How to use the mackup.constants.CUSTOM_APPS_DIR function in mackup

def _parse_directory(self):
        """
        Parse the storage directory in the config.

        Returns:
            str
        """
        if self._parser.has_option("storage", "directory"):
            directory = self._parser.get("storage", "directory")
            # Don't allow CUSTOM_APPS_DIR as a storage directory
            if directory == CUSTOM_APPS_DIR:
                raise ConfigError(
                    "{} cannot be used as a storage directory.".format(CUSTOM_APPS_DIR)
                )
        else:
            directory = MACKUP_BACKUP_PATH

        return str(directory)

"""
        Return the application configuration files.

        Return a list of configuration files describing the apps supported by
        Mackup. The files returned are absolute full path to those files.
        e.g. /usr/lib/mackup/applications/bash.cfg

        Only one config file per application should be returned, custom config
        having a priority over stock config.

        Returns:
            set of strings.
        """
        # Configure the config parser
        apps_dir = os.path.join(os.path.dirname(os.path.realpath(__file__)), APPS_DIR)
        custom_apps_dir = os.path.join(os.environ["HOME"], CUSTOM_APPS_DIR)

        # List of stock application config files
        config_files = set()

        # Temp list of user added app config file names
        custom_files = set()

        # Get the list of custom application config files first
        if os.path.isdir(custom_apps_dir):
            for filename in os.listdir(custom_apps_dir):
                if filename.endswith(".cfg"):
                    config_files.add(os.path.join(custom_apps_dir, filename))
                    # Also add it to the set of custom apps, so that we don't
                    # add the stock config for the same app too
                    custom_files.add(filename)

def _parse_directory(self):
        """
        Parse the storage directory in the config.

        Returns:
            str
        """
        if self._parser.has_option("storage", "directory"):
            directory = self._parser.get("storage", "directory")
            # Don't allow CUSTOM_APPS_DIR as a storage directory
            if directory == CUSTOM_APPS_DIR:
                raise ConfigError(
                    "{} cannot be used as a storage directory.".format(CUSTOM_APPS_DIR)
                )
        else:
            directory = MACKUP_BACKUP_PATH

        return str(directory)