namget.tistory.com/entry/python-%ED%8C%8C%EC%9D%B4%EC%8D%AC-%EB%A6%AC%EC%8A%A4%ED%8A%B8%ED%8A%9C%ED%94%8C-%EA%B8%B8%EC%9D%B4%EA%B5%AC%ED%95%98%EA%B8%B0-len