Called by ``__init()__`` to set the object's ``_prefix`` attribute,
        which determines the prefix app users must use when overriding
        settings associated with this helper. For example:

        If the ``_prefix`` attribute were to be set to "YOURAPP", and there
        exists an app setting called ``SETTING_NAME``, app users would override
        that setting by adding a variable with the name ``YOURAPP_SETTING_NAME``
        to their Django settings.

        Developers can choose their own prefix by setting the ``prefix``
        attribute on their helper class. If no value is specified, a deterministic
        default value is generated, based on the where the helper class is defined.
        For example:

        A helper class defined in ``yourapp/conf/settings.py`` or
        ``yourapp/settings.py`` would be assigned the prefix: ``"YOURAPP"``.

        A helper class is defined in ``yourapp/subapp/conf/settings.py`` or
        ``yourapp/subapp/settings.py`` would be assigned the prefix: ``"YOURAPP_SUBAPP"``.

def _set_prefix(self):
        """
        Called by ``__init()__`` to set the object's ``_prefix`` attribute,
        which determines the prefix app users must use when overriding
        settings associated with this helper. For example:

        If the ``_prefix`` attribute were to be set to "YOURAPP", and there
        exists an app setting called ``SETTING_NAME``, app users would override
        that setting by adding a variable with the name ``YOURAPP_SETTING_NAME``
        to their Django settings.

        Developers can choose their own prefix by setting the ``prefix``
        attribute on their helper class. If no value is specified, a deterministic
        default value is generated, based on the where the helper class is defined.
        For example:

        A helper class defined in ``yourapp/conf/settings.py`` or
        ``yourapp/settings.py`` would be assigned the prefix: ``"YOURAPP"``.

        A helper class is defined in ``yourapp/subapp/conf/settings.py`` or
        ``yourapp/subapp/settings.py`` would be assigned the prefix: ``"YOURAPP_SUBAPP"``.
        """
        if self.prefix is not None:
            value = self.prefix.rstrip('_')
        else:
            module_path_parts = self.__module_path_split[:-1]
            if module_path_parts[-1] == 'conf':
                module_path_parts.pop()
            value = '_'.join(module_path_parts)
        self._prefix = value.upper()