Added definition_order parameter to CrossSelector (#570)

panel/tests/widgets/test_select.py

@@ -302,3 +302,13 @@ def test_cross_select_move_unselected_to_selected():
 
     assert cross_select.value == ['A', 1, 'B', 3]
     assert cross_select._lists[True].options == ['A', 'B', '1', '3']
+
+
+def test_cross_select_move_unselected_to_selected_not_definition_order():
+    cross_select = CrossSelector(options=['B', 'A', 'C', 1, 2, 3], value=['A', 1], size=5, definition_order=False)
+
+    cross_select._lists[False].value = ['B', '3']
+    cross_select._buttons[True].clicks = 1
+
+    assert cross_select.value == ['A', 1, 'B', 3]
+    assert cross_select._lists[True].options == ['A', '1', 'B', '3']

panel/widgets/select.py

@@ -300,6 +300,10 @@ class CrossSelector(CompositeWidget, MultiSelect):
        The number of options shown at once (note this is the
        only way to control the height of this widget)""")
 
+    definition_order = param.Integer(default=True, doc=""" Whether to
+       preserve definition order after filtering. Disable to allow the
+       order of selection to define the order of the selected list.""")
+
     def __init__(self, *args, **kwargs):
         super(CrossSelector, self).__init__(**kwargs)
         # Compute selected and unselected values
@@ -418,7 +422,10 @@ def _apply_query(self, selected):
         query = self._query[selected]
         other = self._lists[not selected].labels
         labels = self.labels
-        options = [k for k in labels if k not in other]
+        if self.definition_order:
+            options = [k for k in labels if k not in other]
+        else:
+            options = self._lists[selected].values
         if not query:
             self._lists[selected].options = options
             self._lists[selected].value = []