From c1bc3a3222d1dcd5ecb93a429c628cc6e1041a48 Mon Sep 17 00:00:00 2001
From: Marc Skov Madsen <marc.skov.madsen@gmail.com>
Date: Tue, 15 Sep 2020 12:47:38 +0200
Subject: [PATCH] fixed issue 1581 (#1584)

* fixed issue 1581

* Handle condition where Param.parameters was set

* Keep track whether parameters was explicitly set

Co-authored-by: Marc Skov Madsen <masma@orsted.dk>
Co-authored-by: Philipp Rudiger <prudiger@anaconda.com>
---
 panel/param.py            | 22 +++++++++++++++++++---
 panel/tests/test_param.py | 31 +++++++++++++++++++++++++++++++
 2 files changed, 50 insertions(+), 3 deletions(-)

diff --git a/panel/param.py b/panel/param.py
index c9ebc626bb..f71e616278 100644
--- a/panel/param.py
+++ b/panel/param.py
@@ -98,7 +98,7 @@ class Param(PaneBase):
     name = param.String(default='', doc="""
         Title of the pane.""")
 
-    parameters = param.List(default=[], doc="""
+    parameters = param.List(default=[], allow_None=True, doc="""
         If set this serves as a whitelist of parameters to display on
         the supplied Parameterized object.""")
 
@@ -154,8 +154,13 @@ def __init__(self, object=None, **params):
             object = object.owner
         if isinstance(object, param.parameterized.Parameters):
             object = object.cls if object.self is None else object.self
+
         if 'parameters' not in params and object is not None:
             params['parameters'] = [p for p in object.param if p != 'name']
+            self._explicit_parameters = False
+        else:
+            self._explicit_parameters = object is not None
+
         if object and 'name' not in params:
             params['name'] = param_name(object.name)
         super(Param, self).__init__(object, **params)
@@ -216,17 +221,28 @@ def _update_widgets(self, *events):
         for event in sorted(events, key=lambda x: x.name):
             if event.name == 'object':
                 if isinstance(event.new, param.parameterized.Parameters):
+                    # Setting object will trigger this method a second time
                     self.object = event.new.cls if event.new.self is None else event.new.self
                     return
-                if event.new is None:
+                
+                if self._explicit_parameters:
+                    parameters = self.parameters
+                elif event.new is None:
                     parameters = []
                 else:
                     parameters = [p for p in event.new.param if p != 'name']
                     self.name = param_name(event.new.name)
             if event.name == 'parameters':
-                parameters = [] if event.new == [] else event.new
+                if event.new is None:
+                    self._explicit_parameters = False
+                    if self.object is not None:
+                        parameters = [p for p in self.object.param if p != 'name']
+                else:
+                    self._explicit_parameters = True
+                    parameters = [] if event.new == [] else event.new
 
         if parameters != [] and parameters != self.parameters:
+            # Setting parameters will trigger this method a second time
             self.parameters = parameters
             return
 
diff --git a/panel/tests/test_param.py b/panel/tests/test_param.py
index e647c7571a..30eefb1264 100644
--- a/panel/tests/test_param.py
+++ b/panel/tests/test_param.py
@@ -1097,3 +1097,34 @@ class Test(param.Parameterized):
 
     assert test.a == 2
     del os.environ['PARAM_JSON_INIT']
+
+def test_change_object_and_keep_parameters():
+    """Test that https://github.com/holoviz/panel/issues/1581 is solved"""
+    # Given
+    class TextModel(param.Parameterized):
+        text = param.String()
+        param2 = param.String()
+
+    class TextView(param.Parameterized):
+        text = param.ClassSelector(class_=TextModel)
+        text_pane = param.Parameter()
+
+        def __init__(self, **params):
+            params["text"] = TextModel(text="Original Text")
+            super().__init__(**params)
+
+            self.text_pane = Param(
+                self.text, parameters=["text"]
+            )
+
+        @param.depends("text", watch=True)
+        def _update_text_pane(self, *_):
+            self.text_pane.object = self.text
+
+    view = TextView()
+    assert view.text_pane.parameters==["text"]
+
+    # When
+    view.text = TextModel(text="New TextModel")
+    # Then
+    assert view.text_pane.parameters==["text"]