nl: call the method parser.add_argument with 6 arguments: string '--extension', string '-e', dest set to string 'extensions',
code: parser . add_argument ( '--extension' , '-e' , dest = 'extensions' , action = 'append' , default = [ 'py' ] , help = 'The file extension(s) to render (default: "py"). ' 'Separate multiple extensions with commas, or use ' '-e multiple times.' )