안녕하세요. 답안에서 rank = movie.select_one('td:nth-child(1) > img')['alt']에서 ['alt']가 있고 없고의 차이가 무엇인가요?

작성한 코드 및 에러 메세지

import requests
from bs4 import BeautifulSoup

headers = {'User-Agent' : 'Mozilla/5.0 (Windows NT 10.0; Win64; x64)AppleWebKit/537.36 (KHTML, like Gecko) Chrome/73.0.3683.86 Safari/537.36'}
data = requests.get('https://movie.naver.com/movie/sdb/rank/rmovie.naver?sel=pnt&date=20210829',headers=headers)

soup = BeautifulSoup(data.text, 'html.parser')

movies = soup.select('#old_content > table > tbody > tr')

#old_content > table > tbody > tr:nth-child(2) > td:nth-child(1) > img
#old_content > table > tbody > tr:nth-child(3) > td:nth-child(1) > img
#old_content > table > tbody > tr:nth-child(2) > td:nth-child(3) > div > div > img
#old_content > table > tbody > tr:nth-child(2) > td:nth-child(3) > div > div > img
for movie in movies:
   a = movie.select_one('td.title > div > a')
   if a is not None:
       title = a.text
       rank = movie.select_one('td:nth-child(1) > img')['alt']
       star = movie.select_one('td.point').text
       print(rank,title,star)