Allow setting new private attributes

user27182 · user27182 · commit a6fa92498a98 · 2025-08-25T23:25:33.000-06:00
diff --git a/pyvista/core/utilities/misc.py b/pyvista/core/utilities/misc.py
@@ -310,23 +310,25 @@ def _no_new_attributes(self, this_class: type) -> None:
 
     def __setattr__(self, key: str, value: Any) -> None:
         """Prevent adding new attributes to classes using "normal" methods."""
-        # Check if this class froze itself. Any frozen state already set by parent classes, e.g.
-        # by calling super().__init__(), will be ignored. This allows subclasses to set attributes
-        # during init without being affect by a parent class init.
-        frozen = self.__dict__.get('__frozen', False)
-        frozen_by = self.__dict__.get('__frozen_by_class', None)
-        if (
-            frozen
-            and frozen_by is type(self)
-            and not (key in type(self).__dict__ or hasattr(self, key))
-        ):
-            from pyvista import PyVistaAttributeError  # noqa: PLC0415
-
-            msg = (
-                f'Attribute {key!r} does not exist and cannot be added to class '
-                f'{self.__class__.__name__!r}\nUse `pv.set_new_attribute` to set new attributes.'
-            )
-            raise PyVistaAttributeError(msg)
+        if not key.startswith('_'):
+            # Check if this class froze itself. Any frozen state already set by parent classes,
+            # e.g. by calling super().__init__(), will be ignored. This allows subclasses to set
+            # attributes during init without being affect by a parent class init.
+            frozen = self.__dict__.get('__frozen', False)
+            frozen_by = self.__dict__.get('__frozen_by_class', None)
+            if (
+                frozen
+                and frozen_by is type(self)
+                and not (key in type(self).__dict__ or hasattr(self, key))
+            ):
+                from pyvista import PyVistaAttributeError  # noqa: PLC0415
+
+                msg = (
+                    f'Attribute {key!r} does not exist and cannot be added to class '
+                    f'{self.__class__.__name__!r}\nUse `pv.set_new_attribute` to set new '
+                    f'attributes or consider setting a private variable (with `_` prefix) instead.'
+                )
+                raise PyVistaAttributeError(msg)
         object.__setattr__(self, key, value)
 
 
diff --git a/tests/core/test_utilities.py b/tests/core/test_utilities.py
@@ -1329,9 +1329,10 @@ def test_no_new_attr_mixin(no_new_attributes_mixin_subclass):
 
     match = (
         "Attribute 'ham' does not exist and cannot be added to class 'A'\n"
-        'Use `pv.set_new_attribute` to set new attributes.'
+        'Use `pv.set_new_attribute` to set new attributes or consider setting a private variable '
+        '(with `_` prefix) instead.'
     )
-    with pytest.raises(pv.PyVistaAttributeError, match=match):
+    with pytest.raises(pv.PyVistaAttributeError, match=re.escape(match)):
         setattr(a, ham, eggs)
 
     match = "Attribute 'ham' does not exist and cannot be added to class 'B'"
@@ -2348,6 +2349,17 @@ def test_vtk_snake_case():
             _ = pv.PolyData().information
 
 
+def test_allow_new_attributes():
+    match = (
+        "Attribute 'foo' does not exist and cannot be added to class 'PolyData'\n"
+        'Use `pv.set_new_attribute` to set new attributes or consider setting a private '
+        'variable (with `_` prefix) instead.'
+    )
+    _ = pv.PolyData()._foo = 42
+    with pytest.raises(pv.PyVistaAttributeError, match=re.escape(match)):
+        _ = pv.PolyData().foo = 42
+
+
 T = TypeVar('T')
 
 
