Doc/howto/urllib2.rst

==============================================
 HOWTO Fetch Internet Resources Using urllib2
==============================================
------------------------------------------
  Fetching URLs With Python
------------------------------------------


.. note::

    There is an French translation of an earlier revision of this
    HOWTO, available at `urllib2 - Le Manuel manquant
    <http://www.voidspace/python/urllib2_francais.shtml>`_.

.. contents:: urllib2 Tutorial
 

Introduction
============

.. sidebar:: Related Articles

    You may also find useful the following article on fetching web
    resources with Python :
    
    * `Basic Authentication <http://www.voidspace.org.uk/python/articles/authentication.shtml>`_
    
        A tutorial on *Basic Authentication*, with examples in Python.
    
    This HOWTO is written by `Michael Foord
    <http://www.voidspace.org.uk/python/index.shtml>`_.

**urllib2** is a Python_ module for fetching URLs (Uniform Resource
Locators). 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.

While urllib2 supports fetching URLs for many "URL schemes"
(identified by the string before the ":" in URL - e.g. "ftp" is the
URL scheme of "ftp://python.org/") using their associated network
protocols (e.g. FTP, HTTP), this tutorial focuses on the most common
case, HTTP.

For straightforward situations *urlopen* is very easy to use. But as
soon as you encounter errors or non-trivial cases when opening HTTP
URLs, you will need some understanding of the HyperText Transfer
Protocol. The most comprehensive and authoritative reference to HTTP
is :RFC:`2616`. This is a technical document and not intended to be
easy to read. This HOWTO aims to illustrate using *urllib2*, with
enough detail about HTTP to help you through. It is not intended to
replace the `urllib2 docs`_ , but is supplementary to them.


Fetching URLs
=============

The simplest way to use urllib2 is as follows : ::

    import urllib2
    response = urllib2.urlopen('http://python.org/')
    html = response.read()

Many uses of urllib2 will be that simple (note that instead of an
'http:' URL we could have used an URL starting with 'ftp:', 'file:',
etc.).  However, it's the purpose of this tutorial to explain the more
complicated cases, concentrating on HTTP.

HTTP is based on requests and responses - the client makes requests
and servers send responses. urllib2 mirrors this with a ``Request``
object which represents the HTTP request you are making. In its
simplest form you create a Request object that specifies the URL you
want to fetch. Calling ``urlopen`` with this Request object returns a
response object for the URL requested. This response is a file-like
object, which means you can for example call .read() on the response :
::

    import urllib2

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

Note that urllib2 makes use of the same Request interface to handle
all URL schemes.  For example, you can make an FTP request like so: ::

    req = urllib2.Request('ftp://example.com/')

In the case of HTTP, there are two extra things that Request objects
allow you to do: First, you can pass data to be sent to the server.
Second, you can pass extra information ("metadata") *about* the data
or the about request itself, to the server - this information is sent
as HTTP "headers".  Let's look at each of these in turn.

Data
----

Sometimes you want to send data to a URL (often the URL will refer to
a CGI (Common Gateway Interface) script [#]_ or other web
application). With HTTP, this is often done using what's known as a
**POST** request. This is often what your browser does when you submit
a HTML form that you filled in on the web. Not all POSTs have to come
from forms: you can use a POST to transmit arbitrary data to your own
application. In the common case of HTML forms, the data needs to be
encoded in a standard way, 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``. ::

    import urllib
    import urllib2  

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

    data = urllib.urlencode(values)
    req = urllib2.Request(url, data)
    response = urllib2.urlopen(req)
    the_page = response.read()    

Note that other encodings are sometimes required (e.g. for file upload
from HTML forms - see `HTML Specification, Form Submission`_ for more
details).

If you do not pass the ``data`` argument, urllib2 uses a **GET**
request.  One way in which GET and POST requests differ is that POST
requests often have "side-effects": they change the state of the
system in some way (for example by placing an order with the website
for a hundredweight of tinned spam to be delivered to your door).
Though the HTTP standard makes it clear that POSTs are intended to
*always* cause side-effects, and GET requests *never* to cause
side-effects, nothing prevents a GET request from having side-effects,
nor a POST requests from having no side-effects.  Data can also be
passed in an HTTP request by encoding it in the URL itself.

Headers
-------

We'll discuss here one particular HTTP header, to illustrate how to
add headers to your HTTP request.

Some websites [#]_ dislike being browsed by programs, or send
different versions to different browsers [#]_ . By default urllib2
identifies itself as ``Python-urllib/x.y`` (where ``x`` and ``y`` are
the major and minor version numbers of the Python release,
e.g. ``Python-urllib/2.5``), which may confuse the site, or just plain
not work. The way a browser identifies itself is through the
``User-Agent`` header [#]_. 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 Internet
Explorer [#]_. ::

    import urllib
    import urllib2  
    
    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(url, data, headers)
    response = urllib2.urlopen(req)
    the_page = response.read()

The response 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.


Handling Exceptions
===================

*urlopen* raises ``URLError`` when it cannot handle a response (though
as usual with Python APIs, builtin exceptions such as ValueError,
TypeError etc. may also be raised).

``HTTPError`` is the subclass of ``URLError`` raised in the specific
case of HTTP URLs.

URLError
--------

Often, URLError is raised 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 URLError, e:
    >>>    print e.reason
    >>>
    (4, 'getaddrinfo failed')


HTTPError
---------

Every HTTP response from the server contains a numeric "status
code". Sometimes the status code indicates that the server is unable
to fulfil the request. The default handlers will handle some of these
responses for you (for example, if the response is a "redirection"
that requests the client fetch the document from a different URL,
urllib2 will handle that 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 section 10 of RFC 2616 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.

Error 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.

``BaseHTTPServer.BaseHTTPRequestHandler.responses`` is a useful
dictionary of response codes in that shows all the response codes used
by RFC 2616. The dictionary is reproduced here for convenience ::

    # Table mapping response codes to messages; entries have the
    # form {code: (shortmessage, longmessage)}.
    responses = {
        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 Content', '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 Timeout', '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 Server 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 Unavailable',
              '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
response 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 URLError, 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:
        response = 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 an ``HTTPError``.

Number 2
~~~~~~~~

::

    from urllib2 import Request, urlopen
    req = Request(someurl)
    try:
        response = urlopen(req)
    except URLError, 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 response 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 opener 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
currently an ``httplib.HTTPMessage`` instance.

Typical headers include 'Content-length', 'Content-type', and so
on. See the `Quick Reference to HTTP Headers`_ for a useful listing of
HTTP headers with brief explanations of their meaning and use.


Openers and Handlers
====================

When you fetch a URL you use an opener (an instance of the perhaps
confusingly-named urllib2.OpenerDirector). Normally we have been using
the default opener - via ``urlopen`` - but you can create custom
openers. Openers use handlers. All the "heavy lifting" is done by the
handlers. Each handler knows how to open URLs for a particular URL
scheme (http, ftp, etc.), or how to handle an aspect of URL opening,
for example HTTP redirections or HTTP cookies.

You will want to create openers if you want to fetch URLs with
specific handlers installed, for example to get an opener that handles
cookies, or to get an opener that does not handle redirections.

To create an opener, instantiate an OpenerDirector, and then call
.add_handler(some_handler_instance) repeatedly.

Alternatively, you can use ``build_opener``, which is a convenience
function for creating opener objects with a single function call.
``build_opener`` adds several handlers by default, but provides a
quick way to add more and/or override the default handlers.

Other sorts of handlers you might want to can handle proxies,
authentication, and other common but slightly specialised
situations.

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

Opener objects have an ``open`` method, which can be called directly
to fetch urls in the same way as the ``urlopen`` function: there's no
need to call ``install_opener``, except as a convenience.


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 Users"


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 URLs 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``. Frequently one doesn't care what the realm is. In
that case, it is convenient to use
``HTTPPasswordMgrWithDefaultRealm``. This allows you to specify a
default username and password for a URL. This will be supplied in the
absence of yoou providing an alternative combination for a specific
realm. We indicate this by providing ``None`` as the realm argument to
the ``add_password`` method.

The top-level URL is the first URL that requires authentication. URLs
"deeper" than the URL you pass to .add_password() will also match. ::

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

    # Add the username and password.
    # If we knew the realm, we could use it instead of ``None``.
    top_level_url = "http://example.com/foo/"
    password_mgr.add_password(None, top_level_url, username, password)

    handler = urllib2.HTTPBasicAuthHandler(password_mgr)                            

    # create "opener" (OpenerDirector instance)
    opener = urllib2.build_opener(handler)                       

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

    # Install the opener.
    # Now all calls to urllib2.urlopen use our opener.
    urllib2.install_opener(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``.

top_level_url is in fact *either* a full URL (including the 'http:'
scheme component and the hostname and optionally the port number)
e.g. "http://example.com/" *or* an "authority" (i.e. the hostname,
optionally including the port number) e.g. "example.com" or
"example.com:8080" (the latter example includes a port number).  The
authority, if present, must NOT contain the "userinfo" component - for
example "joe@password:example.com" is not correct.


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 [#]_. One way to do this is 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)

.. note::

    Currently ``urllib2`` *does not* support fetching of ``https``
    locations through a proxy. 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. Currently, the socket timeout is not exposed at the
httplib or urllib2 levels.  However, you can set the default timeout
globally for all sockets using : ::

    import socket
    import urllib2

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

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


-------


Footnotes
===========

This document was reviewed and revised by John Lee.

.. [#] For an introduction to the CGI protocol see `Writing Web Applications in Python`_. 
.. [#] 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.
.. [#] 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.
.. [#] The user agent for MSIE 6 is *'Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 1.1.4322)'*
.. [#] For details of more HTTP request headers, see `Quick Reference to HTTP Headers`_.

.. [#] 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.  

.. _Python: http://www.python.org
.. _urllib2 docs: http://docs.python.org/lib/module-urllib2.html
.. _HTML Specification, Form Submission: http://www.w3.org/TR/REC-html40/interact/forms.html#h-17.13
.. _Quick Reference to HTTP Headers: http://www.cs.tut.fi/~jkorpela/http.html
.. _PyGoogle: http://pygoogle.sourceforge.net
.. _Voidspace Google: http://www.voidspace.org.uk/python/recipebook.shtml#google
.. _Writing Web Applications in Python: http://www.pyzine.com/Issue008/Section_Articles/article_CGIOne.html
.. _Basic Authentication Tutorial: http://www.voidspace.org.uk/python/articles/authentication.shtml