
``urllib.request`` --- Extensible library for opening URLs
**********************************************************

The ``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.

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

urllib.request.urlopen(url, data=None[, timeout])

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

   Warning: HTTPS requests do not do any verification of the server's
     certificate.

   *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 *application/x-www-form-
   urlencoded* format.  The ``urllib.parse.urlencode()`` function
   takes a mapping or sequence of 2-tuples and returns a string in
   this format.

   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.

   This function returns a file-like object with two additional
   methods from the ``urllib.response`` module

   * ``geturl()`` --- return the URL of the resource retrieved,
     commonly used to determine if a redirect was followed

   * ``info()`` --- return the meta-information of the page, such as
     headers, in the form of an ``email.message_from_string()``
     instance (see Quick Reference to HTTP Headers)

   Raises ``URLError`` on errors.

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

   In addition, default installed ``ProxyHandler`` makes sure the
   requests are handled through the proxy when they are set.

   The legacy ``urllib.urlopen`` function from Python 2.6 and earlier
   has been discontinued; ``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 ``ProxyHandler`` objects.

urllib.request.install_opener(opener)

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

urllib.request.build_opener([handler, ...])

   Return an ``OpenerDirector`` instance, which chains the handlers in
   the order given. *handler*s can be either instances of
   ``BaseHandler``, or subclasses of ``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: ``ProxyHandler``, ``UnknownHandler``,
   ``HTTPHandler``, ``HTTPDefaultErrorHandler``,
   ``HTTPRedirectHandler``, ``FTPHandler``, ``FileHandler``,
   ``HTTPErrorProcessor``.

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

   A ``BaseHandler`` subclass may also change its ``handler_order``
   member variable to modify its position in the handlers list.

urllib.request.urlretrieve(url, filename=None, reporthook=None, data=None)

   Copy a network object denoted by a URL to a local file, if
   necessary. If the URL points to a local file, or a valid cached
   copy of the object exists, the object is not copied.  Return a
   tuple ``(filename, headers)`` where *filename* is the local file
   name under which the object can be found, and *headers* is whatever
   the ``info()`` method of the object returned by ``urlopen()``
   returned (for a remote object, possibly cached). Exceptions are the
   same as for ``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 hook function
   that will be called once on establishment of the network connection
   and once after each block read thereafter.  The hook 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.

   If the *url* uses the ``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 *application/x-www-form-urlencoded* format; see the
   ``urlencode()`` function below.

   ``urlretrieve()`` will raise ``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 ``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.

urllib.request.urlcleanup()

   Clear the cache that may have been built up by previous calls to
   ``urlretrieve()``.

urllib.request.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
   ``quote()`` function.

urllib.request.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 ``unquote()`` to decode *path*.

urllib.request.getproxies()

   This helper function returns a dictionary of scheme to proxy server
   URL mappings. It scans the environment for variables named
   ``<scheme>_proxy`` 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.

The following classes are provided:

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

   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 *application/x-www-form-
   urlencoded* format.  The ``urllib.parse.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
   ``add_header()`` was called with each key and value as arguments.
   This is often used to "spoof" the ``User-Agent`` header, 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
   ``urllib``'s default user agent string is ``"Python-urllib/2.6"``
   (on Python 2.6).

   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.

class class urllib.request.URLopener(proxies=None, **x509)

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

   By default, the ``URLopener`` class sends a *User-Agent* header of
   ``urllib/VVV``, where *VVV* is the ``urllib`` version number.
   Applications can define their own *User-Agent* header by
   subclassing ``URLopener`` or ``FancyURLopener`` and setting the
   class attribute ``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 ``urlopen()``, above.

   Additional keyword parameters, collected in *x509*, may be used for
   authentication of the client when using the ``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.

   ``URLopener`` objects will raise an ``IOError`` exception if the
   server returns an error code.

      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, ``open_unknown()`` is called. The
         *data* argument has the same meaning as the *data* argument
         of ``urlopen()``.

      open_unknown(fullurl, data=None)

         Overridable interface to open unknown URL types.

      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 a ``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 ``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 ``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 *application/x-www-form-urlencoded*
         format; see the ``urlencode()`` function below.

      version

         Variable that specifies the user agent of the opener object.
         To get ``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 class urllib.request.FancyURLopener(...)

   ``FancyURLopener`` subclasses ``URLopener`` providing default
   handling for the following HTTP response codes: 301, 302, 303, 307
   and 401.  For the 30x response codes listed above, the *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 ``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 ``urllib`` reproduces this behaviour.

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

   Note: When performing basic authentication, a ``FancyURLopener``
     instance calls its ``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 ``FancyURLopener`` class offers one additional method that
   should be overloaded to provide the appropriate behavior:

   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.

class class urllib.request.OpenerDirector

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

class class urllib.request.BaseHandler

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

class class urllib.request.HTTPDefaultErrorHandler

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

class class urllib.request.HTTPRedirectHandler

   A class to handle redirections.

class class urllib.request.HTTPCookieProcessor(cookiejar=None)

   A class to handle HTTP Cookies.

class class urllib.request.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 ****. If no proxy environment variables are set, 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.

class class urllib.request.HTTPPasswordMgr

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

class class urllib.request.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 class urllib.request.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 ``HTTPPasswordMgr``; refer to
   section *HTTPPasswordMgr Objects* for information on the interface
   that must be supported.

class class urllib.request.HTTPBasicAuthHandler(password_mgr=None)

   Handle authentication with the remote host. *password_mgr*, if
   given, should be something that is compatible with
   ``HTTPPasswordMgr``; refer to section *HTTPPasswordMgr Objects* for
   information on the interface that must be supported.

class class urllib.request.ProxyBasicAuthHandler(password_mgr=None)

   Handle authentication with the proxy. *password_mgr*, if given,
   should be something that is compatible with ``HTTPPasswordMgr``;
   refer to section *HTTPPasswordMgr Objects* for information on the
   interface that must be supported.

class class urllib.request.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 ``HTTPPasswordMgr``; refer to
   section *HTTPPasswordMgr Objects* for information on the interface
   that must be supported.

class class urllib.request.HTTPDigestAuthHandler(password_mgr=None)

   Handle authentication with the remote host. *password_mgr*, if
   given, should be something that is compatible with
   ``HTTPPasswordMgr``; refer to section *HTTPPasswordMgr Objects* for
   information on the interface that must be supported.

class class urllib.request.ProxyDigestAuthHandler(password_mgr=None)

   Handle authentication with the proxy. *password_mgr*, if given,
   should be something that is compatible with ``HTTPPasswordMgr``;
   refer to section *HTTPPasswordMgr Objects* for information on the
   interface that must be supported.

class class urllib.request.HTTPHandler

   A class to handle opening of HTTP URLs.

class class urllib.request.HTTPSHandler

   A class to handle opening of HTTPS URLs.

class class urllib.request.FileHandler

   Open local files.

class class urllib.request.FTPHandler

   Open FTP URLs.

class class urllib.request.CacheFTPHandler

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

class class urllib.request.UnknownHandler

   A catch-all class to handle unknown URLs.


Request Objects
===============

The following methods describe ``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.

Request.full_url

   The original URL passed to the constructor.

Request.type

   The URI scheme.

Request.host

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

Request.origin_req_host

   The original host for the request, without port.

Request.selector

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

Request.data

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

Request.unverifiable

   boolean, indicates whether the request is unverifiable as defined
   by RFC 2965.

Request.add_data(data)

   Set the ``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``.

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'``.

Request.has_data()

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

Request.get_data()

   Return the instance's data.

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.

Request.add_unredirected_header(key, header)

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

Request.has_header(header)

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

Request.get_full_url()

   Return the URL given in the constructor.

Request.get_type()

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

Request.get_host()

   Return the host to which a connection will be made.

Request.get_selector()

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

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.

Request.get_origin_req_host()

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

Request.is_unverifiable()

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


OpenerDirector Objects
======================

``OpenerDirector`` instances have the following methods:

OpenerDirector.add_handler(handler)

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

   * ``protocol_open()`` --- signal that the handler knows how to open
     *protocol* URLs.

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

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

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

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

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 ``urlopen()`` (which
   simply calls the ``open()`` method on the currently installed
   global ``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).

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 ``http_error_*()`` methods of
   the handler classes.

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

1. Every handler with a method named like ``protocol_request()`` has
   that method called to pre-process the request.

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

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

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

3. Every handler with a method named like ``protocol_response()`` has
   that method called to post-process the response.


BaseHandler Objects
===================

``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:

BaseHandler.add_parent(director)

   Add a director as parent.

BaseHandler.close()

   Remove any parents.

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

Note: The convention has been adopted that subclasses defining
  ``protocol_request()`` or ``protocol_response()`` methods are named
  ``*Processor``; all others are named ``*Handler``.

BaseHandler.parent

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

BaseHandler.default_open(req)

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

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

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

BaseHandler.protocol_open(req)

   This method is *not* defined in ``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
   ``OpenerDirector``. Return values should be the same as for
   ``default_open()``.

BaseHandler.unknown_open(req)

   This method is *not* defined in ``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 ``parent``
   ``OpenerDirector``.  Return values should be the same as for
   ``default_open()``.

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

   This method is *not* defined in ``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  ``OpenerDirector`` getting the error, and should not
   normally be called in other circumstances.

   *req* will be a ``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
   ``urlopen()``.

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 ``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 ``http_error_default()``.

BaseHandler.protocol_request(req)

   This method is *not* defined in ``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
   ``OpenerDirector``. *req* will be a ``Request`` object. The return
   value should be a ``Request`` object.

BaseHandler.protocol_response(req, response)

   This method is *not* defined in ``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
   ``OpenerDirector``. *req* will be a ``Request`` object. *response*
   will be an object implementing the same interface as the return
   value of ``urlopen()``.  The return value should implement the same
   interface as the return value of ``urlopen()``.


HTTPRedirectHandler Objects
===========================

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

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

   Return a ``Request`` or ``None`` in response to a redirect. This is
   called by the default implementations of the ``http_error_30*()``
   methods when a redirection is received from the server.  If a
   redirection should take place, return a new ``Request`` to allow
   ``http_error_30*()`` to perform the redirect to *newurl*.
   Otherwise, raise ``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.

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

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

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

   The same as ``http_error_301()``, but called for the 'found'
   response.

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

   The same as ``http_error_301()``, but called for the 'see other'
   response.

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

   The same as ``http_error_301()``, but called for the 'temporary
   redirect' response.


HTTPCookieProcessor Objects
===========================

``HTTPCookieProcessor`` instances have one attribute:

HTTPCookieProcessor.cookiejar

   The ``http.cookiejar.CookieJar`` in which cookies are stored.


ProxyHandler Objects
====================

ProxyHandler.protocol_open(request)

   The ``ProxyHandler`` will have a method ``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.


HTTPPasswordMgr Objects
=======================

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

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.

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


AbstractBasicAuthHandler Objects
================================

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) ``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).


HTTPBasicAuthHandler Objects
============================

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

   Retry the request with authentication information, if available.


ProxyBasicAuthHandler Objects
=============================

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

   Retry the request with authentication information, if available.


AbstractDigestAuthHandler Objects
=================================

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) ``Request``
   object, and *headers* should be the error headers.


HTTPDigestAuthHandler Objects
=============================

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

   Retry the request with authentication information, if available.


ProxyDigestAuthHandler Objects
==============================

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

   Retry the request with authentication information, if available.


HTTPHandler Objects
===================

HTTPHandler.http_open(req)

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


HTTPSHandler Objects
====================

HTTPSHandler.https_open(req)

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


FileHandler Objects
===================

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 ``parent``.


FTPHandler Objects
==================

FTPHandler.ftp_open(req)

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


CacheFTPHandler Objects
=======================

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

CacheFTPHandler.setTimeout(t)

   Set timeout of connections to *t* seconds.

CacheFTPHandler.setMaxConns(m)

   Set maximum number of cached connections to *m*.


UnknownHandler Objects
======================

UnknownHandler.unknown_open()

   Raise a ``URLError`` exception.


HTTPErrorProcessor Objects
==========================

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
   ``protocol_error_code()`` handler methods, via
   ``OpenerDirector.error()``. Eventually, ``HTTPDefaultErrorHandler``
   will raise an ``HTTPError`` if no other handler handles the error.


Examples
========

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

   >>> import urllib.request
   >>> f = urllib.request.urlopen('http://www.python.org/')
   >>> 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, http://www.w3.org/International/O-charset
, lists the various ways in which a (X)HTML or a XML document could
have specified its encoding information.

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

   >>> 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='This data is passed to stdin of the CGI')
   >>> f = urllib.request.urlopen(req)
   >>> 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)

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')

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

This example replaces the default ``ProxyHandler`` with one that uses
programmatically-supplied proxy URLs, and adds proxy authorization
support with ``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 ``Request`` constructor, or:

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

``OpenerDirector`` automatically adds a *User-Agent* header to every
``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 (*Content-Length*,
*Content-Type* and *Host*) are added when the ``Request`` is passed to
``urlopen()`` (or ``OpenerDirector.open()``).

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})
   >>> f = urllib.request.urlopen("http://www.musi-cal.com/cgi-bin/query?%s" % params)
   >>> print(f.read().decode('utf-8'))

The following example uses the ``POST`` method instead:

   >>> import urllib.request
   >>> import urllib.parse
   >>> params = urllib.parse.urlencode({'spam': 1, 'eggs': 2, 'bacon': 0})
   >>> f = urllib.request.urlopen("http://www.musi-cal.com/cgi-bin/query", params)
   >>> 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)
   >>> f = opener.open("http://www.python.org")
   >>> f.read().decode('utf-8')

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

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


``urllib.request`` Restrictions
===============================

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

* The caching feature of ``urlretrieve()`` has been disabled until I
  find 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 ``urlopen()`` and ``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.

* The data returned by ``urlopen()`` or ``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 *Content-Type* header.  If the returned
  data is HTML, you can use the module ``html.parser`` to parse it.

* 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 ``ftplib`` module, subclassing ``FancyURLopener``, or
  changing *_urlopener* to meet your needs.


``urllib.response`` --- Response classes used by urllib
*******************************************************

The ``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 ``urllib.request`` module.
