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= ROOT|Technical|Code_Examples|Python|python.lib|urllib2.py =

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"""An extensible library for opening URLs using a variety of protocols

The simplest way to use this module is to call the urlopen function,
which accepts a string containing a URL or a Request object (described
below).  It opens the URL and returns the results as file-like
object; the returned object has some extra methods described below.

The OpenerDirector manages a collection of Handler objects that do
all the actual work.  Each Handler implements a particular protocol or
option.  The OpenerDirector is a composite object that invokes the
Handlers needed to open the requested URL.  For example, the
HTTPHandler performs HTTP GET and POST requests and deals with
non-error returns.  The HTTPRedirectHandler automatically deals with
HTTP 301, 302, 303 and 307 redirect errors, and the HTTPDigestAuthHandler
deals with digest authentication.

urlopen(url, data=None) -- basic usage is that same as original
urllib.  pass the url and optionally data to post to an HTTP URL, and
get a file-like object back.  One difference is that you can also pass
a Request instance instead of URL.  Raises a URLError (subclass of
IOError); for HTTP errors, raises an HTTPError, which can also be
treated as a valid response.

build_opener -- function that creates a new OpenerDirector instance.
will install the default handlers.  accepts one or more Handlers as
arguments, either instances or Handler classes that it will
instantiate.  if one of the argument is a subclass of the default
handler, the argument will be installed instead of the default.

install_opener -- installs a new opener as the default opener.

objects of interest:
OpenerDirector --

Request -- an object that encapsulates the state of a request.  the
state can be a simple as the URL.  it can also include extra HTTP
headers, e.g. a User-Agent.

BaseHandler --

exceptions:
URLError-- a subclass of IOError, individual protocols have their own
specific subclass

HTTPError-- also a valid HTTP response, so you can treat an HTTP error
as an exceptional event or valid response

internals:
BaseHandler and parent
_call_chain conventions

Example usage:

import urllib2

# set up authentication info
authinfo = urllib2.HTTPBasicAuthHandler()
authinfo.add_password('realm', 'host', 'username', 'password')

proxy_support = urllib2.ProxyHandler({"http" : "http://ahad-haam:3128"})

# build a new opener that adds authentication and caching FTP handlers
opener = urllib2.build_opener(proxy_support, authinfo, urllib2.CacheFTPHandler)

# install it
urllib2.install_opener(opener)

f = urllib2.urlopen('http://www.python.org/')


"""

# XXX issues:
# If an authentication error handler that tries to perform
# authentication for some reason but fails, how should the error be
# signalled?  The client needs to know the HTTP error code.  But if
# the handler knows that the problem was, e.g., that it didn't know
# that hash algo that requested in the challenge, it would be good to
# pass that information along to the client, too.

# XXX to do:
# name!
# documentation (getting there)
# complex proxies
# abstract factory for opener
# ftp errors aren't handled cleanly
# gopher can return a socket.error
# check digest against correct (i.e. non-apache) implementation

import base64
import ftplib
import gopherlib
import httplib
import inspect
import md5
import mimetypes
import mimetools
import os
import posixpath
import random
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