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11.5. Setting the User-Agent

11.5. Setting the User-Agent

The first step to improving your HTTP web services client is to identify yourself properly with a User-Agent. To do that, you need to move beyond the basic urllib and dive into urllib2.

Example 11.4. Introducing urllib2

>>> import httplib
>>> httplib.HTTPConnection.debuglevel = 1                             1
>>> import urllib2
>>> request = urllib2.Request('http://diveintomark.org/xml/atom.xml') 2
>>> opener = urllib2.build_opener()                                   3
>>> feeddata = opener.open(request).read()                            4
connect: (diveintomark.org, 80)
send: '
GET /xml/atom.xml HTTP/1.0
Host: diveintomark.org
User-agent: Python-urllib/2.1
'
reply: 'HTTP/1.1 200 OK\r\n'
header: Date: Wed, 14 Apr 2004 23:23:12 GMT
header: Server: Apache/2.0.49 (Debian GNU/Linux)
header: Content-Type: application/atom+xml
header: Last-Modified: Wed, 14 Apr 2004 22:14:38 GMT
header: ETag: "e8284-68e0-4de30f80"
header: Accept-Ranges: bytes
header: Content-Length: 26848
header: Connection: close
1 If you still have your Python IDE open from the previous section's example, you can skip this, but this turns on HTTP debugging so you can see what you're actually sending over the wire, and what gets sent back.
2 Fetching an HTTP resource with urllib2 is a three-step process, for good reasons that will become clear shortly. The first step is to create a Request object, which takes the URL of the resource you'll eventually get around to retrieving. Note that this step doesn't actually retrieve anything yet.
3 The second step is to build a URL opener. This can take any number of handlers, which control how responses are handled. But you can also build an opener without any custom handlers, which is what you're doing here. You'll see how to define and use custom handlers later in this chapter when you explore redirects.
4 The final step is to tell the opener to open the URL, using the Request object you created. As you can see from all the debugging information that gets printed, this step actually retrieves the resource and stores the returned data in feeddata.

Example 11.5. Adding headers with the Request

>>> request                                                1
<urllib2.Request instance at 0x00250AA8>
>>> request.get_full_url()
http://diveintomark.org/xml/atom.xml
>>> request.add_header('User-Agent',
...     'OpenAnything/1.0 +http://diveintopython.org/')    2
>>> feeddata = opener.open(request).read()                 3
connect: (diveintomark.org, 80)
send: '
GET /xml/atom.xml HTTP/1.0
Host: diveintomark.org
User-agent: OpenAnything/1.0 +http://diveintopython.org/   4
'
reply: 'HTTP/1.1 200 OK\r\n'
header: Date: Wed, 14 Apr 2004 23:45:17 GMT
header: Server: Apache/2.0.49 (Debian GNU/Linux)
header: Content-Type: application/atom+xml
header: Last-Modified: Wed, 14 Apr 2004 22:14:38 GMT
header: ETag: "e8284-68e0-4de30f80"
header: Accept-Ranges: bytes
header: Content-Length: 26848
header: Connection: close
1 You're continuing from the previous example; you've already created a Request object with the URL you want to access.
2 Using the add_header method on the Request object, you can add arbitrary HTTP headers to the request. The first argument is the header, the second is the value you're providing for that header. Convention dictates that a User-Agent should be in this specific format: an application name, followed by a slash, followed by a version number. The rest is free-form, and you'll see a lot of variations in the wild, but somewhere it should include a URL of your application. The User-Agent is usually logged by the server along with other details of your request, and including a URL of your application allows server administrators looking through their access logs to contact you if something is wrong.
3 The opener object you created before can be reused too, and it will retrieve the same feed again, but with your custom User-Agent header.
4 And here's you sending your custom User-Agent, in place of the generic one that Python sends by default. If you look closely, you'll notice that you defined a User-Agent header, but you actually sent a User-agent header. See the difference? urllib2 changed the case so that only the first letter was capitalized. It doesn't really matter; HTTP specifies that header field names are completely case-insensitive.
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