Add ability to Skip Param<Ref|Function|Method> updates

panel/param.py

@@ -24,9 +24,16 @@
 
 import param
 
+try:
+    from param import Skip
+except Exception:
+    class Skip(Exception):
+        """
+        Exception that allows skipping an update for function-level updates.
+        """
 from param.parameterized import (
-    classlist, discard_events, eval_function_with_deps, get_method_owner,
-    iscoroutinefunction, resolve_ref, resolve_value,
+    Undefined, classlist, discard_events, eval_function_with_deps,
+    get_method_owner, iscoroutinefunction, resolve_ref, resolve_value,
 )
 from param.reactive import rx
 
@@ -829,9 +836,20 @@ async def _eval_async(self, awaitable):
                         self._inner_layout.append(new_obj)
                         self._pane = self._inner_layout[-1]
                     else:
-                        self._update_inner(new_obj)
+                        try:
+                            self._update_inner(new_obj)
+                        except Skip:
+                            pass
             else:
-                self._update_inner(await awaitable)
+                try:
+                    new = await awaitable
+                    if new is Skip or new is Undefined:
+                        raise Skip
+                    self._update_inner(new)
+                except Skip:
+                    self.param.log(
+                        param.DEBUG, 'Skip event was raised, skipping update.'
+                    )
         except Exception as e:
             if not curdoc or (has_context and curdoc.session_context):
                 raise e
@@ -850,7 +868,15 @@ def _replace_pane(self, *args, force=False):
             if self.object is None:
                 new_object = Spacer()
             else:
-                new_object = self.eval(self.object)
+                try:
+                    new_object = self.eval(self.object)
+                    if new_object is Skip and new_object is Undefined:
+                        raise Skip
+                except Skip:
+                    self.param.log(
+                        param.DEBUG, 'Skip event was raised, skipping update.'
+                    )
+                    return
             if inspect.isawaitable(new_object) or isinstance(new_object, types.AsyncGeneratorType):
                 param.parameterized.async_executor(partial(self._eval_async, new_object))
                 return

panel/tests/test_param.py

@@ -6,10 +6,10 @@
 import pytest
 
 from bokeh.models import (
-    AutocompleteInput as BkAutocompleteInput, Button, Checkbox as BkCheckbox,
-    Column as BkColumn, Div, MultiSelect, RangeSlider as BkRangeSlider,
-    Row as BkRow, Select, Slider, Tabs as BkTabs, TextInput,
-    TextInput as BkTextInput, Toggle,
+    AutocompleteInput as BkAutocompleteInput, Button as BkButton,
+    Checkbox as BkCheckbox, Column as BkColumn, Div, MultiSelect,
+    RangeSlider as BkRangeSlider, Row as BkRow, Select, Slider, Tabs as BkTabs,
+    TextInput, TextInput as BkTextInput, Toggle,
 )
 from packaging.version import Version
 
@@ -22,11 +22,11 @@
     HTML, Bokeh, Markdown, Matplotlib, PaneBase, Str, panel,
 )
 from panel.param import (
-    JSONInit, Param, ParamFunction, ParamMethod,
+    JSONInit, Param, ParamFunction, ParamMethod, Skip,
 )
 from panel.tests.util import mpl_available, mpl_figure
 from panel.widgets import (
-    AutocompleteInput, Checkbox, DatePicker, DatetimeInput,
+    AutocompleteInput, Button, Checkbox, DatePicker, DatetimeInput,
     EditableFloatSlider, EditableRangeSlider, LiteralInput, NumberInput,
     RangeSlider,
 )
@@ -392,7 +392,7 @@ class Test(param.Parameterized):
     model = test_pane.get_root(document, comm=comm)
 
     button = model.children[1]
-    assert isinstance(button, Button)
+    assert isinstance(button, BkButton)
 
     # Check that the action is actually executed
     pn_button = test_pane.layout[1]
@@ -1920,3 +1920,48 @@ async def function(value):
     assert root.children[0].text == '<p>5</p>\n'
     await asyncio.sleep(0.1)
     assert root.children[0].text == '<p>6</p>\n'
+
+
+def test_skip_param(document, comm):
+    checkbox = Checkbox(value=False)
+    button = Button()
+
+    def layout(value, click):
+        if not click:
+            raise Skip()
+        return Markdown(f"{value}")
+
+    layout = ParamFunction(bind(layout, checkbox, button))
+
+    root = layout.get_root(document, comm)
+
+    div = root.children[0]
+    assert div.text == '<pre> </pre>'
+    checkbox.value = True
+    assert div.text == '<pre> </pre>'
+    button.param.trigger('value')
+    assert div.text == '<pre> </pre>'
+
+@pytest.mark.asyncio
+async def test_async_skip_param(document, comm):
+    checkbox = Checkbox(value=False)
+    button = Button()
+
+    async def layout(value, click):
+        if not click:
+            raise Skip()
+        return Markdown(f"{value}")
+
+    layout = ParamFunction(bind(layout, checkbox, button))
+
+    root = layout.get_root(document, comm)
+
+    div = root.children[0]
+    await asyncio.sleep(0.01)
+    assert div.text == '<pre> </pre>'
+    checkbox.value = True
+    await asyncio.sleep(0.01)
+    assert div.text == '<pre> </pre>'
+    button.param.trigger('value')
+    await asyncio.sleep(0.01)
+    assert div.text == '<pre> </pre>'