Called by ``__init__()`` to create a dictionary of the relevant
        values from the associated defaults module, and save it to the
        object's ``_defaults`` attribute to improve lookup performance.
        Only variables with upper-case names are included.

        :raises: ImportError

        It is assumed that the defaults module is defined in the same directory
        as ``settings.py`` where the settings helper class is defined. But,
        in cases where this differs, developers can specify an alternative
        import path using the ``defaults_path`` class attribute for their
        helper class.

def _load_defaults(self):
        """
        Called by ``__init__()`` to create a dictionary of the relevant
        values from the associated defaults module, and save it to the
        object's ``_defaults`` attribute to improve lookup performance.
        Only variables with upper-case names are included.

        :raises: ImportError

        It is assumed that the defaults module is defined in the same directory
        as ``settings.py`` where the settings helper class is defined. But,
        in cases where this differs, developers can specify an alternative
        import path using the ``defaults_path`` class attribute for their
        helper class.
        """
        self._defaults_module_path = self.defaults_path or \
            '.'.join(self.__module_path_split[:-1]) + ".defaults"

        module = self._do_import(self._defaults_module_path)
        self._defaults = {
            k: v for k, v in module.__dict__.items()
            if k.isupper()
        }