Fix a regression in #1513 with creating selection range sliders.

The following was broken:

from ipywidgets import SelectionRangeSlider
s=SelectionRangeSlider(options=[1])

Author: jasongrout

ipywidgets/widgets/widget_selection.py

@@ -117,7 +117,7 @@ def _validate_options(self, proposal):
 
     @observe('options')
     def _propagate_options(self, change):
-        "Set the values and labels, and unselect any option if we aren't initializing"
+        "Set the values and labels, and select the first option if we aren't initializing"
         options = self._options_full
         self.set_trait('_options_labels', tuple(i[0] for i in options))
         self._options_values = tuple(i[1] for i in options)
@@ -391,6 +391,15 @@ def _validate_options(self, proposal):
             raise TraitError("Option list must be nonempty")
         return proposal.value
 
+    @observe('options')
+    def _propagate_options(self, change):
+        "Select the first option"
+        options = self._options_full
+        self.set_trait('_options_labels', tuple(i[0] for i in options))
+        self._options_values = tuple(i[1] for i in options)
+        if self._initializing_traits_ is not True:
+            self.index = (0, 0)
+
 @register
 class SelectionSlider(_SelectionNonempty):
     """Slider to select a single item from a list or dictionary."""
@@ -415,13 +424,6 @@ class SelectionRangeSlider(_MultipleSelectionNonempty):
     label = Tuple(help="Min and max selected labels")
     index = Tuple((0,0), help="Min and max selected indices").tag(sync=True)
 
-    @observe('options')
-    def _propagate_options(self, change):
-        "Unselect any option"
-        if self._initializing_traits_ is not True:
-            self.index = (0, 0)
-        self.set_trait('_options_labels', tuple(i[0] for i in change.new))
-        self._options_values = tuple(i[1] for i in change.new)
 
     @validate('index')
     def _validate_index(self, proposal):