Python请求：仅在较新版本时下载

Question

仅当服务器副本比本地副本新时，从服务器下载新文件的标准pythonic方法是什么？

我的python-search-fu今天非常薄弱，或者确实需要滚动自己的日期时间解析器和比较器，如下所示。真的没有requests.header.get_datetime_object('last-modified')吗？还是request.save_to_file(url, outfile, maintain_datetime=True)？

import requests
import datetime

r = requests.head(url)
url_time = r.headers['last-modified']
file_time = datetime.datetime.fromtimestamp(os.path.getmtime(dstFile))
print url_time  #emits 'Sat, 28 Mar 2015 08:05:42 GMT' on my machine
print file_time #emits '2015-03-27 21:53:28.175072' 

if time_is_older(url_time, file_time):
    print 'url modtime is not newer than local file, skipping download'
    return
else:
    do_download(url)
    os.utime(dstFile, url_time) # maintain server's file timestamp

def time_is_older(str_time, time_object):
    ''' Parse str_time and see if is older than time_object.
        This is a fragile function, what if str_time is in different locale?
    '''
    parsed_time = datetime.datetime.strptime(str_time, 
        #Fri, 27 Mar 2015 08:05:42 GMT
        '%a, %d %b %Y %X %Z')
    return parsed_time < time_object

Answer 1

import requests
import datetime
from dateutil.parser import parse as parsedate
r = requests.head(url)
url_time = r.headers['last-modified']
url_date = parsedate(url_time)
file_time = datetime.datetime.fromtimestamp(os.path.getmtime(dstFile))
if url_date > file_time :
    donwload it !