I assume you use python3.7

test_against_stored.py

import urllib.request
with urllib.request.urlopen('http://python.org/') as response:
   html = response.read()
   with open("stored.html", "r+") as file:
       if not (str(file.read()) == str(html)):
           print("THIS IS WHERE YOU ALERT")
       file.seek(0) #sets file pointer to beginning 
       file.truncate() #empties the file
       file.write(str(html))