Guide pratique : récupérer des ressources web en utilisant le module *urllib*
*****************************************************************************

Auteur:
   Michael Foord


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


Related Articles
^^^^^^^^^^^^^^^^

You may also find useful the following article on fetching web
resources with Python:

* Basic Authentication

     A tutorial on *Basic Authentication*, with examples in Python.

**urllib.request** 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.

urllib.request 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 *urllib*, with
enough detail about HTTP to help you through. It is not intended to
replace the "urllib.request" docs, but is supplementary to them.


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

The simplest way to use urllib.request is as follows:

   import urllib.request
   with urllib.request.urlopen('http://python.org/') as response:
      html = response.read()

If you wish to retrieve a resource via URL and store it in a temporary
location, you can do so via the "shutil.copyfileobj()" and
"tempfile.NamedTemporaryFile()" functions:

   import shutil
   import tempfile
   import urllib.request

   with urllib.request.urlopen('http://python.org/') as response:
       with tempfile.NamedTemporaryFile(delete=False) as tmp_file:
           shutil.copyfileobj(response, tmp_file)

   with open(tmp_file.name) as html:
       pass

Many uses of urllib will be that simple (note that instead of an
'http:' URL we could have used a 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. urllib.request 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 urllib.request

   req = urllib.request.Request('http://python.org/')
   with urllib.request.urlopen(req) as response:
      the_page = response.read()

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

   req = urllib.request.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 about the 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.parse" library.

   import urllib.parse
   import urllib.request

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

   data = urllib.parse.urlencode(values)
   data = data.encode('ascii') # data should be bytes
   req = urllib.request.Request(url, data)
   with urllib.request.urlopen(req) as response:
      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, urllib 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 urllib.request
   >>> import urllib.parse
   >>> data = {}
   >>> data['name'] = 'Somebody Here'
   >>> data['location'] = 'Northampton'
   >>> data['language'] = 'Python'
   >>> url_values = urllib.parse.urlencode(data)
   >>> print(url_values)  # The order may differ from below.
   name=Somebody+Here&language=Python&location=Northampton
   >>> url = 'http://www.example.com/example.cgi'
   >>> full_url = url + '?' + url_values
   >>> data = urllib.request.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 [1] dislike being browsed by programs, or send different
versions to different browsers [2]. By default urllib 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
[3]. 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 [4].

   import urllib.parse
   import urllib.request

   url = 'http://www.someserver.com/cgi-bin/register.cgi'
   user_agent = 'Mozilla/5.0 (Windows NT 6.1; Win64; x64)'
   values = {'name': 'Michael Foord',
             'location': 'Northampton',
             'language': 'Python' }
   headers = {'User-Agent': user_agent}

   data = urllib.parse.urlencode(values)
   data = data.encode('ascii')
   req = urllib.request.Request(url, data, headers)
   with urllib.request.urlopen(req) as response:
      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.


Gestion des exceptions
======================

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

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

The exception classes are exported from the "urllib.error" module.


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 = urllib.request.Request('http://www.pretend_server.org')
   >>> try: urllib.request.urlopen(req)
   ... except urllib.error.URLError as 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,
urllib 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.

"http.server.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 as returned by
the "urllib.response" module:

   >>> req = urllib.request.Request('http://www.python.org/fish.html')
   >>> try:
   ...     urllib.request.urlopen(req)
   ... except urllib.error.HTTPError as e:
   ...     print(e.code)
   ...     print(e.read())
   ...
   404
   b'<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
     "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">\n\n\n<html
     ...
     <title>Page Not Found</title>\n
     ...


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 urllib.request import Request, urlopen
   from urllib.error import 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 "HTTPError".


Number 2
~~~~~~~~

   from urllib.request import Request, urlopen
   from urllib.error import 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 "HTTPError" instance) has two
useful methods "info()" and "geturl()" and is defined in the module
"urllib.response".

* **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 "http.client.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 "urllib.request.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 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 = urllib.request.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 = urllib.request.HTTPBasicAuthHandler(password_mgr)

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

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

   # Install the opener.
   # Now all calls to urllib.request.urlopen use our opener.
   urllib.request.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
  "http_proxy" environment variable is set), "UnknownHandler",
  "HTTPHandler", "HTTPDefaultErrorHandler", "HTTPRedirectHandler",
  "FTPHandler", "FileHandler", "DataHandler", "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
=======

**urllib** 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 [5]. 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 = urllib.request.ProxyHandler({})
   >>> opener = urllib.request.build_opener(proxy_support)
   >>> urllib.request.install_opener(opener)

Note:

  Currently "urllib.request" *does not* support fetching of "https"
  locations through a proxy.  However, this can be enabled by
  extending urllib.request as shown in the recipe [6].

Note:

  "HTTP_PROXY" will be ignored if a variable "REQUEST_METHOD" is set;
  see the documentation on "getproxies()".


Sockets and Layers
==================

The Python support for fetching resources from the web is layered.
urllib uses the "http.client" 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
http.client or urllib.request levels. However, you can set the default
timeout globally for all sockets using

   import socket
   import urllib.request

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

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

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


Notes
=====

This document was reviewed and revised by John Lee.

[1] Google for example.

[2] Browser sniffing is a very bad practice for website design -
    building sites using web standards is much more sensible.
    Unfortunately a lot of sites still send different versions to
    different browsers.

[3] The user agent for MSIE 6 is *'Mozilla/4.0 (compatible; MSIE 6.0;
    Windows NT 5.1; SV1; .NET CLR 1.1.4322)'*

[4] For details of more HTTP request headers, see Quick Reference to
    HTTP Headers.

[5] 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 urllib picks up on.
    In order to test scripts with a localhost server, I have to
    prevent urllib from using the proxy.

[6] urllib opener for SSL proxy (CONNECT method): ASPN Cookbook
    Recipe.
