Declare the recorded MGXS generation method in Settings.__init__

Initializing _mgxs_generation_method to None makes it a declared private
attribute rather than one added dynamically outside __init__, and lets
convert_to_multigroup access it directly (type-checking settings first).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

openmc/model/model.py

@@ -2773,7 +2773,10 @@ class Model:
         # mgxs_generation_settings) is used, defaulting to "material_wise".
         # Conflicting specifications are rejected, since the generation
         # defaults differ by method.
-        settings_method = getattr(settings, '_mgxs_generation_method', None)
+        if settings is not None:
+            check_type('settings', settings, openmc.Settings)
+        settings_method = (settings._mgxs_generation_method
+                           if settings is not None else None)
         if method is None:
             method = settings_method or 'material_wise'
         elif settings_method is not None and method != settings_method:
@@ -2817,7 +2820,6 @@ class Model:
             if temperature_settings is not None:
                 settings.temperature = temperature_settings
         else:
-            check_type('settings', settings, openmc.Settings)
             # batches and particles are required for any OpenMC transport
             # run; catch their absence here so a hand-built settings object
             # fails with a pointer to the defaults rather than a mysterious

openmc/settings.py

@@ -501,6 +501,11 @@ class Settings:
 
         self._random_ray = {}
 
+        # MGXS generation method recorded by Model.mgxs_generation_settings()
+        # and read back by Model.convert_to_multigroup; provenance only, not
+        # written to XML
+        self._mgxs_generation_method = None
+
         for key, value in kwargs.items():
             setattr(self, key, value)