format_html called with 3 arguments: string '<link href="{0}" type="text/css" media="{1}" rel="stylesheet" />', return value of the function self.absolute_path called with an argument path and medium, for every medium in media and for every path in value under the medium key of the self._css dictionary. define the method absolute_path with 3 arguments self, path and prefix set to None.
def absolute_path ( self , path , prefix = None ) :