containers and selectioncontainers now only allow one of any single child

IPython/html/widgets/widget_container.py

@@ -24,11 +24,29 b' class ContainerWidget(DOMWidget):'
 
     # Keys, all private and managed by helper methods.  Flexible box model
     # classes...
-    children = List(Instance(DOMWidget), sync=True)
-    
-    description = Unicode(sync=True)
-    button_text = Unicode(sync=True)
+    children = List(Instance(DOMWidget))
+    _children = List(Instance(DOMWidget), sync=True)
+
+    def __init__(self, *pargs, **kwargs):
+        """Constructor"""
+        DOMWidget.__init__(self, *pargs, **kwargs)
+        self.on_trait_change(self._validate, ['children'])
+
+    def _validate(self, name, old, new):
+        """Validate children list.
+
+        Makes sure only one instance of any given model can exist in the 
+        children list."""
+        if new is not None and isinstance(new, list):
+            children = []
+            for child in new:
+                if child not in children:
+                    children.append(child)
+            self._children = children
 
 
 class ModalWidget(ContainerWidget):
     view_name = Unicode('ModalView', sync=True)
+    
+    description = Unicode(sync=True)
+    button_text = Unicode(sync=True)

IPython/html/widgets/widget_selectioncontainer.py

@@ -14,21 +14,19 b' pages.'
 #-----------------------------------------------------------------------------
 # Imports
 #-----------------------------------------------------------------------------
-from .widget import DOMWidget
+from .widget_container import ContainerWidget
 from IPython.utils.traitlets import Unicode, Dict, CInt, List, Instance
 
 #-----------------------------------------------------------------------------
 # Classes
 #-----------------------------------------------------------------------------
-class AccordionWidget(DOMWidget):
+class AccordionWidget(ContainerWidget):
     view_name = Unicode('AccordionView', sync=True)
 
     # Keys
     _titles = Dict(help="Titles of the pages", sync=True)
     selected_index = CInt(0, sync=True)
 
-    children = List(Instance(DOMWidget), sync=True)
-
     # Public methods
     def set_title(self, index, title):
         """Sets the title of a container page.