Switch Param pane widget type when bounds (un)defined (#1953)

* Switch Param pane widget type when bounds (un)defined

* Add test

* Fix test

panel/param.py

@@ -427,29 +427,31 @@ def action(change):
 
         def link(change, watchers=[watcher]):
             updates = {}
+            widget = self._widgets[p_name]
             if change.what == 'constant':
                 updates['disabled'] = change.new
             elif change.what == 'precedence':
                 if (change.new < self.display_threshold and
                     widget in self._widget_box.objects):
                     self._widget_box.pop(widget)
                 elif change.new >= self.display_threshold:
-                    precedence = lambda k: self.object.param['name' if k == '_title' else k].precedence
-                    params = self._ordered_params
-                    if self.show_name:
-                        params.insert(0, '_title')
-                    widgets = []
-                    for k in params:
-                        if precedence(k) is None or precedence(k) >= self.display_threshold:
-                            widgets.append(self._widgets[k])
-                    self._widget_box.objects = widgets
+                    self._rerender()
                 return
             elif change.what == 'objects':
                 updates['options'] = p_obj.get_range()
             elif change.what == 'bounds':
                 start, end = p_obj.get_soft_bounds()
-                updates['start'] = start
-                updates['end'] = end
+                supports_bounds = hasattr(widget, 'start')
+                if start is None or end is None:
+                    rerender = supports_bounds
+                else:
+                    rerender = not supports_bounds
+                if supports_bounds:
+                    updates['start'] = start
+                    updates['end'] = end
+                if rerender:
+                    self._rerender_widget(p_name)
+                    return
             elif change.what == 'step':
                 updates['step'] = p_obj.step
             elif change.what == 'label':
@@ -530,6 +532,27 @@ def _ordered_params(self):
     # Model API
     #----------------------------------------------------------------
 
+    def _rerender(self):
+        precedence = lambda k: self.object.param['name' if k == '_title' else k].precedence
+        params = self._ordered_params
+        if self.show_name:
+            params.insert(0, '_title')
+        widgets = []
+        for k in params:
+            if precedence(k) is None or precedence(k) >= self.display_threshold:
+                widgets.append(self._widgets[k])
+        self._widget_box.objects = widgets
+
+    def _rerender_widget(self, p_name):
+        watchers = []
+        for w in self._callbacks:
+            if w.inst is self._widgets[p_name]:
+                w.inst.param.unwatch(w)
+            else:
+                watchers.append(w)
+        self._widgets[p_name] = self.widget(p_name)
+        self._rerender()
+
     def _get_widgets(self):
         """Return name,widget boxes for all parameters (i.e., a property sheet)"""
         # Format name specially

panel/tests/test_param.py

@@ -5,13 +5,13 @@
 import param
 
 from bokeh.models import (
-    Div, Slider, Select, RangeSlider, MultiSelect, Row as BkRow,
-    CheckboxGroup, Toggle, Button, TextInput as BkTextInput,
-    Tabs as BkTabs, Column as BkColumn, TextInput)
+    Div, Slider, Select, RangeSlider as BkRangeSlider, MultiSelect,
+    Row as BkRow, CheckboxGroup, Toggle, Button, TextInput as
+    BkTextInput,Tabs as BkTabs, Column as BkColumn, TextInput)
 from panel.pane import Pane, PaneBase, Matplotlib, Bokeh, HTML
 from panel.layout import Tabs, Row
 from panel.param import Param, ParamMethod, ParamFunction, JSONInit
-from panel.widgets import LiteralInput
+from panel.widgets import LiteralInput, RangeSlider
 from panel.tests.util import mpl_available, mpl_figure
 
 
@@ -221,7 +221,7 @@ class Test(param.Parameterized):
     model = test_pane.get_root(document, comm=comm)
 
     widget = model.children[1]
-    assert isinstance(widget, RangeSlider)
+    assert isinstance(widget, BkRangeSlider)
     assert widget.start == 0
     assert widget.end == 1.1
     assert widget.value == (0.1, 0.5)
@@ -660,9 +660,9 @@ class Test(param.Parameterized):
     assert number.value != 3
     assert test.a == 3
 
-    test.a = 4
-    assert number.value != 4
-    assert test.a == 4
+    pane._widgets['a']._process_events({'value': 4})
+    assert test.a == 3
+    assert number.value == 4
 
 
 def test_set_show_name(document, comm):
@@ -1182,3 +1182,25 @@ def _update_text_pane(self, *_):
     view.text = TextModel(text="New TextModel")
     # Then
     assert view.text_pane.parameters==["text"]
+
+
+def test_rerender_bounded_widget_when_bounds_set_and_unset():
+    class Test(param.Parameterized):
+        num = param.Range()
+
+    test = Test()
+    p = Param(test)
+
+    assert isinstance(p._widgets['num'], LiteralInput)
+    assert p._widgets['num'] in p._widget_box
+
+    test.param.num.bounds = (0, 5)
+
+    assert isinstance(p._widgets['num'], RangeSlider)
+    assert p._widgets['num'] in p._widget_box
+
+    test.param.num.bounds = (None, 5)
+
+    assert isinstance(p._widgets['num'], LiteralInput)
+    assert p._widgets['num'] in p._widget_box
+