From f9ac89779c0b55882fa6c42b03a953ef0fc329cf Mon Sep 17 00:00:00 2001
From: Maxime Liquet <35924738+maximlt@users.noreply.github.com>
Date: Fri, 21 Feb 2025 12:19:00 +0100
Subject: [PATCH] Ensure edit_constant resets class and instance level
 parameters (#1015)

---
 param/parameterized.py           | 20 ++++++++++++++------
 tests/testparameterizedobject.py | 12 ++++++++++++
 2 files changed, 26 insertions(+), 6 deletions(-)

diff --git a/param/parameterized.py b/param/parameterized.py
index 48855b7d..763a826a 100644
--- a/param/parameterized.py
+++ b/param/parameterized.py
@@ -340,15 +340,23 @@ def edit_constant(parameterized):
     Temporarily set parameters on Parameterized object to constant=False
     to allow editing them.
     """
-    params = parameterized.param.objects('existing').values()
-    constants = [p.constant for p in params]
-    for p in params:
-        p.constant = False
+    kls_params = parameterized.param.objects(instance=False)
+    inst_params = parameterized._param__private.params
+    updated = []
+    for pname, pobj in (kls_params | inst_params).items():
+        if pobj.constant:
+            pobj.constant = False
+            updated.append(pname)
     try:
         yield
     finally:
-        for (p, const) in zip(params, constants):
-            p.constant = const
+        for pname in updated:
+            # Some operations trigger a parameter instantiation (copy),
+            # we ensure both the class and instance parameters are reset.
+            if pname in kls_params:
+                type(parameterized).param[pname].constant=True
+            if pname in inst_params:
+                parameterized.param[pname].constant = True
 
 
 @contextmanager
diff --git a/tests/testparameterizedobject.py b/tests/testparameterizedobject.py
index 9bb85d00..206f8048 100644
--- a/tests/testparameterizedobject.py
+++ b/tests/testparameterizedobject.py
@@ -19,6 +19,7 @@
     ParamOverrides,
     Undefined,
     default_label_formatter,
+    edit_constant,
     no_instance_params,
     shared_parameters,
 )
@@ -313,6 +314,17 @@ def test_constant_parameter(self):
         testpo = TestPO()
         self.assertEqual(testpo.const,9)
 
+    def test_edit_constant(self):
+        testpo = TestPO(const=670)
+        # Checking no parameter was already instantiated
+        assert not testpo._param__private.params
+        with edit_constant(testpo):
+            testpo.const = 891
+        assert testpo.const == 891
+        assert testpo.param['const'].constant
+        assert TestPO.param['const'].constant
+        assert TestPO.param['const'].default not in (670, 891)
+
     def test_readonly_parameter(self):
         """Test that you can't set a read-only parameter on construction or as an attribute."""
         testpo = TestPO()