How to use the whitenoise.string_utils.ensure_leading_trailing_slash function in whitenoise

self.autorefresh = settings.DEBUG
        self.use_finders = settings.DEBUG
        self.static_prefix = urlparse(settings.STATIC_URL or '').path
        if settings.DEBUG:
            self.max_age = 0
        # Allow settings to override default attributes
        for attr in self.config_attrs:
            settings_key = 'WHITENOISE_{0}'.format(attr.upper())
            try:
                value = getattr(settings, settings_key)
            except AttributeError:
                pass
            else:
                value = decode_if_byte_string(value)
                setattr(self, attr, value)
        self.static_prefix = ensure_leading_trailing_slash(self.static_prefix)
        self.static_root = decode_if_byte_string(settings.STATIC_ROOT)

def add_files(self, root, prefix=None):
        root = decode_if_byte_string(root, force_text=True)
        root = os.path.abspath(root)
        root = root.rstrip(os.path.sep) + os.path.sep
        prefix = decode_if_byte_string(prefix)
        prefix = ensure_leading_trailing_slash(prefix)
        if self.autorefresh:
            # Later calls to `add_files` overwrite earlier ones, hence we need
            # to store the list of directories in reverse order so later ones
            # match first when they're checked in "autorefresh" mode
            self.directories.insert(0, (root, prefix))
        else:
            if os.path.isdir(root):
                self.update_files_dictionary(root, prefix)
            else:
                warnings.warn(u"No directory at: {}".format(root))

script_name = settings.FORCE_SCRIPT_NAME.rstrip("/")
            if self.static_prefix.startswith(script_name):
                self.static_prefix = self.static_prefix[len(script_name) :]
        if settings.DEBUG:
            self.max_age = 0
        # Allow settings to override default attributes
        for attr in self.config_attrs:
            settings_key = "WHITENOISE_{0}".format(attr.upper())
            try:
                value = getattr(settings, settings_key)
            except AttributeError:
                pass
            else:
                value = decode_if_byte_string(value)
                setattr(self, attr, value)
        self.static_prefix = ensure_leading_trailing_slash(self.static_prefix)
        self.static_root = decode_if_byte_string(settings.STATIC_ROOT)