.. 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=None)

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=None)

Overridable interface to open unknown URL types.

.. method:: retrieve(url, filename=None, reporthook=None, data=None)

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

is a tuple consisting of a local filename and either an

:class:`email.message.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: A chunk number, the maximum size chunks are read in and the total size of the download

(-1 if unknown). It will be called once at the start and 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:`urllib.parse.urlencode` function.

.. 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.