用Python实现应用Last-Modified和ETag避免下载重复内容

Http 1.1中避免重复下载的标记

使用Http1.1中定义好的头信息来避免重复下载，参考HTTP/1.1 Section 14 Header Field Definitions中的14.19 ETag/14.24 If-Match/14.29 Last-Modified/14.25 If-Modified-Since

开发者把Last-Modified 和ETags请求的http报头一起使用，能够有效利用本地缓存，降低无谓的重复下载。

示例代码逻辑

1. 客户端下载一个链接（Sample）;
2. 服务器返回Sample，Sample中记录Last-Modified/ETag标记;
3. 客户端再次下载这个链接，并将上次请求时服务器返回的Last-Modified/ETag一起传递给服务器;
4. 服务器检查该Last-Modified或ETag，并判断出该页面自上次客户端请求之后还未被修改，直接返回响应304和一个空的响应体。

其实在《Dive Into Python》中就有相当详细的实例代码，强烈建议没看过这本书的python程序员们认真学习一下，会提升面向对象编程和网络编程能力的。

示例代码

?Download HttpDownload.txt

#!/usr/bin/python
# -*- coding: utf-8 -*-
'''
Created on Nov 9, 2011@author: li3huo
'''import urllib, urllib2
import sys
import time
class DefaultErrorHandler(urllib2.HTTPDefaultErrorHandler):"""用来保证请求中记录Http状态
"""def http_error_default(self, req, fp, code, msg, headers):result = urllib2.HTTPError(req.get_full_url(), code, msg, headers, fp)result.status = codereturn resultclass Sample():"""a sample is the url i want to download
"""url = NonecontentLength = 0etag = NonelastModified = Nonedata = Nonepath = Nonedef __init__(self, url, contentLength=0, etag=None, lastModified=None):self.url = urlself.contentLength = 0self.etag = etagself.lastModified = lastModifiedself.status = 200self.file = filedef __repr__(self):return repr("Http Status=%d; Length=%d; Last Modified Time=%s; eTag=%s" % (self.status, self.contentLength, self.lastModified, self.etag))def downloadSample(self):request = urllib2.Request(self.url)request.add_header('User-Agent', "Mozilla/5.0 (Macintosh; U; PPC Mac OS X; en)")if self.lastModified:request.add_header('If-Modified-Since', self.lastModified)if self.etag:request.add_header('If-None-Match', self.etag)conn = urllib2.build_opener(DefaultErrorHandler()).open(request)if hasattr(conn, 'headers'):# save ETag, if the server sent oneself.etag = conn.headers.get('ETag')# save Last-Modified header, if the server sent oneself.lastModified = conn.headers.get('Last-Modified')self.contentLength = conn.headers.get("content-length")if hasattr(conn, 'status'):self.status = conn.statusprint "status=%d" % self.statusself.data = conn.read()if self.status == 304:print "the content is same, so return nothing!"if not self.contentLength:self.contentLength = len(self.data)conn.close()if __name__ == '__main__':url = 'http://www.sina.com.cn'sample = Sample(url)sample.downloadSample()print samplesample.downloadSample()print sample

输出结果

‘Http Status=200; Length=589988; Last Modified Time=Wed, 09 Nov 2011 10:45:55 GMT; eTag=None’
status=304
the content is same, so return nothing!
‘Http Status=304; Length=0; Last Modified Time=Wed, 09 Nov 2011 10:45:55 GMT; eTag=None’