def resolve_filename(self, package_dir, filename):
        """Gets a proper full relative path of Sass source and
        CSS source that will be generated, according to ``package_dir``
        and ``filename``.

        :param package_dir: the path of package directory
        :type package_dir: :class:`str`, :class:`basestring`
        :param filename: the filename of Sass/SCSS source to compile
        :type filename: :class:`str`, :class:`basestring`
        :returns: a pair of (sass, css) path
        :rtype: :class:`tuple`

        """
        sass_path = os.path.join(package_dir, self.sass_path, filename)
        if self.strip_extension:
            filename, _ = os.path.splitext(filename)
        css_filename = filename + '.css'
        css_path = os.path.join(package_dir, self.css_path, css_filename)
        return sass_path, css_path