Setup default selected index behavior

Author: SylvainCorlay

ipywidgets/widgets/widget_selectioncontainer.py

@@ -10,15 +10,16 @@
 from .widget_box import Box
 from .widget import register
 from .widget_core import CoreWidget
-from traitlets import Unicode, Dict, CInt, TraitError, validate
+from traitlets import Unicode, Dict, CInt, TraitError, validate, observe
 
 
 class _SelectionContainer(Box, CoreWidget):
     """Base class used to display multiple child widgets."""
     _titles = Dict(help="Titles of the pages").tag(sync=True)
     selected_index = CInt(
         help="""The index of the selected page. This is either an integer selecting a particular sub-widget, or None to have no widgets selected.""",
-        allow_none=True
+        allow_none=True,
+        default_value=None
     ).tag(sync=True)
 
     @validate('selected_index')
@@ -28,6 +29,11 @@ def _validated_index(self, proposal):
         else:
             raise TraitError('Invalid selection: index out of bounds')
 
+    @observe('children')
+    def _observe_children(self, change):
+        if self.selected_index is not None and len(change.new) < self.selected_index:
+            self.selected_index = None
+
     # Public methods
     def set_title(self, index, title):
         """Sets the title of a container page.
@@ -79,6 +85,15 @@ class Tab(_SelectionContainer):
     _view_name = Unicode('TabView').tag(sync=True)
     _model_name = Unicode('TabModel').tag(sync=True)
 
+    def __init__(self, **kwargs):
+        if 'children' in kwargs and 'selected_index' not in kwargs and len(kwargs['children']) > 0:
+            kwargs['selected_index'] = 0
+        super(Tab, self).__init__(**kwargs)
+
+    @observe('children')
+    def _observe_children(self, change):
+        if len(change.new) > 0:
+            self.selected_index = 0
 
 @register
 class Stacked(_SelectionContainer):

packages/controls/src/widget_selectioncontainer.ts

@@ -31,7 +31,7 @@ export class SelectionContainerModel extends BoxModel {
     return {
       ...super.defaults(),
       _model_name: 'SelectionContainerModel',
-      selected_index: 0,
+      selected_index: null,
       _titles: {}
     };
   }