Tooltip on toggle button

IPython/html/static/widgets/js/widget_bool.js

@@ -76,7 +76,7 b' define(['
                     e.preventDefault();
                     that.handle_click();
                 }));
-
+            this.$el.attr("data-toggle", "tooltip");
             this.model.on('change:button_style', function(model, value) {
                 this.update_button_style();
             }, this);
@@ -113,6 +113,7 b' define(['
                 this.$el.prop('disabled', disabled);
 
                 var description = this.model.get('description');
+                this.$el.attr("title", this.model.get("tooltip"));
                 if (description.trim().length === 0) {
                     this.$el.html(" "); // Preserve button height
                 } else {

IPython/html/widgets/widget_bool.py

@@ -38,6 +38,7 b' class ToggleButton(_Bool):'
     """Displays a boolean `value`."""
     
     _view_name = Unicode('ToggleButtonView', sync=True)
+    tooltip = Unicode(help="Tooltip caption of the toggle button.", sync=True)
 
     button_style = CaselessStrEnum(
         values=['primary', 'success', 'info', 'warning', 'danger', ''],