[Patch #1479977] Revised version of urllib2 HOWTO, edited by John J. Lee

1 files changed, 308 insertions, 143 deletions

diff --git a/Doc/howto/urllib2.rst b/Doc/howto/urllib2.rst
index 675cfd0..6feb7c2 100644
--- a/Doc/howto/urllib2.rst
+++ b/Doc/howto/urllib2.rst
@@ -8,7 +8,9 @@
 
 .. note::
 
-    There is an French translation of this HOWTO, available at `urllib2 - Le Manuel manquant <http://www.voidspace/python/urllib2_francais.shtml>`_.
+    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
  
@@ -18,56 +20,143 @@ Introduction
 
 .. sidebar:: Related Articles
 
-    You may also find useful the following articles on fetching web resources with Python :
+    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 exampels in Python.
+        A tutorial on *Basic Authentication*, with examples in Python.
     
-    * `cookielib and ClientCookie <http://www.voidspace.org.uk/python/articles/cookielib.shtml>`_
-    
-        How to handle cookies, when fetching web pages with 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.
-
-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 HyperText Transfer Protocol. The most comprehensive 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. 
+    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
 =============
 
-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 [#]_. Calling ``urlopen`` with this Request object returns a handle on the page requested. This handle is a file like object : ::
+The simplest way to use urllib2 is as follows : ::
 
     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 (Common Gateway Interface) [#]_ 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``. ::
+    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  
-    
-    the_url = 'http://www.someserver.com/cgi-bin/register.cgi'
+
+    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()    
+    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
+-------
 
-Some websites [#]_ dislike being browsed by programs, or send different versions to different browsers [#]_ . 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 [#]_. 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 [#]_. ::
+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  
     
-    the_url = 'http://www.someserver.com/cgi-bin/register.cgi'
+    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',
@@ -75,38 +164,38 @@ Some websites [#]_ dislike being browsed by programs, or send different versions
     headers = { 'User-Agent' : user_agent }
     
     data = urllib.urlencode(values)
-    req = urllib2.Request(the_url, data, headers)
-    handle = urllib2.urlopen(req)
-    the_page = handle.read()
+    req = urllib2.Request(url, data, headers)
+    response = urllib2.urlopen(req)
+    the_page = response.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.
+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.
 
 
-Coping With Errors
-==================
+Handling Exceptions
+===================
 
-*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. ::
+*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).
 
-    req = urllib2.Request(some_url)
-    try:
-        handle = urllib2.urlopen(req)
-    except IOError:
-        print 'Something went wrong'
-    else:
-        print handle.read()
+``HTTPError`` is the subclass of ``URLError`` raised in the specific
+case of HTTP URLs.
 
 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. 
+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 IOError, e:
+    >>> except URLError, e:
     >>>    print e.reason
     >>>
     (4, 'getaddrinfo failed')
@@ -115,26 +204,36 @@ e.g. ::
 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.
+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).
 
-The ``HTTPError`` instance raised will have an integer 'code' attribute, which corresponds to the error sent by the server.
+See section 10 of RFC 2616 for a reference on all the HTTP error
+codes.
 
-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.
+The ``HTTPError`` instance raised will have an integer 'code'
+attribute, which corresponds to the error sent by the server.
 
 Error Codes
 ~~~~~~~~~~~
 
-.. note::
-
-    As of Python 2.5 a dictionary like this one has become part of ``urllib2``.
+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)}.
-    httpresponses = {
+    responses = {
         100: ('Continue', 'Request received, please continue'),
         101: ('Switching Protocols',
               'Switching to new protocol; obey Upgrade header'),
@@ -143,78 +242,72 @@ Error Codes
         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'),
+        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'),
+        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',
+        304: ('Not Modified',
               'Document has not changed since given time'),
         305: ('Use Proxy',
-                'You must use proxy specified in Location'
-                ' to access this resource.'),
+              'You must use proxy specified in Location to access this '
+              'resource.'),
         307: ('Temporary Redirect',
               'Object moved temporarily -- see URI list'),
-              
-        400: ('Bad request',
+
+        400: ('Bad Request',
               'Bad request syntax or unsupported method'),
         401: ('Unauthorized',
               'No permission -- see authorization schemes'),
-        402: ('Payment required',
+        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.'),
+        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.'),
+        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.'),
+        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'),
+        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 temporarily overloaded',
-              'The server cannot '
-              'process the request due to a high load'),
-        504: ('Gateway timeout',
+        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.'),
+        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. ::
+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 IOError, e:
+    >>> except URLError, e:
     >>>     print e.code
     >>>     print e.read()
     >>> 
@@ -229,8 +322,8 @@ When an error is raised the server responds by returning an http error code *and
 Wrapping it Up
 --------------
 
-So if you want to be prepared for ``HTTPError`` *or* ``URLError`` there are two
-basic approaches. I prefer the second approach.
+So if you want to be prepared for ``HTTPError`` *or* ``URLError``
+there are two basic approaches. I prefer the second approach.
 
 Number 1
 ~~~~~~~~
@@ -241,7 +334,7 @@ Number 1
     from urllib2 import Request, urlopen, URLError, HTTPError
     req = Request(someurl)
     try:
-        handle = urlopen(req)
+        response = urlopen(req)
     except HTTPError, e:
         print 'The server couldn\'t fulfill the request.'
         print 'Error code: ', e.code
@@ -254,7 +347,8 @@ Number 1
 
 .. note::
 
-    The ``except HTTPError`` *must* come first, otherwise ``except URLError`` will *also* catch an ``HTTPError``.
+    The ``except HTTPError`` *must* come first, otherwise ``except URLError``
+    will *also* catch an ``HTTPError``.
 
 Number 2
 ~~~~~~~~
@@ -264,8 +358,8 @@ Number 2
     from urllib2 import Request, urlopen
     req = Request(someurl)
     try:
-        handle = urlopen(req)
-    except IOError, e:
+        response = urlopen(req)
+    except URLError, e:
         if hasattr(e, 'reason'):
             print 'We failed to reach a server.'
             print 'Reason: ', e.reason
@@ -279,110 +373,180 @@ Number 2
 info and geturl
 ===============
 
-The handle returned by urlopen (or the ``HTTPError`` instance) has two useful methods ``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 openener object used) may have followed a redirect. The url of the page fetched may not be the same as the url requested.
+**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 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.
+**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 reference on the different sort of headers.
+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
 ====================
 
-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.
+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.
 
-``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
+(global) default opener. This means that calls to ``urlopen`` will use
+the opener you have installed.
 
-``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.
+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`_.
+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"``. 
+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"
+    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 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 yoou providing an alternative combination for a specific realm. We signify this by providing ``None`` as the realm argument to the ``add_password`` method.
+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 toplevelurl is the first url that requires authentication. This is usually a 'super-url' of any others in the same realm. ::
+The top-level URL is the first URL that requires authentication. URLs
+"deeper" than the URL you pass to .add_password() will also match. ::
 
-    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``
-    
+    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 the handler
-    
+
+    # create "opener" (OpenerDirector instance)
     opener = urllib2.build_opener(handler)                       
-    # from handler to opener
 
-    opener.open(a_url)      
     # 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)                               
-    # 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:// !!
+    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``.
 
-It took me a long time to track that down the first time I tried to use handlers.
+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 [#]_. 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 : ::
+**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)
 
-.. caution::
+.. note::
 
-    Currently ``urllib2`` *does not* support fetching of ``https`` locations through
-    a proxy. This can be a problem.
+    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.
+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 : ::
+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 = 10
+
     # 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')
-    handle = urllib2.urlopen(req)
-    # this call to urllib2.urlopen
-    # now uses the default timeout
-    # we have set in the socket module    
+    response = urllib2.urlopen(req)
 
 
 -------
@@ -391,8 +555,8 @@ As of Python 2.3 you can specify how long a socket should wait for a response be
 Footnotes
 ===========
 
-.. [#] Possibly some of this tutorial will make it into the standard library docs for versions of Python after 2.4.1.
-.. [#] 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.
+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.
@@ -403,8 +567,9 @@ Footnotes
 
 .. _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
\ No newline at end of file
+.. _Basic Authentication Tutorial: http://www.voidspace.org.uk/python/articles/authentication.shtml