Fix edit_constant to prevent class value mutation when not constant

param/parameterized.py

@@ -428,6 +428,7 @@ def edit_constant(parameterized: 'Parameterized') -> Generator[None, None, None]
     """
     kls_params = parameterized.param.objects(instance=False)
     inst_params = parameterized._param__private.params
+    init_inst_params = list(inst_params)
     updated = []
     for pname, pobj in (kls_params | inst_params).items():
         if pobj.constant:
@@ -439,7 +440,7 @@ def edit_constant(parameterized: 'Parameterized') -> Generator[None, None, None]
         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:
+            if pname in kls_params and pname not in init_inst_params:
                 type(parameterized).param[pname].constant=True
             if pname in inst_params:
                 parameterized.param[pname].constant = True

tests/testparameterizedobject.py

@@ -329,6 +329,26 @@ def test_edit_constant(self):
         assert TestPO.param['const'].constant
         assert TestPO.param['const'].default not in (670, 891)
 
+    def test_edit_constant_does_not_mutate_cls_value_if_not_constant(self):
+        # Regression https://github.com/holoviz/param/issues/1100
+        class P(param.Parameterized):
+            a = param.Number()
+
+
+        p = P()
+
+        # Manually set p.param.a.constant to True
+        p.param.a.constant = True
+
+        assert p.param.a.constant is True
+        assert P.param.a.constant is False
+
+        with param.parameterized.edit_constant(p):
+            pass
+
+        assert p.param.a.constant is True
+        assert P.param.a.constant is False
+
     def test_readonly_parameter(self):
         """Test that you can't set a read-only parameter on construction or as an attribute."""
         testpo = TestPO()