urllib2 - The Missing Manual

Fetching URLs With Python

Note

There is an French translation of this article - urllib2 - Le Manuel manquant.

urllib2 Tutorial

Introduction

urllib2 is a Python module for fetching URLs. It offers a very simple interface, in the form of the urlopen function. This is capable of fetching URLs using a variety of different protocols. It also offers a slightly more complex interface for handling common situations - like basic authentication, cookies, proxies, and so on. These are provided by objects called handlers and openers.

For straightforward situations urlopen is very easy to use. But as soon as you encounter errors, or non-trivial cases, you will need some understanding of the HTTP protocol. The most comprehensive reference to HTTP is RFC 2616. This is a technical document and not intended to be easy to read Laughing . This tutorial aims to illustrate using urllib2, with enough detail about HTTP to help you through. It is not intended to replace the urllib2 docs [1], but is supplementary to them.

Fetching URLs

HTTP is based on requests and responses - the client makes requests and servers send responses. Python mirrors this by having you form a Request object which represents the request you are making. In it's simplest form you create a Request object that specifies the URL you want to fetch [2]. Calling urlopen with this Request object returns a handle on the page requested. This handle is a file like object :

import urllib2

the_url = 'http://www.voidspace.org.uk'
req = urllib2.Request(the_url)
handle = urllib2.urlopen(req)
the_page = handle.read()

There are two extra things that Request objects allow you to do. Sometimes you want to POST data to a CGI [3] or other web application. This is what your browser does when you fill in a FORM on the web. You may be mimicking a FORM submission, or transmitting data to your own application. In either case the data needs to be encoded for safe transmission over HTTP, and then passed to the Request object as the data argument. The encoding is done using a function from the urllib library not from urllib2 Razz .

import urllib
import urllib2

the_url = 'http://www.someserver.com/cgi-bin/register.cgi'
values = {'name' : 'Michael Foord',
          'location' : 'Northampton',
          'language' : 'Python' }

data = urllib.urlencode(values)
req = urllib2.Request(the_url, data)
handle = urllib2.urlopen(req)
the_page = handle.read()

Some websites [4] dislike being browsed by programs, or send different versions to different browsers [5] . By default urllib2 identifies itself as Python-urllib/2.4, which may confuse the site, or just plain not work. The way a browser identifies itself is through the User-Agent header [6]. When you create a Request object you can pass a dictionary of headers in. The following example makes the same request as above, but identifies itself as a version of IE [7].

import urllib
import urllib2

the_url = 'http://www.someserver.com/cgi-bin/register.cgi'
user_agent = 'Mozilla/4.0 (compatible; MSIE 5.5; Windows NT)'
values = {'name' : 'Michael Foord',
          'location' : 'Northampton',
          'language' : 'Python' }
headers = { 'User-Agent' : user_agent }

data = urllib.urlencode(values)
req = urllib2.Request(the_url, data, headers)
handle = urllib2.urlopen(req)
the_page = handle.read()

The handle also has two useful methods. See the section on info and geturl which comes after we have a look at what happens when things go wrong.

Coping With Errors

urlopen raises URLError or HTTPError in the event of an error. HTTPError is a subclass of URLError, which is a subclass of IOError. This means you can trap for IOError if you want.

req = urllib2.Request(some_url)
try:
    handle = urllib2.urlopen(req)
except IOError:
    print 'Something went wrong'
else:
    print handle.read()

URLError

If the request fails to reach a server then urlopen will raise a URLError. This will usually be because there is no network connection (no route to the specified server), or the specified server doesn't exist.

In this case, the exception raised will have a 'reason' attribute, which is a tuple containing an error code and a text error message.

e.g.

>>> req = urllib2.Request('http://www.pretend_server.org')
>>> try: urllib2.urlopen(req)
>>> except IOError, e:
>>>    print e.reason
>>>
(4, 'getaddrinfo failed')

HTTPError

If the request reaches a server, but the server is unable to fulfil the request, it returns an error code. The default handlers will hande some of these errors for you. For those it can't handle, urlopen will raise an HTTPError. Typical errors include '404' (page not found), '403' (request forbidden), and '401' (authentication required).

See http://www.w3.org/Protocols/HTTP/HTRESP.html for a reference on all the http error codes.

The HTTPError instance raised will have an integer 'code' attribute, which corresponds to the error sent by the server.

There is a useful dictionary of response codes in HTTPBaseServer, that shows all the defined response codes. Because the default handlers handle redirects (codes in the 300 range), and codes in the 100-299 range indicate success, you will usually only see error codes in the 400-599 range.

Error Codes

Note

As of Python 2.5 a dictionary like this one has become part of urllib2. Very Happy

# Table mapping response codes to messages; entries have the
# form {code: (shortmessage, longmessage)}.
httpresponses = {
    100: ('Continue', 'Request received, please continue'),
    101: ('Switching Protocols',
          'Switching to new protocol; obey Upgrade header'),

    200: ('OK', 'Request fulfilled, document follows'),
    201: ('Created', 'Document created, URL follows'),
    202: ('Accepted',
          'Request accepted, processing continues off-line'),
    203: ('Non-Authoritative Information',
            'Request fulfilled from cache'),
    204: ('No response', 'Request fulfilled, nothing follows'),
    205: ('Reset Content', 'Clear input form for further input.'),
    206: ('Partial Content', 'Partial content follows.'),

    300: ('Multiple Choices',
          'Object has several resources -- see URI list'),
    301: ('Moved Permanently',
            'Object moved permanently -- see URI list'),
    302: ('Found', 'Object moved temporarily -- see URI list'),
    303: ('See Other', 'Object moved -- see Method and URL list'),
    304: ('Not modified',
          'Document has not changed since given time'),
    305: ('Use Proxy',
            'You must use proxy specified in Location'
            ' to access this resource.'),
    307: ('Temporary Redirect',
          'Object moved temporarily -- see URI list'),

    400: ('Bad request',
          'Bad request syntax or unsupported method'),
    401: ('Unauthorized',
          'No permission -- see authorization schemes'),
    402: ('Payment required',
          'No payment -- see charging schemes'),
    403: ('Forbidden',
          'Request forbidden -- authorization will not help'),
    404: ('Not Found', 'Nothing matches the given URI'),
    405: ('Method Not Allowed',
          'Specified method is invalid for this server.'),
    406: ('Not Acceptable',
            'URI not available in preferred format.'),
    407: ('Proxy Authentication Required',
            'You must authenticate with '
            'this proxy before proceeding.'),
    408: ('Request Time-out',
            'Request timed out; try again later.'),
    409: ('Conflict', 'Request conflict.'),
    410: ('Gone',
          'URI no longer exists and has been permanently removed.'),
    411: ('Length Required', 'Client must specify Content-Length.'),
    412: ('Precondition Failed',
            'Precondition in headers is false.'),
    413: ('Request Entity Too Large', 'Entity is too large.'),
    414: ('Request-URI Too Long', 'URI is too long.'),
    415: ('Unsupported Media Type',
            'Entity body in unsupported format.'),
    416: ('Requested Range Not Satisfiable',
          'Cannot satisfy request range.'),
    417: ('Expectation Failed',
          'Expect condition could not be satisfied.'),

    500: ('Internal error', 'Server got itself in trouble'),
    501: ('Not Implemented',
          'Server does not support this operation'),
    502: ('Bad Gateway',
            'Invalid responses from another server/proxy.'),
    503: ('Service temporarily overloaded',
          'The server cannot '
          'process the request due to a high load'),
    504: ('Gateway timeout',
          'The gateway server did not receive a timely response'),
    505: ('HTTP Version not supported', 'Cannot fulfill request.'),
    }

When an error is raised the server responds by returning an http error code and an error page. You can use the HTTPError instance as a handle on the page returned. This means that as well as the code attribute, it also has read, geturl, and info, methods.

>>> req = urllib2.Request('http://www.python.org/fish.html')
>>> try:
>>>     urllib2.urlopen(req)
>>> except IOError, e:
>>>     print e.code
>>>     print e.read()
>>>
404
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
    "http://www.w3.org/TR/html4/loose.dtd">
<?xml-stylesheet href="./css/ht2html.css"
    type="text/css"?>
<html><head><title>Error 404: File Not Found</title>
...... etc...

Wrapping it Up

So if you want to be prepared for HTTPError or URLError there are two basic approaches. I prefer the second approach.

Number 1

from urllib2 import Request, urlopen, URLError, HTTPError
req = Request(someurl)
try:
    handle = urlopen(req)
except HTTPError, e:
    print 'The server couldn\'t fulfill the request.'
    print 'Error code: ', e.code
except URLError, e:
    print 'We failed to reach a server.'
    print 'Reason: ', e.reason
else:
    # everything is fine

Note

The except HTTPError must come first, otherwise except URLError will also catch HTTPErrors. Surprised

Number 2

from urllib2 import Request, urlopen
req = Request(someurl)
try:
    handle = urlopen(req)
except IOError, e:
    if hasattr(e, 'reason'):
        print 'We failed to reach a server.'
        print 'Reason: ', e.reason
    elif hasattr(e, 'code'):
        print 'The server couldn\'t fulfill the request.'
        print 'Error code: ', e.code
else:
    # everything is fine

info and geturl

The handle returned by urlopen (or the HTTPError instance) has two useful methods info and geturl.

geturl - this returns the real url of the page fetched. This is useful because urlopen (or the openener object used) may have followed a redirect. The url of the page fetched may not be the same as the url requested.

info - this returns a dictionary like object that describes the page fetched, particularly the headers sent by the server. It is actually an httplib.HTTPMessage instance. In versions of Python prior to 2.3.4 it wasn't safe to iterate over the object directly, so you should iterate over the list returned by msg.keys() instead.

Typical headers include 'content-length', 'content-type', and so on. See the Quick Reference to HTTP Headers for a useful reference on the different sort of headers.

Openers and Handlers

Openers and handlers are slightly esoteric parts of urllib2. When you fetch a URL you use an opener. Normally we have been using the default opener - via urlopen - but you can create custom openers. Openers use handlers.

build_opener is used to create opener objects - for fetching URLs with specific handlers installed. Handlers can handle cookies, authentication, and other common but slightly specialised situations. Opener objects have an open method, which can be called directly to fetch urls in the same way as the urlopen function.

install_opener can be used to make an opener object the default opener. This means that calls to urlopen will use the opener you have installed.

Basic Authentication

To illustrate creating and installing a handler we will use the HTTPBasicAuthHandler. For a more detailed discussion of this subject - including an explanation of how Basic Authentication works - see the Basic Authentication Tutorial.

When authentication is required, the server sends a header (as well as the 401 error code) requesting authentication. This specifies the authentication scheme and a 'realm'. The header looks like : www-authenticate: SCHEME realm="REALM".

e.g.

www-authenticate: Basic realm="cPanel"

The client should then retry the request with the appropriate name and password for the realm included as a header in the request. This is 'basic authentication'. In order to simplify this process we can create an instance of HTTPBasicAuthHandler and an opener to use this handler.

The HTTPBasicAuthHandler uses an object called a password manager to handle the mapping of URIs and realms to passwords and usernames. If you know what the realm is (from the authentication header sent by the server), then you can use a HTTPPasswordMgr. Generally there is only one realm per URI, so it is possible to use HTTPPasswordMgrWithDefaultRealm. This allows you to specify a default username and password for a URI. This will be supplied in the absence of you providing an alternative combination for a specific realm. We signify this by providing None as the realm argument to the add_password method.

The toplevelurl is the first url that requires authentication. This is usually a 'super-url' of any others in the same realm.

password_mgr = urllib2.HTTPPasswordMgrWithDefaultRealm()
# create a password manager

password_mgr.add_password(None,
    top_level_url, username, password)
# add the username and password
# if we knew the realm, we could
# use it instead of ``None``

handler = urllib2.HTTPBasicAuthHandler(password_mgr)
# create the handler

opener = urllib2.build_opener(handler)
# from handler to opener

opener.open(a_url)
# use the opener to fetch a URL

urllib2.install_opener(opener)
# install the opener
# now all calls to urllib2.urlopen use our opener

Note

In the above example we only supplied our HHTPBasicAuthHandler to build_opener. By default openers have the handlers for normal situations - ProxyHandler, UnknownHandler, HTTPHandler, HTTPDefaultErrorHandler, HTTPRedirectHandler, FTPHandler, FileHandler, HTTPErrorProcessor. The only reason to explicitly supply these to build_opener (which chains handlers provided as a list), would be to change the order they appear in the chain.

One thing not to get bitten by is that the top_level_url in the code above must not contain the protocol - the http:// part. So if the URL we are trying to access is http://www.someserver.com/path/page.html, then we set :

top_level_url = "www.someserver.com/path/page.html"
# *no* http:// !!

It took me a long time to track that down the first time I tried to use handlers Embarassed .

Proxies

urllib2 will auto-detect your proxy settings and use those. This is through the ProxyHandler which is part of the normal handler chain. Normally that's a good thing, but there are occasions when it may not be helpful [8]. In order to do this we need to setup our own ProxyHandler, with no proxies defined. This is done using similar steps to setting up a Basic Authentication handler :

>>> proxy_support = urllib2.ProxyHandler({})
>>> opener = urllib2.build_opener(proxy_support)
>>> urllib2.install_opener(opener)

Caution!

Currently urllib2 does not support fetching of https locations through a proxy. Sad This can be a problem.

Sockets and Layers

The Python support for fetching resources from the web is layered. urllib2 uses the httplib library, which in turn uses the socket library.

As of Python 2.3 you can specify how long a socket should wait for a response before timing out. This can be useful in applications which have to fetch web pages. By default the socket module has no timeout and can hang. To set the timeout use :

import socket
import urllib2

timeout = 10
# timeout in seconds
socket.setdefaulttimeout(timeout)

req = urllib2.Request('http://www.voidspace.org.uk')
handle = urllib2.urlopen(req)
# this call to urllib2.urlopen
# now uses the default timeout
# we have set in the socket module

Footnotes

[1]Possibly some of this tutorial will make it into the standard library docs for versions of Python after 2.4.1.
[2]You can fetch URLs directly with urlopen, without using a request object. It's more explicit, and therefore more Pythonic, to use urllib2.Request though. It also makes it easier to add headers to your request.
[3]For an introduction to the CGI protocol see Writing Web Applications in Python.
[4]Like Google for example. The proper way to use google from a program is to use PyGoogle of course. See Voidspace Google for some examples of using the Google API.
[5]Browser sniffing is a very bad practise for website design - building sites using web standards is much more sensible. Unfortunately a lot of sites still send different versions to different browsers.
[6]The user agent for MSIE 6 is 'Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 1.1.4322)'
[7]For details of more HTTP request headers, see Quick Reference to HTTP Headers.
[8]In my case I have to use a proxy to access the internet at work. If you attempt to fetch localhost URLs through this proxy it blocks them. IE is set to use the proxy, which urllib2 picks up on. In order to test scripts with a localhost server, I have to prevent urllib2 from using the proxy.

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Last edited Fri Feb 15 13:42:08 2008.