Tooltips in togglebuttons

IPython/html/static/widgets/js/widget_selection.js

@@ -325,6 +325,7 b' define(['
                             .html(item_html)
                             .appendTo(that.$buttongroup)
                             .attr('data-value', encodeURIComponent(item))
+                            .attr('data-toggle', 'tooltip')
                             .attr('value', item)
                             .on('click', $.proxy(that.handle_click, that));
                         that.update_style_traits($item_element);
@@ -334,7 +335,8 b' define(['
                     } else {
                         $item_element.removeClass('active');
                     }
-                    $item_element.prop('disabled', disabled); 
+                    $item_element.prop('disabled', disabled);
+                    $item_element.attr('title', that.model.get('tooltips')[index]); 
                 });
                 
                 // Remove items that no longer exist.

IPython/html/widgets/widget_selection.py

@@ -19,7 +19,7 b' from threading import Lock'
 
 from .widget import DOMWidget, register
 from IPython.utils.traitlets import (
-    Unicode, Bool, Any, Dict, TraitError, CaselessStrEnum, Tuple
+    Unicode, Bool, Any, Dict, TraitError, CaselessStrEnum, Tuple, List
 )
 from IPython.utils.py3compat import unicode_type
 from IPython.utils.warn import DeprecatedClass
@@ -32,6 +32,12 b' class _Selection(DOMWidget):'
     
     ``options`` can be specified as a list or dict. If given as a list,
     it will be transformed to a dict of the form ``{str(value):value}``.
+
+    When programmatically setting the value, a reverse lookup is performed
+    among the options to set the value of ``selected_label`` accordingly. The
+    reverse lookup uses the equality operator by default, but an other
+    predicate may be provided via the ``equals`` argument. For example, when
+    dealing with numpy arrays, one may set equals=np.array_equal.
     """
     
     value = Any(help="Selected value")
@@ -194,6 +200,7 b' class ToggleButtons(_Selection):'
     """Group of toggle buttons that represent an enumeration.  Only one toggle
     button can be toggled at any point in time.""" 
     _view_name = Unicode('ToggleButtonsView', sync=True)
+    tooltips = List(Unicode(), sync=True)
 
     button_style = CaselessStrEnum(
         values=['primary', 'success', 'info', 'warning', 'danger', ''],