Urllib2 urlopen and read - difference

J. Cliff Dyer jcd at sdf.lonestar.org
Thu Apr 15 20:55:20 CEST 2010


On Thu, 2010-04-15 at 11:25 -0700, koranthala wrote:
> Hi,
>    Suppose I am doing the following:
> req = urllib2.urlopen('http://www.python.org')
> data = req.read()
> 
>    When is the actual data received? is it done by the first line? or
> is it done only when req.read() is used?
>   My understanding is that when urlopen is done itself, we would have
> received all the data, and req.read() just reads it from the file
> descriptor.
>   But, when I read the source code of pylot, it mentioned the
> following:
>             resp = opener.open(request)  # this sends the HTTP request
> and returns as soon as it is done connecting and sending
>             connect_end_time = self.default_timer()
>             content = resp.read()
>             req_end_time = self.default_timer()
> 
> Here, it seems to suggest that the data is received only after you do
> resp.read(), which made me all confused.
> 
> If someone could help me out, it would be much helpful.

My understanding (please correct me if I'm wrong), is that when you call
open, you send a request to the server, and get a response object back.
The server immediately begins sending data (you can't control when they
send it, once you've requested it).  When you call read() on your
response object, it reads all the data it has already received, and if
that amount of data isn't sufficient to handle your read call, it blocks
until it has enough.

So your opener returns as soon as the request is sent, and read() blocks
if it doesn't have enough data to handle your request.

Cheers,
Cliff




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