requests 库的（stream）

查找requests的相应资料

By default, when you make a request, the body of the response is 
downloaded immediately. You can override this behaviour and defer 
downloading the response body until you access the Response.content 
attribute with the stream parameter:

tarball_url = 'https://github.com/kennethreitz/requests/tarball/master'
r = requests.get(tarball_url, stream=True)

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At this point only the response headers have been downloaded and the 
connection remains open, hence allowing us to make content retrieval 
conditional:

if int(r.headers['content-length']) < TOO_LONG:
  content = r.content
  ...

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只有headers头被下载了，body中的数据还没有被下载，这样就能避免不必要的流量开销，只有当你使用r.content 的时候，所有body内容才会被下载

You can further control the workflow by use of the 
Response.iter_content() and Response.iter_lines() methods. 
Alternatively, you can read the undecoded body from the underlying 
urllib3 urllib3.HTTPResponse at Response.raw.

实时上还可以使用Response.iter_content() Response.iter_lines() 
Response.raw()来自己决定要读取多少数据

最后要注意的是，使用stream=True以后需要自己执行Response的 

关闭操作close

默认情况下requests对URL的访问是阻塞式的，可以通过使用 
1）grequests 
2）requests-futures 
来实现非阻塞式的访问

参考资料 
1.http://docs.python-requests.org/en/master/user/advanced/#body-content-workflow 
2.http://docs.python-requests.org/en/master/user/advanced/#blocking-or-non-blocking 
3.http://docs.python-requests.org/en/master/user/advanced/#blocking-or-non-blocking