xref: /external/python/cpython3/Doc/library/urllib.request.rst

:mod:`urllib.request` --- Extensible library for opening URLs
=============================================================

.. module:: urllib.request
   :synopsis: Extensible library for opening URLs.

.. moduleauthor:: Jeremy Hylton <jeremy (a] alum.mit.edu>
.. sectionauthor:: Moshe Zadka <moshez (a] users.sourceforge.net>
.. sectionauthor:: Senthil Kumaran <senthil (a] uthcode.com>

**Source code:** :source:`Lib/urllib/request.py`

--------------

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

.. seealso::

    The `Requests package <http://docs.python-requests.org/>`_
    is recommended for a higher-level HTTP client interface.


The :mod:`urllib.request` module defines the following functions:


.. function:: urlopen(url, data=None[, timeout], *, cafile=None, capath=None, cadefault=False, context=None)

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

   *data* must be an object specifying additional data to be sent to the
   server, or ``None`` if no such data is needed.  See :class:`Request`
   for details.

   urllib.request module uses HTTP/1.1 and includes ``Connection:close`` header
   in its HTTP requests.

   The optional *timeout* parameter specifies a timeout in seconds for
   blocking operations like the connection attempt (if not specified,
   the global default timeout setting will be used).  This actually
   only works for HTTP, HTTPS and FTP connections.

   If *context* is specified, it must be a :class:`ssl.SSLContext` instance
   describing the various SSL options. See :class:`~http.client.HTTPSConnection`
   for more details.

   The optional *cafile* and *capath* parameters specify a set of trusted
   CA certificates for HTTPS requests.  *cafile* should point to a single
   file containing a bundle of CA certificates, whereas *capath* should
   point to a directory of hashed certificate files.  More information can
   be found in :meth:`ssl.SSLContext.load_verify_locations`.

   The *cadefault* parameter is ignored.

   This function always returns an object which can work as a
   :term:`context manager` and has methods such as

   * :meth:`~urllib.response.addinfourl.geturl` --- return the URL of the resource retrieved,
     commonly used to determine if a redirect was followed

   * :meth:`~urllib.response.addinfourl.info` --- return the meta-information of the page, such as headers,
     in the form of an :func:`email.message_from_string` instance (see
     `Quick Reference to HTTP Headers <http://jkorpela.fi/http.html>`_)

   * :meth:`~urllib.response.addinfourl.getcode` -- return the HTTP status code of the response.

   For HTTP and HTTPS URLs, this function returns a
   :class:`http.client.HTTPResponse` object slightly modified. In addition
   to the three new methods above, the msg attribute contains the
   same information as the :attr:`~http.client.HTTPResponse.reason`
   attribute --- the reason phrase returned by server --- instead of
   the response headers as it is specified in the documentation for
   :class:`~http.client.HTTPResponse`.

   For FTP, file, and data URLs and requests explicitly handled by legacy
   :class:`URLopener` and :class:`FancyURLopener` classes, this function
   returns a :class:`urllib.response.addinfourl` object.

   Raises :exc:`~urllib.error.URLError` on protocol 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).

   In addition, if proxy settings are detected (for example, when a ``*_proxy``
   environment variable like :envvar:`http_proxy` is set),
   :class:`ProxyHandler` is default installed and makes sure the requests are
   handled through the proxy.

   The legacy ``urllib.urlopen`` function from Python 2.6 and earlier has been
   discontinued; :func:`urllib.request.urlopen` corresponds to the old
   ``urllib2.urlopen``.  Proxy handling, which was done by passing a dictionary
   parameter to ``urllib.urlopen``, can be obtained by using
   :class:`ProxyHandler` objects.

   .. versionchanged:: 3.2
      *cafile* and *capath* were added.

   .. versionchanged:: 3.2
      HTTPS virtual hosts are now supported if possible (that is, if
      :data:`ssl.HAS_SNI` is true).

   .. versionadded:: 3.2
      *data* can be an iterable object.

   .. versionchanged:: 3.3
      *cadefault* was added.

   .. versionchanged:: 3.4.3
      *context* was added.

   .. deprecated:: 3.6

       *cafile*, *capath* and *cadefault* are deprecated in favor of *context*.
       Please use :meth:`ssl.SSLContext.load_cert_chain` instead, or let
       :func:`ssl.create_default_context` select the system's trusted CA
       certificates for you.

.. 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:`~urllib.request.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` (if proxy
   settings are detected), :class:`UnknownHandler`, :class:`HTTPHandler`,
   :class:`HTTPDefaultErrorHandler`, :class:`HTTPRedirectHandler`,
   :class:`FTPHandler`, :class:`FileHandler`, :class:`HTTPErrorProcessor`.

   If the Python installation has SSL support (i.e., if the :mod:`ssl` module
   can be imported), :class:`HTTPSHandler` will also be added.

   A :class:`BaseHandler` subclass may also change its :attr:`handler_order`
   attribute to modify its position in the handlers list.


.. function:: pathname2url(path)

   Convert the pathname *path* from the local syntax for a path to the form used in
   the path component of a URL.  This does not produce a complete URL.  The return
   value will already be quoted using the :func:`~urllib.parse.quote` function.


.. function:: url2pathname(path)

   Convert the path component *path* from a percent-encoded URL to the local syntax for a
   path.  This does not accept a complete URL.  This function uses
   :func:`~urllib.parse.unquote` to decode *path*.

.. function:: getproxies()

   This helper function returns a dictionary of scheme to proxy server URL
   mappings. It scans the environment for variables named ``<scheme>_proxy``,
   in a case insensitive approach, for all operating systems first, and when it
   cannot find it, looks for proxy information from Mac OSX System
   Configuration for Mac OS X and Windows Systems Registry for Windows.
   If both lowercase and uppercase environment variables exist (and disagree),
   lowercase is preferred.

   .. 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).


The following classes are provided:

.. class:: Request(url, data=None, headers={}, origin_req_host=None, unverifiable=False, method=None)

   This class is an abstraction of a URL request.

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

   *data* must be an object 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 supported object
   types include bytes, file-like objects, and iterables.  If no
   ``Content-Length`` nor ``Transfer-Encoding`` header field
   has been provided, :class:`HTTPHandler` will set these headers according
   to the type of *data*.  ``Content-Length`` will be used to send
   bytes objects, while ``Transfer-Encoding: chunked`` as specified in
   :rfc:`7230`, Section 3.3.1 will be used to send files and other iterables.

   For an HTTP POST request method, *data* should be a buffer in the
   standard :mimetype:`application/x-www-form-urlencoded` format.  The
   :func:`urllib.parse.urlencode` function takes a mapping or sequence
   of 2-tuples and returns an ASCII string in this format. It should
   be encoded to bytes before being used as the *data* parameter.

   *headers* should be a dictionary, and will be treated as if
   :meth:`add_header` was called with each key and value as arguments.
   This is often used to "spoof" the ``User-Agent`` header value, which is
   used by a browser to identify itself -- some HTTP servers only
   allow requests coming from common browsers as opposed to scripts.
   For example, Mozilla Firefox may identify itself as ``"Mozilla/5.0
   (X11; U; Linux i686) Gecko/20071127 Firefox/2.0.0.11"``, while
   :mod:`urllib`'s default user agent string is
   ``"Python-urllib/2.6"`` (on Python 2.6).

   An appropriate ``Content-Type`` header should be included if the *data*
   argument is present.  If this header has not been provided and *data*
   is not None, ``Content-Type: application/x-www-form-urlencoded`` will
   be added as a default.

   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
   ``http.cookiejar.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.

   *method* should be a string that indicates the HTTP request method that
   will be used (e.g. ``'HEAD'``).  If provided, its value is stored in the
   :attr:`~Request.method` attribute and is used by :meth:`get_method()`.
   The default is ``'GET'`` if *data* is ``None`` or ``'POST'`` otherwise.
   Subclasses may indicate a different default method by setting the
   :attr:`~Request.method` attribute in the class itself.

   .. note::
      The request will not work as expected if the data object is unable
      to deliver its content more than once (e.g. a file or an iterable
      that can produce the content only once) and the request is retried
      for HTTP redirects or authentication.  The *data* is sent to the
      HTTP server right away after the headers.  There is no support for
      a 100-continue expectation in the library.

   .. versionchanged:: 3.3
      :attr:`Request.method` argument is added to the Request class.

   .. versionchanged:: 3.4
      Default :attr:`Request.method` may be indicated at the class level.

   .. versionchanged:: 3.6
      Do not raise an error if the ``Content-Length`` has not been
      provided and *data* is neither ``None`` nor a bytes object.
      Fall back to use chunked transfer encoding instead.

.. 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:`~urllib.error.HTTPError` exceptions.


.. class:: HTTPRedirectHandler()

   A class to handle redirections.


.. class:: HTTPCookieProcessor(cookiejar=None)

   A class to handle HTTP Cookies.


.. class:: ProxyHandler(proxies=None)

   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
   ``<protocol>_proxy``.  If no proxy environment variables are set, then
   in a Windows environment proxy settings are obtained from the registry's
   Internet Settings section, and in a Mac OS X environment proxy information
   is retrieved from the OS X System Configuration Framework.

   To disable autodetected proxy pass an empty dictionary.

   The :envvar:`no_proxy` environment variable can be used to specify hosts
   which shouldn't be reached via proxy; if set, it should be a comma-separated
   list of hostname suffixes, optionally with ``:port`` appended, for example
   ``cern.ch,ncsa.uiuc.edu,some.host:8080``.

    .. note::

       ``HTTP_PROXY`` will be ignored if a variable ``REQUEST_METHOD`` is set;
       see the documentation on :func:`~urllib.request.getproxies`.


.. 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:: HTTPPasswordMgrWithPriorAuth()

   A variant of :class:`HTTPPasswordMgrWithDefaultRealm` that also has a
   database of ``uri -> is_authenticated`` mappings.  Can be used by a
   BasicAuth handler to determine when to send authentication credentials
   immediately instead of waiting for a ``401`` response first.

   .. versionadded:: 3.5


.. class:: AbstractBasicAuthHandler(password_mgr=None)

   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.  If *passwd_mgr* also provides ``is_authenticated`` and
   ``update_authenticated`` methods (see
   :ref:`http-password-mgr-with-prior-auth`), then the handler will use the
   ``is_authenticated`` result for a given URI to determine whether or not to
   send authentication credentials with the request.  If ``is_authenticated``
   returns ``True`` for the URI, credentials are sent.  If ``is_authenticated``
   is ``False``, credentials are not sent, and then if a ``401`` response is
   received the request is re-sent with the authentication credentials.  If
   authentication succeeds, ``update_authenticated`` is called to set
   ``is_authenticated`` ``True`` for the URI, so that subsequent requests to
   the URI or any of its super-URIs will automatically include the
   authentication credentials.

   .. versionadded:: 3.5
      Added ``is_authenticated`` support.


.. class:: HTTPBasicAuthHandler(password_mgr=None)

   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. HTTPBasicAuthHandler will raise a :exc:`ValueError` when
   presented with a wrong Authentication scheme.


.. class:: ProxyBasicAuthHandler(password_mgr=None)

   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=None)

   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=None)

   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. When both Digest Authentication Handler and Basic
   Authentication Handler are both added, Digest Authentication is always tried
   first. If the Digest Authentication returns a 40x response again, it is sent
   to Basic Authentication handler to Handle.  This Handler method will raise a
   :exc:`ValueError` when presented with an authentication scheme other than
   Digest or Basic.

   .. versionchanged:: 3.3
      Raise :exc:`ValueError` on unsupported Authentication Scheme.



.. class:: ProxyDigestAuthHandler(password_mgr=None)

   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(debuglevel=0, context=None, check_hostname=None)

   A class to handle opening of HTTPS URLs.  *context* and *check_hostname*
   have the same meaning as in :class:`http.client.HTTPSConnection`.

   .. versionchanged:: 3.2
      *context* and *check_hostname* were added.


.. class:: FileHandler()

   Open local files.

.. class:: DataHandler()

   Open data URLs.

   .. versionadded:: 3.4

.. class:: FTPHandler()

   Open FTP URLs.


.. class:: CacheFTPHandler()

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


.. class:: UnknownHandler()

   A catch-all class to handle unknown URLs.


.. class:: HTTPErrorProcessor()

   Process HTTP error responses.


.. _request-objects:

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

The following methods describe :class:`Request`'s public interface,
and so all may be overridden in subclasses.  It also defines several
public attributes that can be used by clients to inspect the parsed
request.

.. attribute:: Request.full_url

   The original URL passed to the constructor.

   .. versionchanged:: 3.4

   Request.full_url is a property with setter, getter and a deleter. Getting
   :attr:`~Request.full_url` returns the original request URL with the
   fragment, if it was present.

.. attribute:: Request.type

   The URI scheme.

.. attribute:: Request.host

   The URI authority, typically a host, but may also contain a port
   separated by a colon.

.. attribute:: Request.origin_req_host

   The original host for the request, without port.

.. attribute:: Request.selector

   The URI path.  If the :class:`Request` uses a proxy, then selector
   will be the full URL that is passed to the proxy.

.. attribute:: Request.data

   The entity body for the request, or ``None`` if not specified.

   .. versionchanged:: 3.4
      Changing value of :attr:`Request.data` now deletes "Content-Length"
      header if it was previously set or calculated.

.. attribute:: Request.unverifiable

   boolean, indicates whether the request is unverifiable as defined
   by :rfc:`2965`.

.. attribute:: Request.method

   The HTTP request method to use.  By default its value is :const:`None`,
   which means that :meth:`~Request.get_method` will do its normal computation
   of the method to be used.  Its value can be set (thus overriding the default
   computation in :meth:`~Request.get_method`) either by providing a default
   value by setting it at the class level in a :class:`Request` subclass, or by
   passing a value in to the :class:`Request` constructor via the *method*
   argument.

   .. versionadded:: 3.3

   .. versionchanged:: 3.4
      A default value can now be set in subclasses; previously it could only
      be set via the constructor argument.


.. method:: Request.get_method()

   Return a string indicating the HTTP request method.  If
   :attr:`Request.method` is not ``None``, return its value, otherwise return
   ``'GET'`` if :attr:`Request.data` is ``None``, or ``'POST'`` if it's not.
   This is only meaningful for HTTP requests.

   .. versionchanged:: 3.3
      get_method now looks at the value of :attr:`Request.method`.


.. 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.


.. method:: Request.has_header(header)

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


.. method:: Request.remove_header(header)

   Remove named header from the request instance (both from regular and
   unredirected headers).

   .. versionadded:: 3.4


.. method:: Request.get_full_url()

   Return the URL given in the constructor.

   .. versionchanged:: 3.4

   Returns :attr:`Request.full_url`


.. 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_header(header_name, default=None)

   Return the value of the given header. If the header is not present, return
   the default value.


.. method:: Request.header_items()

   Return a list of tuples (header_name, header_value) of the Request headers.

.. versionchanged:: 3.4
   The request methods add_data, has_data, get_data, get_type, get_host,
   get_selector, get_origin_req_host and is_unverifiable that were deprecated
   since 3.3 have been removed.


.. _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=None[, timeout])

   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`).  The
   optional *timeout* parameter specifies a timeout in seconds for blocking
   operations like the connection attempt (if not specified, the global default
   timeout setting will be used). The timeout feature actually works only for
   HTTP, HTTPS and FTP connections).


.. method:: OpenerDirector.error(proto, *args)

   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:`~urllib.error.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:`~OpenerDirector.open` and
   :meth:`~OpenerDirector.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 attribute 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:`~urllib.error.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:`~urllib.error.HTTPError` is raised.  See :rfc:`2616` for
   details of the precise meanings of the various redirection codes.

   An :class:`HTTPError` exception raised as a security consideration if the
   HTTPRedirectHandler is presented with a redirected URL which is not an HTTP,
   HTTPS or FTP URL.


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

   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 to *newurl*.  Otherwise, raise :exc:`~urllib.error.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:`` or ``URI:`` 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
---------------------------

:class:`HTTPCookieProcessor` instances have one attribute:

.. attribute:: HTTPCookieProcessor.cookiejar

   The :class:`http.cookiejar.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.


.. _http-password-mgr-with-prior-auth:

HTTPPasswordMgrWithPriorAuth Objects
------------------------------------

This password manager extends :class:`HTTPPasswordMgrWithDefaultRealm` to support
tracking URIs for which authentication credentials should always be sent.


.. method:: HTTPPasswordMgrWithPriorAuth.add_password(realm, uri, user, \
            passwd, is_authenticated=False)

   *realm*, *uri*, *user*, *passwd* are as for
   :meth:`HTTPPasswordMgr.add_password`.  *is_authenticated* sets the initial
   value of the ``is_authenticated`` flag for the given URI or list of URIs.
   If *is_authenticated* is specified as ``True``, *realm* is ignored.


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

   Same as for :class:`HTTPPasswordMgrWithDefaultRealm` objects


.. method:: HTTPPasswordMgrWithPriorAuth.update_authenticated(self, uri, \
            is_authenticated=False)

   Update the ``is_authenticated`` flag for the given *uri* or list
   of URIs.


.. method:: HTTPPasswordMgrWithPriorAuth.is_authenticated(self, authuri)

   Returns the current state of the ``is_authenticated`` flag for
   the given URI.


.. _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 (a] 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'``.

   .. versionchanged:: 3.2
      This method is applicable only for local hostnames.  When a remote
      hostname is given, an :exc:`~urllib.error.URLError` is raised.


.. _data-handler-objects:

DataHandler Objects
-------------------

.. method:: DataHandler.data_open(req)

   Read a data URL. This kind of URL contains the content encoded in the URL
   itself. The data URL syntax is specified in :rfc:`2397`. This implementation
   ignores white spaces in base64 encoded data URLs so the URL may be wrapped
   in whatever source file it comes from. But even though some browsers don't
   mind about a missing padding at the end of a base64 encoded data URL, this
   implementation will raise an :exc:`ValueError` in that case.


.. _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*.


.. _unknown-handler-objects:

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


.. method:: UnknownHandler.unknown_open()

   Raise a :exc:`~urllib.error.URLError` exception.


.. _http-error-processor-objects:

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

.. method:: HTTPErrorProcessor.http_response(request, response)

   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:`HTTPDefaultErrorHandler` will raise an
   :exc:`~urllib.error.HTTPError` if no other handler handles the error.


.. method:: HTTPErrorProcessor.https_response(request, response)

   Process HTTPS error responses.

   The behavior is same as :meth:`http_response`.


.. _urllib-request-examples:

Examples
--------

In addition to the examples below, more examples are given in
:ref:`urllib-howto`.

This example gets the python.org main page and displays the first 300 bytes of
it. ::

   >>> import urllib.request
   >>> with urllib.request.urlopen('http://www.python.org/') as f:
   ...     print(f.read(300))
   ...
   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
   xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">\n\n<head>\n
   <meta http-equiv="content-type" content="text/html; charset=utf-8" />\n
   <title>Python Programming '

Note that urlopen returns a bytes object.  This is because there is no way
for urlopen to automatically determine the encoding of the byte stream
it receives from the HTTP server. In general, a program will decode
the returned bytes object to string once it determines or guesses
the appropriate encoding.

The following W3C document, https://www.w3.org/International/O-charset\ , lists
the various ways in which an (X)HTML or an XML document could have specified its
encoding information.

As the python.org website uses *utf-8* encoding as specified in its meta tag, we
will use the same for decoding the bytes object. ::

   >>> with urllib.request.urlopen('http://www.python.org/') as f:
   ...     print(f.read(100).decode('utf-8'))
   ...
   <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
   "http://www.w3.org/TR/xhtml1/DTD/xhtm

It is also possible to achieve the same result without using the
:term:`context manager` approach. ::

   >>> import urllib.request
   >>> f = urllib.request.urlopen('http://www.python.org/')
   >>> print(f.read(100).decode('utf-8'))
   <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
   "http://www.w3.org/TR/xhtml1/DTD/xhtm

In the following example, 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 urllib.request
   >>> req = urllib.request.Request(url='https://localhost/cgi-bin/test.cgi',
   ...                       data=b'This data is passed to stdin of the CGI')
   >>> with urllib.request.urlopen(req) as f:
   ...     print(f.read().decode('utf-8'))
   ...
   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)

Here is an example of doing a ``PUT`` request using :class:`Request`::

    import urllib.request
    DATA = b'some data'
    req = urllib.request.Request(url='http://localhost:8080', data=DATA,method='PUT')
    with urllib.request.urlopen(req) as f:
        pass
    print(f.status)
    print(f.reason)

Use of Basic HTTP Authentication::

   import urllib.request
   # Create an OpenerDirector with support for Basic HTTP Authentication...
   auth_handler = urllib.request.HTTPBasicAuthHandler()
   auth_handler.add_password(realm='PDQ Application',
                             uri='https://mahler:8092/site-updates.py',
                             user='klem',
                             passwd='kadidd!ehopper')
   opener = urllib.request.build_opener(auth_handler)
   # ...and install it globally so it can be used with urlopen.
   urllib.request.install_opener(opener)
   urllib.request.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
programmatically-supplied proxy URLs, and adds proxy authorization support with
:class:`ProxyBasicAuthHandler`. ::

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

   opener = urllib.request.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 urllib.request
   req = urllib.request.Request('http://www.example.com/')
   req.add_header('Referer', 'http://www.python.org/')
   # Customize the default User-Agent header value:
   req.add_header('User-Agent', 'urllib-example/0.1 (Contact: . . .)')
   r = urllib.request.urlopen(req)

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

   import urllib.request
   opener = urllib.request.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`).

.. _urllib-examples:

Here is an example session that uses the ``GET`` method to retrieve a URL
containing parameters::

   >>> import urllib.request
   >>> import urllib.parse
   >>> params = urllib.parse.urlencode({'spam': 1, 'eggs': 2, 'bacon': 0})
   >>> url = "http://www.musi-cal.com/cgi-bin/query?%s" % params
   >>> with urllib.request.urlopen(url) as f:
   ...     print(f.read().decode('utf-8'))
   ...

The following example uses the ``POST`` method instead. Note that params output
from urlencode is encoded to bytes before it is sent to urlopen as data::

   >>> import urllib.request
   >>> import urllib.parse
   >>> data = urllib.parse.urlencode({'spam': 1, 'eggs': 2, 'bacon': 0})
   >>> data = data.encode('ascii')
   >>> with urllib.request.urlopen("http://requestb.in/xrbl82xr", data) as f:
   ...     print(f.read().decode('utf-8'))
   ...

The following example uses an explicitly specified HTTP proxy, overriding
environment settings::

   >>> import urllib.request
   >>> proxies = {'http': 'http://proxy.example.com:8080/'}
   >>> opener = urllib.request.FancyURLopener(proxies)
   >>> with opener.open("http://www.python.org") as f:
   ...     f.read().decode('utf-8')
   ...

The following example uses no proxies at all, overriding environment settings::

   >>> import urllib.request
   >>> opener = urllib.request.FancyURLopener({})
   >>> with opener.open("http://www.python.org/") as f:
   ...     f.read().decode('utf-8')
   ...


Legacy interface
----------------

The following functions and classes are ported from the Python 2 module
``urllib`` (as opposed to ``urllib2``).  They might become deprecated at
some point in the future.

.. function:: urlretrieve(url, filename=None, reporthook=None, data=None)

   Copy a network object denoted by a URL to a local file. If the URL
   points to a local file, the object will not be copied unless filename is supplied.
   Return a tuple ``(filename, headers)`` where *filename* is the
   local file name under which the object can be found, and *headers* is whatever
   the :meth:`info` method of the object returned by :func:`urlopen` returned (for
   a remote object). Exceptions are the same as for :func:`urlopen`.

   The second argument, if present, specifies the file location to copy to (if
   absent, the location will be a tempfile with a generated name). The third
   argument, if present, is a callable that will be called once on
   establishment of the network connection and once after each block read
   thereafter.  The callable will be passed three arguments; a count of blocks
   transferred so far, a block size in bytes, and the total size of the file.  The
   third argument may be ``-1`` on older FTP servers which do not return a file
   size in response to a retrieval request.

   The following example illustrates the most common usage scenario::

      >>> import urllib.request
      >>> local_filename, headers = urllib.request.urlretrieve('http://python.org/')
      >>> html = open(local_filename)
      >>> html.close()

   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 be a bytes object in standard
   :mimetype:`application/x-www-form-urlencoded` format; see the
   :func:`urllib.parse.urlencode` function.

   :func:`urlretrieve` will raise :exc:`ContentTooShortError` when it detects that
   the amount of data available  was less than the expected amount (which is the
   size reported by a  *Content-Length* header). This can occur, for example, when
   the  download is interrupted.

   The *Content-Length* is treated as a lower bound: if there's more data  to read,
   urlretrieve reads more data, but if less data is available,  it raises the
   exception.

   You can still retrieve the downloaded data in this case, it is stored  in the
   :attr:`content` attribute of the exception instance.

   If no *Content-Length* header was supplied, urlretrieve can not check the size
   of the data it has downloaded, and just returns it.  In this case you just have
   to assume that the download was successful.

.. function:: urlcleanup()

   Cleans up temporary files that may have been left behind by previous
   calls to :func:`urlretrieve`.

.. class:: URLopener(proxies=None, **x509)

   .. deprecated:: 3.3

   Base class for opening and reading URLs.  Unless you need to support opening
   objects using schemes other than :file:`http:`, :file:`ftp:`, or :file:`file:`,
   you probably want to use :class:`FancyURLopener`.

   By default, the :class:`URLopener` class sends a :mailheader:`User-Agent` header
   of ``urllib/VVV``, where *VVV* is the :mod:`urllib` version number.
   Applications can define their own :mailheader:`User-Agent` header by subclassing
   :class:`URLopener` or :class:`FancyURLopener` and setting the class attribute
   :attr:`version` to an appropriate string value in the subclass definition.

   The optional *proxies* parameter should be a dictionary mapping scheme names to
   proxy URLs, where an empty dictionary turns proxies off completely.  Its default
   value is ``None``, in which case environmental proxy settings will be used if
   present, as discussed in the definition of :func:`urlopen`, above.

   Additional keyword parameters, collected in *x509*, may be used for
   authentication of the client when using the :file:`https:` scheme.  The keywords
   *key_file* and *cert_file* are supported to provide an  SSL key and certificate;
   both are needed to support client authentication.

   :class:`URLopener` objects will raise an :exc:`OSError` exception if the server
   returns an error code.

   .. 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.


.. class:: FancyURLopener(...)

   .. deprecated:: 3.3

   :class:`FancyURLopener` subclasses :class:`URLopener` providing default handling
   for the following HTTP response codes: 301, 302, 303, 307 and 401.  For the 30x
   response codes listed above, the :mailheader:`Location` header is used to fetch
   the actual URL.  For 401 response codes (authentication required), basic HTTP
   authentication is performed.  For the 30x response codes, recursion is bounded
   by the value of the *maxtries* attribute, which defaults to 10.

   For all other response codes, the method :meth:`http_error_default` is called
   which you can override in subclasses to handle the error appropriately.

   .. note::

      According to the letter of :rfc:`2616`, 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 :mod:`urllib` reproduces this behaviour.

   The parameters to the constructor are the same as those for :class:`URLopener`.

   .. note::

      When performing basic authentication, a :class:`FancyURLopener` instance calls
      its :meth:`prompt_user_passwd` method.  The default implementation asks the
      users for the required information on the controlling terminal.  A subclass may
      override this method to support more appropriate behavior if needed.

   The :class:`FancyURLopener` class offers one additional method that should be
   overloaded to provide the appropriate behavior:

   .. method:: prompt_user_passwd(host, realm)

      Return information needed to authenticate the user at the given host in the
      specified security realm.  The return value should be a tuple, ``(user,
      password)``, which can be used for basic authentication.

      The implementation prompts for this information on the terminal; an application
      should override this method to use an appropriate interaction model in the local
      environment.


:mod:`urllib.request` Restrictions
----------------------------------

  .. index::
     pair: HTTP; protocol
     pair: FTP; protocol

* Currently, only the following protocols are supported: HTTP (versions 0.9 and
  1.0), FTP, local files, and data URLs.

  .. versionchanged:: 3.4 Added support for data URLs.

* The caching feature of :func:`urlretrieve` has been disabled until someone
  finds the time to hack proper processing of Expiration time headers.

* There should be a function to query whether a particular URL is in the cache.

* For backward compatibility, if a URL appears to point to a local file but the
  file can't be opened, the URL is re-interpreted using the FTP protocol.  This
  can sometimes cause confusing error messages.

* The :func:`urlopen` and :func:`urlretrieve` functions can cause arbitrarily
  long delays while waiting for a network connection to be set up.  This means
  that it is difficult to build an interactive Web client using these functions
  without using threads.

  .. index::
     single: HTML
     pair: HTTP; protocol

* The data returned by :func:`urlopen` or :func:`urlretrieve` is the raw data
  returned by the server.  This may be binary data (such as an image), plain text
  or (for example) HTML.  The HTTP protocol provides type information in the reply
  header, which can be inspected by looking at the :mailheader:`Content-Type`
  header.  If the returned data is HTML, you can use the module
  :mod:`html.parser` to parse it.

  .. index:: single: FTP

* The code handling the FTP protocol cannot differentiate between a file and a
  directory.  This can lead to unexpected behavior when attempting to read a URL
  that points to a file that is not accessible.  If the URL ends in a ``/``, it is
  assumed to refer to a directory and will be handled accordingly.  But if an
  attempt to read a file leads to a 550 error (meaning the URL cannot be found or
  is not accessible, often for permission reasons), then the path is treated as a
  directory in order to handle the case when a directory is specified by a URL but
  the trailing ``/`` has been left off.  This can cause misleading results when
  you try to fetch a file whose read permissions make it inaccessible; the FTP
  code will try to read it, fail with a 550 error, and then perform a directory
  listing for the unreadable file. If fine-grained control is needed, consider
  using the :mod:`ftplib` module, subclassing :class:`FancyURLopener`, or changing
  *_urlopener* to meet your needs.



:mod:`urllib.response` --- Response classes used by urllib
==========================================================

.. module:: urllib.response
   :synopsis: Response classes used by urllib.

The :mod:`urllib.response` module defines functions and classes which define a
minimal file like interface, including ``read()`` and ``readline()``. The
typical response object is an addinfourl instance, which defines an ``info()``
method and that returns headers and a ``geturl()`` method that returns the url.
Functions defined by this module are used internally by the
:mod:`urllib.request` module.