def is_partial(self, filename):
        """Check if a file is a partial.

        Partial files are not rendered, but they are used in rendering
        templates.

        A file is considered a partial if it or any of its parent directories
        are prefixed with an ``'_'``.

        :param filename: the name of the file to check
        """
        return any((x.startswith("_") for x in filename.split(os.path.sep)))