from bs4 import BeautifulSoup

with open('index.html', mode='rt', encoding='utf-8') as f:
    soup = BeautifulSoup(f.read(), 'html.parser')

    print(soup)

    nt = soup.new_tag('div', id='hoge') # 新しい要素を作る
    nt.string = 'ほげ'

    soup.find(class_ = 'footer').append(nt) # UL要素を検索して追加


    print(soup)

with open('test.html', mode = 'w', encoding = 'utf-8') as fw:
    fw.write(soup.prettify())