************************************************

  HOWTO Fetch Internet Resources Using urllib2

************************************************

:Author: `Michael Foord <http://www.voidspace.org.uk/python/index.shtml>`_

.. note::

    There is an French translation of an earlier revision of this

    HOWTO, available at `urllib2 - Le Manuel manquant

    <http://www.voidspace.org.uk/python/articles/urllib2_francais.shtml>`_.

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.

**urllib2** is a `Python <http://www.python.org>`_ 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.

urllib2 supports fetching URLs for many "URL schemes" (identified by the string

before the ":" in URL - for example "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 :mod:`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

<http://www.w3.org/TR/REC-html40/interact/forms.html#h-17.13>`_ 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

GET request by encoding it in the URL itself.

This is done as follows::

    >>> import urllib2

    >>> import urllib

    >>> data = {}

    >>> data['name'] = 'Somebody Here'

    >>> data['location'] = 'Northampton'

    >>> data['language'] = 'Python'

    >>> url_values = urllib.urlencode(data)

    >>> print url_values

    name=Somebody+Here&language=Python&location=Northampton

    >>> url = 'http://www.example.com/example.cgi'

    >>> full_url = url + '?' + url_values

    >>> data = urllib2.open(full_url)

Notice that the full URL is created by adding a ``?`` to the URL, followed by

the encoded values.

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 :exc:`URLError` when it cannot handle a response (though as usual

with Python APIs, builtin exceptions such as 

:exc:`ValueError`, :exc:`TypeError` etc. may also

be raised).

:exc:`HTTPError` is the subclass of :exc:`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 :exc:`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 :exc:`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 :exc:`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 :exc:`HTTPError` *or* :exc:`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 :exc:`HTTPError`.

Number 2

~~~~~~~~

::

    from urllib2 import Request, urlopen, URLError

    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 :exc:`HTTPError` instance) has two useful

methods :meth:`info` and :meth:`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 <http://www.cs.tut.fi/~jkorpela/http.html>`_

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 :class:`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

<http://www.voidspace.org.uk/python/articles/authentication.shtml>`_.

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 you 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.  However, this can be enabled by extending urllib2 as

    shown in the recipe [#]_.

Sockets and Layers

==================