docs: def context_class(self):
          """
          Context as a :class:`shelter.core.context.Context` class or subclass.
          """
          if 'context_class' not in self._cached_values:
              context_cls_name = getattr(self.settings, 'CONTEXT_CLASS', '')
              if context_cls_name:
                  context_class = import_object(context_cls_name)
              else:
                  context_class = Context
              self._cached_values['context_class'] = context_class
          return self._cached_values['context_class']