Deprecate setting a parameter value before full instance initialization

doc/reference/deprecations.md

@@ -4,6 +4,7 @@ List of currently deprecated APIs:
 
 | Warning | Description |
 |-|-|
+| `ParamPendingDeprecationWarning` since `2.3.0` | `param.parameterized` module / Setting a parameter value before full instance initialization |
 | `ParamFutureWarning` since `2.2.0`, `ParamDeprecationWarning` since `2.0.0` | Parameter slots / `List._class`: use instead `item_type` |
 | `ParamFutureWarning` since `2.2.0`, `ParamDeprecationWarning` since `2.0.0` | Parameter slots / `Number.set_hook`: no replacement |
 | `ParamFutureWarning` since `2.2.0`, `ParamDeprecationWarning` since `2.0.0` | `param.__init__` module / `param.produce_value`: no replacement |

param/parameterized.py

@@ -39,6 +39,7 @@
 from ._utils import (
     DEFAULT_SIGNATURE,
     ParamFutureWarning as _ParamFutureWarning,
+    ParamPendingDeprecationWarning as _ParamPendingDeprecationWarning,
     Skip,
     _deprecated,
     _deprecate_positional_args,
@@ -1574,6 +1575,15 @@ def __set__(self, obj, val):
             else:
                 # When setting a Parameter before calling super.
                 if not isinstance(obj._param__private, _InstancePrivate):
+                    warnings.warn(
+                        f"Setting the Parameter {self.name!r} to {val!r} before "
+                        f"the Parameterized class {type(obj).__name__!r} is fully "
+                        "instantiated is deprecated and will raise an error in "
+                        "a future version. Ensure the value is set after calling "
+                        "`super().__init__(**params)` in the constructor.",
+                        category=_ParamPendingDeprecationWarning,
+                        stacklevel=4,
+                    )
                     obj._param__private = _InstancePrivate(
                         explicit_no_refs=type(obj)._param__private.explicit_no_refs
                     )

tests/testdeprecations.py

@@ -97,6 +97,16 @@ def test_deprecate_all_equal(self):
         with pytest.raises(param._utils.ParamFutureWarning):
             param.parameterized.all_equal(1, 1)
 
+    def test_deprecate_setting_parameter_before_init(self):
+        class P(param.Parameterized):
+            x = param.Parameter()
+
+            def __init__(self, **params):
+                self.x = 10
+                super().__init__(**params)
+        with pytest.raises(param._utils.ParamPendingDeprecationWarning):
+            P()
+
 
 class TestDeprecateParameters:
 

tests/testparameterizedobject.py

@@ -2,6 +2,7 @@
 import inspect
 import re
 import unittest
+import warnings
 
 import param
 import numbergen
@@ -400,7 +401,9 @@ def cb(self):
                 nonlocal count
                 count += 1
 
-        p = P()
+        with warnings.catch_warnings():
+            warnings.filterwarnings('ignore', message='Setting the Parameter', category=param._utils.ParamPendingDeprecationWarning)
+            p = P()
 
         assert p.x == 1
         assert count == 0
@@ -417,7 +420,9 @@ def __init__(self, depth=0):
                     self.sub = P(depth+1)
                 super().__init__()
 
-        p = P()
+        with warnings.catch_warnings():
+            warnings.filterwarnings('ignore', message='Setting the Parameter', category=param._utils.ParamPendingDeprecationWarning)
+            p = P()
 
         assert p.value == 'B'
         assert p.sub.value == 'B'
@@ -442,7 +447,9 @@ def __init__(self, batched=True, **params):
 
         # When b is instantiated the `batched` Parameter of B is set before
         # Parameterized.__init__ is called.
-        b = B()
+        with warnings.catch_warnings():
+            warnings.filterwarnings('ignore', message='Setting the Parameter', category=param._utils.ParamPendingDeprecationWarning)
+            b = B()
         assert b.batched is True
 
     def test_instantiation_param_objects_before_super_subclass(self):