
:mod:`urllib2` --- extensible library for opening URLs
======================================================

.. module:: urllib2
.. moduleauthor:: Jeremy Hylton <jhylton@users.sourceforge.net>
.. sectionauthor:: Moshe Zadka <moshez@users.sourceforge.net>




The :mod:`urllib2` module defines functions and classes which help in opening
URLs (mostly HTTP) in a complex world --- basic and digest authentication,
redirections, cookies and more.

The :mod:`urllib2` module defines the following functions:


.. function:: urlopen(url[, data])

   Open the URL *url*, which can be either a string or a :class:`Request` object.

   *data* may be a string specifying additional data to send to the server, or
   ``None`` if no such data is needed.  Currently HTTP requests are the only ones
   that use *data*; the HTTP request will be a POST instead of a GET when the
   *data* parameter is provided.  *data* should be a buffer in the standard
   :mimetype:`application/x-www-form-urlencoded` format.  The
   :func:`urllib.urlencode` function takes a mapping or sequence of 2-tuples and
   returns a string in this format.

   This function returns a file-like object with two additional methods:

* :meth:`geturl` --- return the URL of the resource retrieved

* :meth:`info` --- return the meta-information of the page, as a dictionary-like
     object

   Raises :exc:`URLError` on errors.

   Note that ``None`` may be returned if no handler handles the request (though the
   default installed global :class:`OpenerDirector` uses :class:`UnknownHandler` to
   ensure this never happens).


.. function:: install_opener(opener)

   Install an :class:`OpenerDirector` instance as the default global opener.
   Installing an opener is only necessary if you want urlopen to use that opener;
   otherwise, simply call :meth:`OpenerDirector.open` instead of :func:`urlopen`.
   The code does not check for a real :class:`OpenerDirector`, and any class with
   the appropriate interface will work.


.. function:: build_opener([handler, ...])

   Return an :class:`OpenerDirector` instance, which chains the handlers in the
   order given. *handler*s can be either instances of :class:`BaseHandler`, or
   subclasses of :class:`BaseHandler` (in which case it must be possible to call
   the constructor without any parameters).  Instances of the following classes
   will be in front of the *handler*s, unless the *handler*s contain them,
   instances of them or subclasses of them: :class:`ProxyHandler`,
   :class:`UnknownHandler`, :class:`HTTPHandler`, :class:`HTTPDefaultErrorHandler`,
   :class:`HTTPRedirectHandler`, :class:`FTPHandler`, :class:`FileHandler`,
   :class:`HTTPErrorProcessor`.

   If the Python installation has SSL support (:func:`socket.ssl` exists),
   :class:`HTTPSHandler` will also be added.

   Beginning in Python 2.3, a :class:`BaseHandler` subclass may also change its
   :attr:`handler_order` member variable to modify its position in the handlers
   list.

The following exceptions are raised as appropriate:


.. exception:: URLError

   The handlers raise this exception (or derived exceptions) when they run into a
   problem.  It is a subclass of :exc:`IOError`.


.. exception:: HTTPError

   A subclass of :exc:`URLError`, it can also function as a  non-exceptional file-
   like return value (the same thing that :func:`urlopen` returns).  This is useful
   when handling exotic HTTP errors, such as requests for authentication.


.. exception:: GopherError

   A subclass of :exc:`URLError`, this is the error raised by the Gopher handler.

The following classes are provided:


.. class:: Request(url[, data][, headers] [, origin_req_host][, unverifiable])

   This class is an abstraction of a URL request.

   *url* should be a string containing a valid URL.

   *data* may be a string specifying additional data to send to the server, or
   ``None`` if no such data is needed.  Currently HTTP requests are the only ones
   that use *data*; the HTTP request will be a POST instead of a GET when the
   *data* parameter is provided.  *data* should be a buffer in the standard
   :mimetype:`application/x-www-form-urlencoded` format.  The
   :func:`urllib.urlencode` function takes a mapping or sequence of 2-tuples and
   returns a string in this format.

   *headers* should be a dictionary, and will be treated as if :meth:`add_header`
   was called with each key and value as arguments.

   The final two arguments are only of interest for correct handling of third-party
   HTTP cookies:

   *origin_req_host* should be the request-host of the origin transaction, as
   defined by :rfc:`2965`.  It defaults to ``cookielib.request_host(self)``.  This
   is the host name or IP address of the original request that was initiated by the
   user.  For example, if the request is for an image in an HTML document, this
   should be the request-host of the request for the page containing the image.

   *unverifiable* should indicate whether the request is unverifiable, as defined
   by RFC 2965.  It defaults to False.  An unverifiable request is one whose URL
   the user did not have the option to approve.  For example, if the request is for
   an image in an HTML document, and the user had no option to approve the
   automatic fetching of the image, this should be true.


.. class:: OpenerDirector()

   The :class:`OpenerDirector` class opens URLs via :class:`BaseHandler`\ s chained
   together. It manages the chaining of handlers, and recovery from errors.


.. class:: BaseHandler()

   This is the base class for all registered handlers --- and handles only the
   simple mechanics of registration.


.. class:: HTTPDefaultErrorHandler()

   A class which defines a default handler for HTTP error responses; all responses
   are turned into :exc:`HTTPError` exceptions.


.. class:: HTTPRedirectHandler()

   A class to handle redirections.


.. class:: HTTPCookieProcessor([cookiejar])

   A class to handle HTTP Cookies.


.. class:: ProxyHandler([proxies])

   Cause requests to go through a proxy. If *proxies* is given, it must be a
   dictionary mapping protocol names to URLs of proxies. The default is to read the
   list of proxies from the environment variables :envvar:`<protocol>_proxy`.


.. class:: HTTPPasswordMgr()

   Keep a database of  ``(realm, uri) -> (user, password)`` mappings.


.. class:: HTTPPasswordMgrWithDefaultRealm()

   Keep a database of  ``(realm, uri) -> (user, password)`` mappings. A realm of
   ``None`` is considered a catch-all realm, which is searched if no other realm
   fits.


.. class:: AbstractBasicAuthHandler([password_mgr])

   This is a mixin class that helps with HTTP authentication, both to the remote
   host and to a proxy. *password_mgr*, if given, should be something that is
   compatible with :class:`HTTPPasswordMgr`; refer to section
   :ref:`http-password-mgr` for information on the interface that must be
   supported.


.. class:: HTTPBasicAuthHandler([password_mgr])

   Handle authentication with the remote host. *password_mgr*, if given, should be
   something that is compatible with :class:`HTTPPasswordMgr`; refer to section
   :ref:`http-password-mgr` for information on the interface that must be
   supported.


.. class:: ProxyBasicAuthHandler([password_mgr])

   Handle authentication with the proxy. *password_mgr*, if given, should be
   something that is compatible with :class:`HTTPPasswordMgr`; refer to section
   :ref:`http-password-mgr` for information on the interface that must be
   supported.


.. class:: AbstractDigestAuthHandler([password_mgr])

   This is a mixin class that helps with HTTP authentication, both to the remote
   host and to a proxy. *password_mgr*, if given, should be something that is
   compatible with :class:`HTTPPasswordMgr`; refer to section
   :ref:`http-password-mgr` for information on the interface that must be
   supported.


.. class:: HTTPDigestAuthHandler([password_mgr])

   Handle authentication with the remote host. *password_mgr*, if given, should be
   something that is compatible with :class:`HTTPPasswordMgr`; refer to section
   :ref:`http-password-mgr` for information on the interface that must be
   supported.


.. class:: ProxyDigestAuthHandler([password_mgr])

   Handle authentication with the proxy. *password_mgr*, if given, should be
   something that is compatible with :class:`HTTPPasswordMgr`; refer to section
   :ref:`http-password-mgr` for information on the interface that must be
   supported.


.. class:: HTTPHandler()

   A class to handle opening of HTTP URLs.


.. class:: HTTPSHandler()

   A class to handle opening of HTTPS URLs.


.. class:: FileHandler()

   Open local files.


.. class:: FTPHandler()

   Open FTP URLs.


.. class:: CacheFTPHandler()

   Open FTP URLs, keeping a cache of open FTP connections to minimize delays.


.. class:: GopherHandler()

   Open gopher URLs.


.. class:: UnknownHandler()

   A catch-all class to handle unknown URLs.


.. _request-objects:

Request Objects
---------------

The following methods describe all of :class:`Request`'s public interface, and
so all must be overridden in subclasses.


.. method:: Request.add_data(data)

   Set the :class:`Request` data to *data*.  This is ignored by all handlers except
   HTTP handlers --- and there it should be a byte string, and will change the
   request to be ``POST`` rather than ``GET``.


.. method:: Request.get_method()

   Return a string indicating the HTTP request method.  This is only meaningful for
   HTTP requests, and currently always returns ``'GET'`` or ``'POST'``.


.. method:: Request.has_data()

   Return whether the instance has a non-\ ``None`` data.


.. method:: Request.get_data()

   Return the instance's data.


.. method:: Request.add_header(key, val)

   Add another header to the request.  Headers are currently ignored by all
   handlers except HTTP handlers, where they are added to the list of headers sent
   to the server.  Note that there cannot be more than one header with the same
   name, and later calls will overwrite previous calls in case the *key* collides.
   Currently, this is no loss of HTTP functionality, since all headers which have
   meaning when used more than once have a (header-specific) way of gaining the
   same functionality using only one header.


.. method:: Request.add_unredirected_header(key, header)

   Add a header that will not be added to a redirected request.

   .. versionadded:: 2.4


.. method:: Request.has_header(header)

   Return whether the instance has the named header (checks both regular and
   unredirected).

   .. versionadded:: 2.4


.. method:: Request.get_full_url()

   Return the URL given in the constructor.


.. method:: Request.get_type()

   Return the type of the URL --- also known as the scheme.


.. method:: Request.get_host()

   Return the host to which a connection will be made.


.. method:: Request.get_selector()

   Return the selector --- the part of the URL that is sent to the server.


.. method:: Request.set_proxy(host, type)

   Prepare the request by connecting to a proxy server. The *host* and *type* will
   replace those of the instance, and the instance's selector will be the original
   URL given in the constructor.


.. method:: Request.get_origin_req_host()

   Return the request-host of the origin transaction, as defined by :rfc:`2965`.
   See the documentation for the :class:`Request` constructor.


.. method:: Request.is_unverifiable()

   Return whether the request is unverifiable, as defined by RFC 2965. See the
   documentation for the :class:`Request` constructor.


.. _opener-director-objects:

OpenerDirector Objects
----------------------

:class:`OpenerDirector` instances have the following methods:


.. method:: OpenerDirector.add_handler(handler)

   *handler* should be an instance of :class:`BaseHandler`.  The following methods
   are searched, and added to the possible chains (note that HTTP errors are a
   special case).

* :meth:`protocol_open` --- signal that the handler knows how to open *protocol*
     URLs.

* :meth:`http_error_type` --- signal that the handler knows how to handle HTTP
     errors with HTTP error code *type*.

* :meth:`protocol_error` --- signal that the handler knows how to handle errors
     from (non-\ ``http``) *protocol*.

* :meth:`protocol_request` --- signal that the handler knows how to pre-process
     *protocol* requests.

* :meth:`protocol_response` --- signal that the handler knows how to post-
     process *protocol* responses.


.. method:: OpenerDirector.open(url[, data])

   Open the given *url* (which can be a request object or a string), optionally
   passing the given *data*. Arguments, return values and exceptions raised are the
   same as those of :func:`urlopen` (which simply calls the :meth:`open` method on
   the currently installed global :class:`OpenerDirector`).


.. method:: OpenerDirector.error(proto[, arg[, ...]])

   Handle an error of the given protocol.  This will call the registered error
   handlers for the given protocol with the given arguments (which are protocol
   specific).  The HTTP protocol is a special case which uses the HTTP response
   code to determine the specific error handler; refer to the :meth:`http_error_\*`
   methods of the handler classes.

   Return values and exceptions raised are the same as those of :func:`urlopen`.

OpenerDirector objects open URLs in three stages:

The order in which these methods are called within each stage is determined by
sorting the handler instances.

#. Every handler with a method named like :meth:`protocol_request` has that
   method called to pre-process the request.

#. Handlers with a method named like :meth:`protocol_open` are called to handle
   the request. This stage ends when a handler either returns a non-\ :const:`None`
   value (ie. a response), or raises an exception (usually :exc:`URLError`).
   Exceptions are allowed to propagate.

   In fact, the above algorithm is first tried for methods named
   :meth:`default_open`.  If all such methods return :const:`None`, the algorithm
   is repeated for methods named like :meth:`protocol_open`.  If all such methods
   return :const:`None`, the algorithm is repeated for methods named
   :meth:`unknown_open`.

   Note that the implementation of these methods may involve calls of the parent
   :class:`OpenerDirector` instance's :meth:`.open` and :meth:`.error` methods.

#. Every handler with a method named like :meth:`protocol_response` has that
   method called to post-process the response.


.. _base-handler-objects:

BaseHandler Objects
-------------------

:class:`BaseHandler` objects provide a couple of methods that are directly
useful, and others that are meant to be used by derived classes.  These are
intended for direct use:


.. method:: BaseHandler.add_parent(director)

   Add a director as parent.


.. method:: BaseHandler.close()

   Remove any parents.

The following members and methods should only be used by classes derived from
:class:`BaseHandler`.

.. note::

   The convention has been adopted that subclasses defining
   :meth:`protocol_request` or :meth:`protocol_response` methods are named
   :class:`\*Processor`; all others are named :class:`\*Handler`.


.. attribute:: BaseHandler.parent

   A valid :class:`OpenerDirector`, which can be used to open using a different
   protocol, or handle errors.


.. method:: BaseHandler.default_open(req)

   This method is *not* defined in :class:`BaseHandler`, but subclasses should
   define it if they want to catch all URLs.

   This method, if implemented, will be called by the parent
   :class:`OpenerDirector`.  It should return a file-like object as described in
   the return value of the :meth:`open` of :class:`OpenerDirector`, or ``None``.
   It should raise :exc:`URLError`, unless a truly exceptional thing happens (for
   example, :exc:`MemoryError` should not be mapped to :exc:`URLError`).

   This method will be called before any protocol-specific open method.


.. method:: BaseHandler.protocol_open(req)
   :noindex:

   This method is *not* defined in :class:`BaseHandler`, but subclasses should
   define it if they want to handle URLs with the given protocol.

   This method, if defined, will be called by the parent :class:`OpenerDirector`.
   Return values should be the same as for  :meth:`default_open`.


.. method:: BaseHandler.unknown_open(req)

   This method is *not* defined in :class:`BaseHandler`, but subclasses should
   define it if they want to catch all URLs with no specific registered handler to
   open it.

   This method, if implemented, will be called by the :attr:`parent`
   :class:`OpenerDirector`.  Return values should be the same as for
   :meth:`default_open`.


.. method:: BaseHandler.http_error_default(req, fp, code, msg, hdrs)

   This method is *not* defined in :class:`BaseHandler`, but subclasses should
   override it if they intend to provide a catch-all for otherwise unhandled HTTP
   errors.  It will be called automatically by the  :class:`OpenerDirector` getting
   the error, and should not normally be called in other circumstances.

   *req* will be a :class:`Request` object, *fp* will be a file-like object with
   the HTTP error body, *code* will be the three-digit code of the error, *msg*
   will be the user-visible explanation of the code and *hdrs* will be a mapping
   object with the headers of the error.

   Return values and exceptions raised should be the same as those of
   :func:`urlopen`.


.. method:: BaseHandler.http_error_nnn(req, fp, code, msg, hdrs)

   *nnn* should be a three-digit HTTP error code.  This method is also not defined
   in :class:`BaseHandler`, but will be called, if it exists, on an instance of a
   subclass, when an HTTP error with code *nnn* occurs.

   Subclasses should override this method to handle specific HTTP errors.

   Arguments, return values and exceptions raised should be the same as for
   :meth:`http_error_default`.


.. method:: BaseHandler.protocol_request(req)
   :noindex:

   This method is *not* defined in :class:`BaseHandler`, but subclasses should
   define it if they want to pre-process requests of the given protocol.

   This method, if defined, will be called by the parent :class:`OpenerDirector`.
   *req* will be a :class:`Request` object. The return value should be a
   :class:`Request` object.


.. method:: BaseHandler.protocol_response(req, response)
   :noindex:

   This method is *not* defined in :class:`BaseHandler`, but subclasses should
   define it if they want to post-process responses of the given protocol.

   This method, if defined, will be called by the parent :class:`OpenerDirector`.
   *req* will be a :class:`Request` object. *response* will be an object
   implementing the same interface as the return value of :func:`urlopen`.  The
   return value should implement the same interface as the return value of
   :func:`urlopen`.


.. _http-redirect-handler:

HTTPRedirectHandler Objects
---------------------------

.. note::

   Some HTTP redirections require action from this module's client code.  If this
   is the case, :exc:`HTTPError` is raised.  See :rfc:`2616` for details of the
   precise meanings of the various redirection codes.


.. method:: HTTPRedirectHandler.redirect_request(req, fp, code, msg, hdrs)

   Return a :class:`Request` or ``None`` in response to a redirect. This is called
   by the default implementations of the :meth:`http_error_30\*` methods when a
   redirection is received from the server.  If a redirection should take place,
   return a new :class:`Request` to allow :meth:`http_error_30\*` to perform the
   redirect.  Otherwise, raise :exc:`HTTPError` if no other handler should try to
   handle this URL, or return ``None`` if you can't but another handler might.

   .. note::

      The default implementation of this method does not strictly follow :rfc:`2616`,
      which says that 301 and 302 responses to ``POST`` requests must not be
      automatically redirected without confirmation by the user.  In reality, browsers
      do allow automatic redirection of these responses, changing the POST to a
      ``GET``, and the default implementation reproduces this behavior.


.. method:: HTTPRedirectHandler.http_error_301(req, fp, code, msg, hdrs)

   Redirect to the ``Location:`` URL.  This method is called by the parent
   :class:`OpenerDirector` when getting an HTTP 'moved permanently' response.


.. method:: HTTPRedirectHandler.http_error_302(req, fp, code, msg, hdrs)

   The same as :meth:`http_error_301`, but called for the 'found' response.


.. method:: HTTPRedirectHandler.http_error_303(req, fp, code, msg, hdrs)

   The same as :meth:`http_error_301`, but called for the 'see other' response.


.. method:: HTTPRedirectHandler.http_error_307(req, fp, code, msg, hdrs)

   The same as :meth:`http_error_301`, but called for the 'temporary redirect'
   response.


.. _http-cookie-processor:

HTTPCookieProcessor Objects
---------------------------

.. versionadded:: 2.4

:class:`HTTPCookieProcessor` instances have one attribute:


.. attribute:: XXX Class.cookiejar

   The :class:`cookielib.CookieJar` in which cookies are stored.


.. _proxy-handler:

ProxyHandler Objects
--------------------


.. method:: ProxyHandler.protocol_open(request)
   :noindex:

   The :class:`ProxyHandler` will have a method :meth:`protocol_open` for every
   *protocol* which has a proxy in the *proxies* dictionary given in the
   constructor.  The method will modify requests to go through the proxy, by
   calling ``request.set_proxy()``, and call the next handler in the chain to
   actually execute the protocol.


.. _http-password-mgr:

HTTPPasswordMgr Objects
-----------------------

These methods are available on :class:`HTTPPasswordMgr` and
:class:`HTTPPasswordMgrWithDefaultRealm` objects.


.. method:: HTTPPasswordMgr.add_password(realm, uri, user, passwd)

   *uri* can be either a single URI, or a sequence of URIs. *realm*, *user* and
   *passwd* must be strings. This causes ``(user, passwd)`` to be used as
   authentication tokens when authentication for *realm* and a super-URI of any of
   the given URIs is given.


.. method:: HTTPPasswordMgr.find_user_password(realm, authuri)

   Get user/password for given realm and URI, if any.  This method will return
   ``(None, None)`` if there is no matching user/password.

   For :class:`HTTPPasswordMgrWithDefaultRealm` objects, the realm ``None`` will be
   searched if the given *realm* has no matching user/password.


.. _abstract-basic-auth-handler:

AbstractBasicAuthHandler Objects
--------------------------------


.. method:: AbstractBasicAuthHandler.http_error_auth_reqed(authreq, host, req, headers)

   Handle an authentication request by getting a user/password pair, and re-trying
   the request.  *authreq* should be the name of the header where the information
   about the realm is included in the request, *host* specifies the URL and path to
   authenticate for, *req* should be the (failed) :class:`Request` object, and
   *headers* should be the error headers.

   *host* is either an authority (e.g. ``"python.org"``) or a URL containing an
   authority component (e.g. ``"http://python.org/"``). In either case, the
   authority must not contain a userinfo component (so, ``"python.org"`` and
   ``"python.org:80"`` are fine, ``"joe:password@python.org"`` is not).


.. _http-basic-auth-handler:

HTTPBasicAuthHandler Objects
----------------------------


.. method:: HTTPBasicAuthHandler.http_error_401(req, fp, code,  msg, hdrs)

   Retry the request with authentication information, if available.


.. _proxy-basic-auth-handler:

ProxyBasicAuthHandler Objects
-----------------------------


.. method:: ProxyBasicAuthHandler.http_error_407(req, fp, code,  msg, hdrs)

   Retry the request with authentication information, if available.


.. _abstract-digest-auth-handler:

AbstractDigestAuthHandler Objects
---------------------------------


.. method:: AbstractDigestAuthHandler.http_error_auth_reqed(authreq, host, req, headers)

   *authreq* should be the name of the header where the information about the realm
   is included in the request, *host* should be the host to authenticate to, *req*
   should be the (failed) :class:`Request` object, and *headers* should be the
   error headers.


.. _http-digest-auth-handler:

HTTPDigestAuthHandler Objects
-----------------------------


.. method:: HTTPDigestAuthHandler.http_error_401(req, fp, code,  msg, hdrs)

   Retry the request with authentication information, if available.


.. _proxy-digest-auth-handler:

ProxyDigestAuthHandler Objects
------------------------------


.. method:: ProxyDigestAuthHandler.http_error_407(req, fp, code,  msg, hdrs)

   Retry the request with authentication information, if available.


.. _http-handler-objects:

HTTPHandler Objects
-------------------


.. method:: HTTPHandler.http_open(req)

   Send an HTTP request, which can be either GET or POST, depending on
   ``req.has_data()``.


.. _https-handler-objects:

HTTPSHandler Objects
--------------------


.. method:: HTTPSHandler.https_open(req)

   Send an HTTPS request, which can be either GET or POST, depending on
   ``req.has_data()``.


.. _file-handler-objects:

FileHandler Objects
-------------------


.. method:: FileHandler.file_open(req)

   Open the file locally, if there is no host name, or the host name is
   ``'localhost'``. Change the protocol to ``ftp`` otherwise, and retry opening it
   using :attr:`parent`.


.. _ftp-handler-objects:

FTPHandler Objects
------------------


.. method:: FTPHandler.ftp_open(req)

   Open the FTP file indicated by *req*. The login is always done with empty
   username and password.


.. _cacheftp-handler-objects:

CacheFTPHandler Objects
-----------------------

:class:`CacheFTPHandler` objects are :class:`FTPHandler` objects with the
following additional methods:


.. method:: CacheFTPHandler.setTimeout(t)

   Set timeout of connections to *t* seconds.


.. method:: CacheFTPHandler.setMaxConns(m)

   Set maximum number of cached connections to *m*.


.. _gopher-handler:

GopherHandler Objects
---------------------


.. method:: GopherHandler.gopher_open(req)

   Open the gopher resource indicated by *req*.


.. _unknown-handler-objects:

UnknownHandler Objects
----------------------


.. method:: UnknownHandler.unknown_open()

   Raise a :exc:`URLError` exception.


.. _http-error-processor-objects:

HTTPErrorProcessor Objects
--------------------------

.. versionadded:: 2.4


.. method:: HTTPErrorProcessor.unknown_open()

   Process HTTP error responses.

   For 200 error codes, the response object is returned immediately.

   For non-200 error codes, this simply passes the job on to the
   :meth:`protocol_error_code` handler methods, via :meth:`OpenerDirector.error`.
   Eventually, :class:`urllib2.HTTPDefaultErrorHandler` will raise an
   :exc:`HTTPError` if no other handler handles the error.


.. _urllib2-examples:

Examples
--------

This example gets the python.org main page and displays the first 100 bytes of
it::

   >>> import urllib2
   >>> f = urllib2.urlopen('http://www.python.org/')
   >>> print f.read(100)
   <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
   <?xml-stylesheet href="./css/ht2html

Here we are sending a data-stream to the stdin of a CGI and reading the data it
returns to us. Note that this example will only work when the Python
installation supports SSL. ::

   >>> import urllib2
   >>> req = urllib2.Request(url='https://localhost/cgi-bin/test.cgi',
   ...                       data='This data is passed to stdin of the CGI')
   >>> f = urllib2.urlopen(req)
   >>> print f.read()
   Got Data: "This data is passed to stdin of the CGI"

The code for the sample CGI used in the above example is::

   #!/usr/bin/env python
   import sys
   data = sys.stdin.read()
   print 'Content-type: text-plain\n\nGot Data: "%s"' % data

Use of Basic HTTP Authentication::

   import urllib2
   # Create an OpenerDirector with support for Basic HTTP Authentication...
   auth_handler = urllib2.HTTPBasicAuthHandler()
   auth_handler.add_password('realm', 'host', 'username', 'password')
   opener = urllib2.build_opener(auth_handler)
   # ...and install it globally so it can be used with urlopen.
   urllib2.install_opener(opener)
   urllib2.urlopen('http://www.example.com/login.html')

:func:`build_opener` provides many handlers by default, including a
:class:`ProxyHandler`.  By default, :class:`ProxyHandler` uses the environment
variables named ``<scheme>_proxy``, where ``<scheme>`` is the URL scheme
involved.  For example, the :envvar:`http_proxy` environment variable is read to
obtain the HTTP proxy's URL.

This example replaces the default :class:`ProxyHandler` with one that uses
programatically-supplied proxy URLs, and adds proxy authorization support with
:class:`ProxyBasicAuthHandler`. ::

   proxy_handler = urllib2.ProxyHandler({'http': 'http://www.example.com:3128/'})
   proxy_auth_handler = urllib2.HTTPBasicAuthHandler()
   proxy_auth_handler.add_password('realm', 'host', 'username', 'password')

   opener = build_opener(proxy_handler, proxy_auth_handler)
   # This time, rather than install the OpenerDirector, we use it directly:
   opener.open('http://www.example.com/login.html')

Adding HTTP headers:

Use the *headers* argument to the :class:`Request` constructor, or::

   import urllib2
   req = urllib2.Request('http://www.example.com/')
   req.add_header('Referer', 'http://www.python.org/')
   r = urllib2.urlopen(req)

:class:`OpenerDirector` automatically adds a :mailheader:`User-Agent` header to
every :class:`Request`.  To change this::

   import urllib2
   opener = urllib2.build_opener()
   opener.addheaders = [('User-agent', 'Mozilla/5.0')]
   opener.open('http://www.example.com/')

Also, remember that a few standard headers (:mailheader:`Content-Length`,
:mailheader:`Content-Type` and :mailheader:`Host`) are added when the
:class:`Request` is passed to :func:`urlopen` (or :meth:`OpenerDirector.open`).

