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************************************************
  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  # The order may differ. #doctest: +SKIP
    name=Somebody+Here&language=Python&location=Northampton
    >>> url = 'http://www.example.com/example.cgi'
    >>> full_url = url + '?' + url_values
    >>> data = urllib2.urlopen(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, built-in 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 as e:
    ...    print e.reason   #doctest: +SKIP
    ...
    (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 urllib2.HTTPError as e:
    ...     print e.code
    ...     print e.read() #doctest: +ELLIPSIS, +NORMALIZE_WHITESPACE
    ...
    404
    <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
    "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
    ...
    <title>Page Not Found</title>
    ...

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 as e:
        print 'The server couldn\'t fulfill the request.'
        print 'Error code: ', e.code
    except URLError as 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 as 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 ``HTTPBasicAuthHandler`` to
    ``build_opener``. By default openers have the handlers for normal situations
    -- ``ProxyHandler`` (if a proxy setting such as an :envvar:`http_proxy`
    environment variable is set), ``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 when a proxy
setting is detected.  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
==================

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 <http://www.pyzine.com/Issue008/Section_Articles/article_CGIOne.html>`_.
.. [#] Like Google for example. The *proper* way to use google from a program
       is to use `PyGoogle <http://pygoogle.sourceforge.net>`_ of course. See
       `Voidspace Google <http://www.voidspace.org.uk/python/recipebook.shtml#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.
.. [#] urllib2 opener for SSL proxy (CONNECT method): `ASPN Cookbook Recipe
       <http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/456195>`_.

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