python writing in file

Please can someone help me? I want to write (i['src']) in a file.txt I do not arrive

from bs4 import BeautifulSoup
import requests
import re
 a = requests.get('domaine.com')
    soup = BeautifulSoup(a.content, 'lxml')
    links = soup.find_all('iframe')
    for i in links:
        print(i['src'])