import email.utils
import os
import datetime
import shutil
import requests
 
def parse_datetime(text):
    dtz = email.utils.parsedate_tz(text)
    dt = datetime.datetime(*dtz[:6])
    if dtz[9]:  # utc offset
        dt += datetime.timedelta(hours=dtz[9])
    return dt
 
def download_if_modified(url, filename):
    resp = requests.get(url, prefetch=False)
    need_to_download = True
    if os.path.exists(filename):
        urldate = parse_datetime(resp.headers['last-modified'])
        filedate = datetime.datetime.utcfromtimestamp(os.path.getmtime(filename))
        need_to_download = urldate > filedate
    if need_to_download:
        with open(filename, "wb") as fp:
            shutil.copyfileobj(resp.raw, fp)
    return need_to_download
         
download_if_modified('http://python.su/static/images/logo.gif', 'logo.gif')
download_if_modified('http://yandex.st/www/1.400/yaru/i/logo.png', 'logo.png')