.. note::

If the environment variable ``REQUEST_METHOD`` is set, which usually

indicates your script is running in a CGI environment, the environment

variable ``HTTP_PROXY`` (uppercase ``_PROXY``) will be ignored. This is

because that variable can be injected by a client using the "Proxy:"

HTTP header. If you need to use an HTTP proxy in a CGI environment,

either use ``ProxyHandler`` explicitly, or make sure the variable name

is in lowercase (or at least the ``_proxy`` suffix).

.. method:: open(fullurl[, data])

Open *fullurl* using the appropriate protocol. This method sets up cache and

proxy information, then calls the appropriate open method with its input

arguments. If the scheme is not recognized, :meth:`open_unknown` is called.

The *data* argument has the same meaning as the *data* argument of

:func:`urlopen`.

.. method:: open_unknown(fullurl[, data])

Overridable interface to open unknown URL types.

.. method:: retrieve(url[, filename[, reporthook[, data]]])

Retrieves the contents of *url* and places it in *filename*. The return value

is a tuple consisting of a local filename and either a

:class:`mimetools.Message` object containing the response headers (for remote

URLs) or ``None`` (for local URLs). The caller must then open and read the

contents of *filename*. If *filename* is not given and the URL refers to a

local file, the input filename is returned. If the URL is non-local and

*filename* is not given, the filename is the output of :func:`tempfile.mktemp`

with a suffix that matches the suffix of the last path component of the input

URL. If *reporthook* is given, it must be a function accepting three numeric

parameters. It will be called after each chunk of data is read from the

network. *reporthook* is ignored for local URLs.

If the *url* uses the :file:`http:` scheme identifier, the optional *data*

argument may be given to specify a ``POST`` request (normally the request type

is ``GET``). The *data* argument must in standard

:mimetype:`application/x-www-form-urlencoded` format; see the :func:`urlencode`

function below.

.. attribute:: version

Variable that specifies the user agent of the opener object. To get

:mod:`urllib` to tell servers that it is a particular user agent, set this in a

subclass as a class variable or in the constructor before calling the base

constructor.