下载较大文件时分段下载会加速下载过程，几乎所有下载软件都有类似的特性。在python中如何实现分段下载文件呢？

>>> import urllib2
>>> req = urllib2.Request('http://www.python.org/')
#下载19000到20000字节的片段
>>> req.headers['Range'] = 'bytes=%s-%s' % (19000, 20000)
>>> f = urllib2.urlopen(req)
>>> pagerange = f.headers.get('Content-Range')
>>> print pagerange
bytes 19000-19189/19190
>>> print repr(f.read())
'</div>\n\tCopyright &copy; 1990-2012, <a href=\'/psf/\'>Python Software Foundation</a><br/>\n\t<a href="/about/legal">Legal Statements</a>\n      </div>\n\n\n    </div>\n  </div>\n</body>\n</html>\n\n\n\n\n\n\n'
>>>