
 
     def validate(self, obj, value):
         if value in self.values:
@@ -1372,7 +1372,7 @@ class Container(Instance):
     _valid_defaults = SequenceTypes
     _trait = None
 
-    def __init__(self, trait=None, default_value=None, allow_none=True,
+    def __init__(self, trait=None, default_value=None, allow_none=False,
                 **metadata):
         """Create a container trait type from a list, set, or tuple.
 
@@ -1398,7 +1398,7 @@ class Container(Instance):
             The default value for the Trait.  Must be list/tuple/set, and
             will be cast to the container type.
 
-        allow_none : Bool [ default True ]
+        allow_none : bool [ default False ]
             Whether to allow the value to be None
 
         **metadata : any
@@ -1468,8 +1468,7 @@ class List(Container):
     klass = list
     _cast_types = (tuple,)
 
-    def __init__(self, trait=None, default_value=None, minlen=0, maxlen=sys.maxsize,
-                allow_none=True, **metadata):
+    def __init__(self, trait=None, default_value=None, minlen=0, maxlen=sys.maxsize, **metadata):
         """Create a List trait type from a list, set, or tuple.
 
         The default value is created by doing ``List(default_value)``,
@@ -1500,7 +1499,7 @@ class List(Container):
         maxlen : Int [ default sys.maxsize ]
             The maximum length of the input list
 
-        allow_none : Bool [ default True ]
+        allow_none : bool [ default False ]
             Whether to allow the value to be None
 
         **metadata : any
@@ -1510,7 +1509,7 @@ class List(Container):
         self._minlen = minlen
         self._maxlen = maxlen
         super(List, self).__init__(trait=trait, default_value=default_value,
-                                allow_none=allow_none, **metadata)
+                                **metadata)
 
     def length_error(self, obj, value):
         e = "The '%s' trait of %s instance must be of length %i <= L <= %i, but a value of %s was specified." \
@@ -1544,7 +1543,7 @@ class Tuple(Container):
     _cast_types = (list,)
 
     def __init__(self, *traits, **metadata):
-        """Tuple(*traits, default_value=None, allow_none=True, **medatata)
+        """Tuple(*traits, default_value=None, **medatata)
 
         Create a tuple from a list, set, or tuple.
 
@@ -1575,7 +1574,7 @@ class Tuple(Container):
             will be cast to a tuple. If `traits` are specified, the
             `default_value` must conform to the shape and type they specify.
 
-        allow_none : Bool [ default True ]
+        allow_none : bool [ default False ]
             Whether to allow the value to be None
 
         **metadata : any
@@ -1606,8 +1605,7 @@ class Tuple(Container):
         if self._traits and default_value is None:
             # don't allow default to be an empty container if length is specified
             args = None
-        super(Container,self).__init__(klass=self.klass, args=args,
-                                  allow_none=allow_none, **metadata)
+        super(Container,self).__init__(klass=self.klass, args=args, **metadata)
 
     def validate_elements(self, obj, value):
         if not self._traits:
@@ -1640,7 +1638,7 @@ class Tuple(Container):
 class Dict(Instance):
     """An instance of a Python dict."""
 
-    def __init__(self, default_value={}, allow_none=True, **metadata):
+    def __init__(self, default_value={}, allow_none=False, **metadata):
         """Create a dict trait type from a dict.
 
         The default value is created by doing ``dict(default_value)``,
@@ -1662,7 +1660,7 @@ class Dict(Instance):
 class EventfulDict(Instance):
     """An instance of an EventfulDict."""
 
-    def __init__(self, default_value={}, allow_none=True, **metadata):
+    def __init__(self, default_value={}, allow_none=False, **metadata):
         """Create a EventfulDict trait type from a dict.
 
         The default value is created by doing
@@ -1685,7 +1683,7 @@ class EventfulDict(Instance):
 class EventfulList(Instance):
     """An instance of an EventfulList."""
 
-    def __init__(self, default_value=None, allow_none=True, **metadata):
+    def __init__(self, default_value=None, allow_none=False, **metadata):
         """Create a EventfulList trait type from a dict.
 
         The default value is created by doing