cpython/Lib/urllib2.py

"""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 the 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.
# ftp errors aren't handled cleanly
# check digest against correct (i.e. non-apache) implementation

# Possible extensions:
# complex proxies  XXX not sure what exactly was meant by this
# abstract factory for opener

import base64
import hashlib
import httplib
import mimetools
import os
import posixpath
import random
import re
import socket
import sys
import time
import urlparse
import bisect

try:
    from cStringIO import StringIO
except ImportError:
    from StringIO import StringIO

from urllib import (unwrap, unquote, splittype, splithost, quote,
     addinfourl, splitport, splitgophertype, splitquery,
     splitattr, ftpwrapper, noheaders, splituser, splitpasswd, splitvalue)

# support for FileHandler, proxies via environment variables
from urllib import localhost, url2pathname, getproxies

# used in User-Agent header sent
__version__ = sys.version[:3]

_opener = None
def urlopen(url, data=None):
    global _opener
    if _opener is None:
        _opener = build_opener()
    return _opener.open(url, data)

def install_opener(opener):
    global _opener
    _opener = opener

# do these error classes make sense?
# make sure all of the IOError stuff is overridden.  we just want to be
# subtypes.

class URLError(IOError):
    # URLError is a sub-type of IOError, but it doesn't share any of
    # the implementation.  need to override __init__ and __str__.
    # It sets self.args for compatibility with other EnvironmentError
    # subclasses, but args doesn't have the typical format with errno in
    # slot 0 and strerror in slot 1.  This may be better than nothing.
    def __init__(self, reason):
        self.args = reason,
        self.reason = reason

    def __str__(self):
        return '<urlopen error %s>' % self.reason

class HTTPError(URLError, addinfourl):
    """Raised when HTTP error occurs, but also acts like non-error return"""
    __super_init = addinfourl.__init__

    def __init__(self, url, code, msg, hdrs, fp):
        self.code = code
        self.msg = msg
        self.hdrs = hdrs
        self.fp = fp
        self.filename = url
        # The addinfourl classes depend on fp being a valid file
        # object.  In some cases, the HTTPError may not have a valid
        # file object.  If this happens, the simplest workaround is to
        # not initialize the base classes.
        if fp is not None:
            self.__super_init(fp, hdrs, url)

    def __str__(self):
        return 'HTTP Error %s: %s' % (self.code, self.msg)

class GopherError(URLError):
    pass

# copied from cookielib.py
_cut_port_re = re.compile(r":\d+$")
def request_host(request):
    """Return request-host, as defined by RFC 2965.

    Variation from RFC: returned value is lowercased, for convenient
    comparison.

    """
    url = request.get_full_url()
    host = urlparse.urlparse(url)[1]
    if host == "":
        host = request.get_header("Host", "")

    # remove port, if present
    host = _cut_port_re.sub("", host, 1)
    return host.lower()

class Request:

    def __init__(self, url, data=None, headers={},
                 origin_req_host=None, unverifiable=False):
        # unwrap('<URL:type://host/path>') --> 'type://host/path'
        self.__original = unwrap(url)
        self.type = None
        # self.__r_type is what's left after doing the splittype
        self.host = None
        self.port = None
        self.data = data
        self.headers = {}
        for key, value in headers.items():
            self.add_header(key, value)
        self.unredirected_hdrs = {}
        if origin_req_host is None:
            origin_req_host = request_host(self)
        self.origin_req_host = origin_req_host
        self.unverifiable = unverifiable

    def __getattr__(self, attr):
        # XXX this is a fallback mechanism to guard against these
        # methods getting called in a non-standard order.  this may be
        # too complicated and/or unnecessary.
        # XXX should the __r_XXX attributes be public?
        if attr[:12] == '_Request__r_':
            name = attr[12:]
            if hasattr(Request, 'get_' + name):
                getattr(self, 'get_' + name)()
                return getattr(self, attr)
        raise AttributeError, attr

    def get_method(self):
        if self.has_data():
            return "POST"
        else:
            return "GET"

    # XXX these helper methods are lame

    def add_data(self, data):
        self.data = data

    def has_data(self):
        return self.data is not None

    def get_data(self):
        return self.data

    def get_full_url(self):
        return self.__original

    def get_type(self):
        if self.type is None:
            self.type, self.__r_type = splittype(self.__original)
            if self.type is None:
                raise ValueError, "unknown url type: %s" % self.__original
        return self.type

    def get_host(self):
        if self.host is None:
            self.host, self.__r_host = splithost(self.__r_type)
            if self.host:
                self.host = unquote(self.host)
        return self.host

    def get_selector(self):
        return self.__r_host

    def set_proxy(self, host, type):
        self.host, self.type = host, type
        self.__r_host = self.__original

    def get_origin_req_host(self):
        return self.origin_req_host

    def is_unverifiable(self):
        return self.unverifiable

    def add_header(self, key, val):
        # useful for something like authentication
        self.headers[key.capitalize()] = val

    def add_unredirected_header(self, key, val):
        # will not be added to a redirected request
        self.unredirected_hdrs[key.capitalize()] = val

    def has_header(self, header_name):
        return (header_name in self.headers or
                header_name in self.unredirected_hdrs)

    def get_header(self, header_name, default=None):
        return self.headers.get(
            header_name,
            self.unredirected_hdrs.get(header_name, default))

    def header_items(self):
        hdrs = self.unredirected_hdrs.copy()
        hdrs.update(self.headers)
        return hdrs.items()

class OpenerDirector:
    def __init__(self):
        client_version = "Python-urllib/%s" % __version__
        self.addheaders = [('User-agent', client_version)]
        # manage the individual handlers
        self.handlers = []
        self.handle_open = {}
        self.handle_error = {}
        self.process_response = {}
        self.process_request = {}

    def add_handler(self, handler):
        added = False
        for meth in dir(handler):
            if meth in ["redirect_request", "do_open", "proxy_open"]:
                # oops, coincidental match
                continue

            i = meth.find("_")
            protocol = meth[:i]
            condition = meth[i+1:]

            if condition.startswith("error"):
                j = condition.find("_") + i + 1
                kind = meth[j+1:]
                try:
                    kind = int(kind)
                except ValueError:
                    pass
                lookup = self.handle_error.get(protocol, {})
                self.handle_error[protocol] = lookup
            elif condition == "open":
                kind = protocol
                lookup = self.handle_open
            elif condition == "response":
                kind = protocol
                lookup = self.process_response
            elif condition == "request":
                kind = protocol
                lookup = self.process_request
            else:
                continue

            handlers = lookup.setdefault(kind, [])
            if handlers:
                bisect.insort(handlers, handler)
            else:
                handlers.append(handler)
            added = True

        if added:
            # XXX why does self.handlers need to be sorted?
            bisect.insort(self.handlers, handler)
            handler.add_parent(self)

    def close(self):
        # Only exists for backwards compatibility.
        pass

    def _call_chain(self, chain, kind, meth_name, *args):
        # Handlers raise an exception if no one else should try to handle
        # the request, or return None if they can't but another handler
        # could.  Otherwise, they return the response.
        handlers = chain.get(kind, ())
        for handler in handlers:
            func = getattr(handler, meth_name)

            result = func(*args)
            if result is not None:
                return result

    def open(self, fullurl, data=None):
        # accept a URL or a Request object
        if isinstance(fullurl, basestring):
            req = Request(fullurl, data)
        else:
            req = fullurl
            if data is not None:
                req.add_data(data)

        protocol = req.get_type()

        # pre-process request
        meth_name = protocol+"_request"
        for processor in self.process_request.get(protocol, []):
            meth = getattr(processor, meth_name)
            req = meth(req)

        response = self._open(req, data)

        # post-process response
        meth_name = protocol+"_response"
        for processor in self.process_response.get(protocol, []):
            meth = getattr(processor, meth_name)
            response = meth(req, response)

        return response

    def _open(self, req, data=None):
        result = self._call_chain(self.handle_open, 'default',
                                  'default_open', req)
        if result:
            return result

        protocol = req.get_type()
        result = self._call_chain(self.handle_open, protocol, protocol +
                                  '_open', req)
        if result:
            return result

        return self._call_chain(self.handle_open, 'unknown',
                                'unknown_open', req)

    def error(self, proto, *args):
        if proto in ('http', 'https'):
            # XXX http[s] protocols are special-cased
            dict = self.handle_error['http'] # https is not different than http
            proto = args[2]  # YUCK!
            meth_name = 'http_error_%s' % proto
            http_err = 1
            orig_args = args
        else:
            dict = self.handle_error
            meth_name = proto + '_error'
            http_err = 0
        args = (dict, proto, meth_name) + args
        result = self._call_chain(*args)
        if result:
            return result

        if http_err:
            args = (dict, 'default', 'http_error_default') + orig_args
            return self._call_chain(*args)

# XXX probably also want an abstract factory that knows when it makes
# sense to skip a superclass in favor of a subclass and when it might
# make sense to include both

def build_opener(*handlers):
    """Create an opener object from a list of handlers.

    The opener will use several default handlers, including support
    for HTTP and FTP.

    If any of the handlers passed as arguments are subclasses of the
    default handlers, the default handlers will not be used.
    """
    import types
    def isclass(obj):
        return isinstance(obj, types.ClassType) or hasattr(obj, "__bases__")

    opener = OpenerDirector()
    default_classes = [ProxyHandler, UnknownHandler, HTTPHandler,
                       HTTPDefaultErrorHandler, HTTPRedirectHandler,
                       FTPHandler, FileHandler, HTTPErrorProcessor]
    if hasattr(httplib, 'HTTPS'):
        default_classes.append(HTTPSHandler)
    skip = []
    for klass in default_classes:
        for check in handlers:
            if isclass(check):
                if issubclass(check, klass):
                    skip.append(klass)
            elif isinstance(check, klass):
                skip.append(klass)
    for klass in skip:
        default_classes.remove(klass)

    for klass in default_classes:
        opener.add_handler(klass())

    for h in handlers:
        if isclass(h):
            h = h()
        opener.add_handler(h)
    return opener

class BaseHandler:
    handler_order = 500

    def add_parent(self, parent):
        self.parent = parent

    def close(self):
        # Only exists for backwards compatibility
        pass

    def __lt__(self, other):
        if not hasattr(other, "handler_order"):
            # Try to preserve the old behavior of having custom classes
            # inserted after default ones (works only for custom user
            # classes which are not aware of handler_order).
            return True
        return self.handler_order < other.handler_order


class HTTPErrorProcessor(BaseHandler):
    """Process HTTP error responses."""
    handler_order = 1000  # after all other processing

    def http_response(self, request, response):
        code, msg, hdrs = response.code, response.msg, response.info()

        if code not in (200, 206):
            response = self.parent.error(
                'http', request, response, code, msg, hdrs)

        return response

    https_response = http_response

class HTTPDefaultErrorHandler(BaseHandler):
    def http_error_default(self, req, fp, code, msg, hdrs):
        raise HTTPError(req.get_full_url(), code, msg, hdrs, fp)

class HTTPRedirectHandler(BaseHandler):
    # maximum number of redirections to any single URL
    # this is needed because of the state that cookies introduce
    max_repeats = 4
    # maximum total number of redirections (regardless of URL) before
    # assuming we're in a loop
    max_redirections = 10

    def redirect_request(self, req, fp, code, msg, headers, newurl):
        """Return a Request or None in response to a redirect.

        This is called by the http_error_30x methods when a
        redirection response is received.  If a redirection should
        take place, return a new Request to allow http_error_30x to
        perform the redirect.  Otherwise, raise HTTPError if no-one
        else should try to handle this url.  Return None if you can't
        but another Handler might.
        """
        m = req.get_method()
        if (code in (301, 302, 303, 307) and m in ("GET", "HEAD")
            or code in (301, 302, 303) and m == "POST"):
            # Strictly (according to RFC 2616), 301 or 302 in response
            # to a POST MUST NOT cause a redirection without confirmation
            # from the user (of urllib2, in this case).  In practice,
            # essentially all clients do redirect in this case, so we
            # do the same.
            # be conciliant with URIs containing a space
            newurl = newurl.replace(' ', '%20')
            return Request(newurl,
                           headers=req.headers,
                           origin_req_host=req.get_origin_req_host(),
                           unverifiable=True)
        else:
            raise HTTPError(req.get_full_url(), code, msg, headers, fp)

    # Implementation note: To avoid the server sending us into an
    # infinite loop, the request object needs to track what URLs we
    # have already seen.  Do this by adding a handler-specific
    # attribute to the Request object.
    def http_error_302(self, req, fp, code, msg, headers):
        # Some servers (incorrectly) return multiple Location headers
        # (so probably same goes for URI).  Use first header.
        if 'location' in headers:
            newurl = headers.getheaders('location')[0]
        elif 'uri' in headers:
            newurl = headers.getheaders('uri')[0]
        else:
            return
        newurl = urlparse.urljoin(req.get_full_url(), newurl)

        # XXX Probably want to forget about the state of the current
        # request, although that might interact poorly with other
        # handlers that also use handler-specific request attributes
        new = self.redirect_request(req, fp, code, msg, headers, newurl)
        if new is None:
            return

        # loop detection
        # .redirect_dict has a key url if url was previously visited.
        if hasattr(req, 'redirect_dict'):
            visited = new.redirect_dict = req.redirect_dict
            if (visited.get(newurl, 0) >= self.max_repeats or
                len(visited) >= self.max_redirections):
                raise HTTPError(req.get_full_url(), code,
                                self.inf_msg + msg, headers, fp)
        else:
            visited = new.redirect_dict = req.redirect_dict = {}
        visited[newurl] = visited.get(newurl, 0) + 1

        # Don't close the fp until we are sure that we won't use it
        # with HTTPError.
        fp.read()
        fp.close()

        return self.parent.open(new)

    http_error_301 = http_error_303 = http_error_307 = http_error_302

    inf_msg = "The HTTP server returned a redirect error that would " \
              "lead to an infinite loop.\n" \
              "The last 30x error message was:\n"


def _parse_proxy(proxy):
    """Return (scheme, user, password, host/port) given a URL or an authority.

    If a URL is supplied, it must have an authority (host:port) component.
    According to RFC 3986, having an authority component means the URL must
    have two slashes after the scheme:

    >>> _parse_proxy('file:/ftp.example.com/')
    Traceback (most recent call last):
    ValueError: proxy URL with no authority: 'file:/ftp.example.com/'

    The first three items of the returned tuple may be None.

    Examples of authority parsing:

    >>> _parse_proxy('proxy.example.com')
    (None, None, None, 'proxy.example.com')
    >>> _parse_proxy('proxy.example.com:3128')
    (None, None, None, 'proxy.example.com:3128')

    The authority component may optionally include userinfo (assumed to be
    username:password):

    >>> _parse_proxy('joe:password@proxy.example.com')
    (None, 'joe', 'password', 'proxy.example.com')
    >>> _parse_proxy('joe:password@proxy.example.com:3128')
    (None, 'joe', 'password', 'proxy.example.com:3128')

    Same examples, but with URLs instead:

    >>> _parse_proxy('http://proxy.example.com/')
    ('http', None, None, 'proxy.example.com')
    >>> _parse_proxy('http://proxy.example.com:3128/')
    ('http', None, None, 'proxy.example.com:3128')
    >>> _parse_proxy('http://joe:password@proxy.example.com/')
    ('http', 'joe', 'password', 'proxy.example.com')
    >>> _parse_proxy('http://joe:password@proxy.example.com:3128')
    ('http', 'joe', 'password', 'proxy.example.com:3128')

    Everything after the authority is ignored:

    >>> _parse_proxy('ftp://joe:password@proxy.example.com/rubbish:3128')
    ('ftp', 'joe', 'password', 'proxy.example.com')

    Test for no trailing '/' case:

    >>> _parse_proxy('http://joe:password@proxy.example.com')
    ('http', 'joe', 'password', 'proxy.example.com')

    """
    scheme, r_scheme = splittype(proxy)
    if not r_scheme.startswith("/"):
        # authority
        scheme = None
        authority = proxy
    else:
        # URL
        if not r_scheme.startswith("//"):
            raise ValueError("proxy URL with no authority: %r" % proxy)
        # We have an authority, so for RFC 3986-compliant URLs (by ss 3.
        # and 3.3.), path is empty or starts with '/'
        end = r_scheme.find("/", 2)
        if end == -1:
            end = None
        authority = r_scheme[2:end]
    userinfo, hostport = splituser(authority)
    if userinfo is not None:
        user, password = splitpasswd(userinfo)
    else:
        user = password = None
    return scheme, user, password, hostport

class ProxyHandler(BaseHandler):
    # Proxies must be in front
    handler_order = 100

    def __init__(self, proxies=None):
        if proxies is None:
            proxies = getproxies()
        assert hasattr(proxies, 'has_key'), "proxies must be a mapping"
        self.proxies = proxies
        for type, url in proxies.items():
            setattr(self, '%s_open' % type,
                    lambda r, proxy=url, type=type, meth=self.proxy_open: \
                    meth(r, proxy, type))

    def proxy_open(self, req, proxy, type):
        orig_type = req.get_type()
        proxy_type, user, password, hostport = _parse_proxy(proxy)
        if proxy_type is None:
            proxy_type = orig_type
        if user and password:
            user_pass = '%s:%s' % (unquote(user), unquote(password))
            creds = base64.encodestring(user_pass).strip()
            req.add_header('Proxy-authorization', 'Basic ' + creds)
        hostport = unquote(hostport)
        req.set_proxy(hostport, proxy_type)
        if orig_type == proxy_type:
            # let other handlers take care of it
            return None
        else:
            # need to start over, because the other handlers don't
            # grok the proxy's URL type
            # e.g. if we have a constructor arg proxies like so:
            # {'http': 'ftp://proxy.example.com'}, we may end up turning
            # a request for http://acme.example.com/a into one for
            # ftp://proxy.example.com/a
            return self.parent.open(req)

class HTTPPasswordMgr:

    def __init__(self):
        self.passwd = {}

    def add_password(self, realm, uri, user, passwd):
        # uri could be a single URI or a sequence
        if isinstance(uri, basestring):
            uri = [uri]
        if not realm in self.passwd:
            self.passwd[realm] = {}
        for default_port in True, False:
            reduced_uri = tuple(
                [self.reduce_uri(u, default_port) for u in uri])
            self.passwd[realm][reduced_uri] = (user, passwd)

    def find_user_password(self, realm, authuri):
        domains = self.passwd.get(realm, {})
        for default_port in True, False:
            reduced_authuri = self.reduce_uri(authuri, default_port)
            for uris, authinfo in domains.iteritems():
                for uri in uris:
                    if self.is_suburi(uri, reduced_authuri):
                        return authinfo
        return None, None

    def reduce_uri(self, uri, default_port=True):
        """Accept authority or URI and extract only the authority and path."""
        # note HTTP URLs do not have a userinfo component
        parts = urlparse.urlsplit(uri)
        if parts[1]:
            # URI
            scheme = parts[0]
            authority = parts[1]
            path = parts[2] or '/'
        else:
            # host or host:port
            scheme = None
            authority = uri
            path = '/'
        host, port = splitport(authority)
        if default_port and port is None and scheme is not None:
            dport = {"http": 80,
                     "https": 443,
                     }.get(scheme)
            if dport is not None:
                authority = "%s:%d" % (host, dport)
        return authority, path

    def is_suburi(self, base, test):
        """Check if test is below base in a URI tree

        Both args must be URIs in reduced form.
        """
        if base == test:
            return True
        if base[0] != test[0]:
            return False
        common = posixpath.commonprefix((base[1], test[1]))
        if len(common) == len(base[1]):
            return True
        return False


class HTTPPasswordMgrWithDefaultRealm(HTTPPasswordMgr):

    def find_user_password(self, realm, authuri):
        user, password = HTTPPasswordMgr.find_user_password(self, realm,
                                                            authuri)
        if user is not None:
            return user, password
        return HTTPPasswordMgr.find_user_password(self, None, authuri)


class AbstractBasicAuthHandler:

    rx = re.compile('[ \t]*([^ \t]+)[ \t]+realm="([^"]*)"', re.I)

    # XXX there can actually be multiple auth-schemes in a
    # www-authenticate header.  should probably be a lot more careful
    # in parsing them to extract multiple alternatives

    # XXX could pre-emptively send auth info already accepted (RFC 2617,
    # end of section 2, and section 1.2 immediately after "credentials"
    # production).

    def __init__(self, password_mgr=None):
        if password_mgr is None:
            password_mgr = HTTPPasswordMgr()
        self.passwd = password_mgr
        self.add_password = self.passwd.add_password

    def http_error_auth_reqed(self, authreq, host, req, headers):
        # host may be an authority (without userinfo) or a URL with an
        # authority
        # XXX could be multiple headers
        authreq = headers.get(authreq, None)
        if authreq:
            mo = AbstractBasicAuthHandler.rx.search(authreq)
            if mo:
                scheme, realm = mo.groups()
                if scheme.lower() == 'basic':
                    return self.retry_http_basic_auth(host, req, realm)

    def retry_http_basic_auth(self, host, req, realm):
        user, pw = self.passwd.find_user_password(realm, host)
        if pw is not None:
            raw = "%s:%s" % (user, pw)
            auth = 'Basic %s' % base64.encodestring(raw).strip()
            if req.headers.get(self.auth_header, None) == auth:
                return None
            req.add_header(self.auth_header, auth)
            return self.parent.open(req)
        else:
            return None


class HTTPBasicAuthHandler(AbstractBasicAuthHandler, BaseHandler):

    auth_header = 'Authorization'

    def http_error_401(self, req, fp, code, msg, headers):
        url = req.get_full_url()
        return self.http_error_auth_reqed('www-authenticate',
                                          url, req, headers)


class ProxyBasicAuthHandler(AbstractBasicAuthHandler, BaseHandler):

    auth_header = 'Proxy-authorization'

    def http_error_407(self, req, fp, code, msg, headers):
        # http_error_auth_reqed requires that there is no userinfo component in
        # authority.  Assume there isn't one, since urllib2 does not (and
        # should not, RFC 3986 s. 3.2.1) support requests for URLs containing
        # userinfo.
        authority = req.get_host()
        return self.http_error_auth_reqed('proxy-authenticate',
                                          authority, req, headers)


def randombytes(n):
    """Return n random bytes."""
    # Use /dev/urandom if it is available.  Fall back to random module
    # if not.  It might be worthwhile to extend this function to use
    # other platform-specific mechanisms for getting random bytes.
    if os.path.exists("/dev/urandom"):
        f = open("/dev/urandom")
        s = f.read(n)
        f.close()
        return s
    else:
        L = [chr(random.randrange(0, 256)) for i in range(n)]
        return "".join(L)

class AbstractDigestAuthHandler:
    # Digest authentication is specified in RFC 2617.

    # XXX The client does not inspect the Authentication-Info header
    # in a successful response.

    # XXX It should be possible to test this implementation against
    # a mock server that just generates a static set of challenges.

    # XXX qop="auth-int" supports is shaky

    def __init__(self, passwd=None):
        if passwd is None:
            passwd = HTTPPasswordMgr()
        self.passwd = passwd
        self.add_password = self.passwd.add_password
        self.retried = 0
        self.nonce_count = 0

    def reset_retry_count(self):
        self.retried = 0

    def http_error_auth_reqed(self, auth_header, host, req, headers):
        authreq = headers.get(auth_header, None)
        if self.retried > 5:
            # Don't fail endlessly - if we failed once, we'll probably
            # fail a second time. Hm. Unless the Password Manager is
            # prompting for the information. Crap. This isn't great
            # but it's better than the current 'repeat until recursion
            # depth exceeded' approach <wink>
            raise HTTPError(req.get_full_url(), 401, "digest auth failed",
                            headers, None)
        else:
            self.retried += 1
        if authreq:
            scheme = authreq.split()[0]
            if scheme.lower() == 'digest':
                return self.retry_http_digest_auth(req, authreq)

    def retry_http_digest_auth(self, req, auth):
        token, challenge = auth.split(' ', 1)
        chal = parse_keqv_list(parse_http_list(challenge))
        auth = self.get_authorization(req, chal)
        if auth:
            auth_val = 'Digest %s' % auth
            if req.headers.get(self.auth_header, None) == auth_val:
                return None
            req.add_unredirected_header(self.auth_header, auth_val)
            resp = self.parent.open(req)
            return resp

    def get_cnonce(self, nonce):
        # The cnonce-value is an opaque
        # quoted string value provided by the client and used by both client
        # and server to avoid chosen plaintext attacks, to provide mutual
        # authentication, and to provide some message integrity protection.
        # This isn't a fabulous effort, but it's probably Good Enough.
        dig = hashlib.sha1("%s:%s:%s:%s" % (self.nonce_count, nonce, time.ctime(),
                                            randombytes(8))).hexdigest()
        return dig[:16]

    def get_authorization(self, req, chal):
        try:
            realm = chal['realm']
            nonce = chal['nonce']
            qop = chal.get('qop')
            algorithm = chal.get('algorithm', 'MD5')
            # mod_digest doesn't send an opaque, even though it isn't
            # supposed to be optional
            opaque = chal.get('opaque', None)
        except KeyError:
            return None

        H, KD = self.get_algorithm_impls(algorithm)
        if H is None:
            return None

        user, pw = self.passwd.find_user_password(realm, req.get_full_url())
        if user is None:
            return None

        # XXX not implemented yet
        if req.has_data():
            entdig = self.get_entity_digest(req.get_data(), chal)
        else:
            entdig = None

        A1 = "%s:%s:%s" % (user, realm, pw)
        A2 = "%s:%s" % (req.get_method(),
                        # XXX selector: what about proxies and full urls
                        req.get_selector())
        if qop == 'auth':
            self.nonce_count += 1
            ncvalue = '%08x' % self.nonce_count
            cnonce = self.get_cnonce(nonce)
            noncebit = "%s:%s:%s:%s:%s" % (nonce, ncvalue, cnonce, qop, H(A2))
            respdig = KD(H(A1), noncebit)
        elif qop is None:
            respdig = KD(H(A1), "%s:%s" % (nonce, H(A2)))
        else:
            # XXX handle auth-int.
            pass

        # XXX should the partial digests be encoded too?

        base = 'username="%s", realm="%s", nonce="%s", uri="%s", ' \
               'response="%s"' % (user, realm, nonce, req.get_selector(),
                                  respdig)
        if opaque:
            base += ', opaque="%s"' % opaque
        if entdig:
            base += ', digest="%s"' % entdig
        base += ', algorithm="%s"' % algorithm
        if qop:
            base += ', qop=auth, nc=%s, cnonce="%s"' % (ncvalue, cnonce)
        return base

    def get_algorithm_impls(self, algorithm):
        # lambdas assume digest modules are imported at the top level
        if algorithm == 'MD5':
            H = lambda x: hashlib.md5(x).hexdigest()
        elif algorithm == 'SHA':
            H = lambda x: hashlib.sha1(x).hexdigest()
        # XXX MD5-sess
        KD = lambda s, d: H("%s:%s" % (s, d))
        return H, KD

    def get_entity_digest(self, data, chal):
        # XXX not implemented yet
        return None


class HTTPDigestAuthHandler(BaseHandler, AbstractDigestAuthHandler):
    """An authentication protocol defined by RFC 2069

    Digest authentication improves on basic authentication because it
    does not transmit passwords in the clear.
    """

    auth_header = 'Authorization'
    handler_order = 490  # before Basic auth

    def http_error_401(self, req, fp, code, msg, headers):
        host = urlparse.urlparse(req.get_full_url())[1]
        retry = self.http_error_auth_reqed('www-authenticate',
                                           host, req, headers)
        self.reset_retry_count()
        return retry


class ProxyDigestAuthHandler(BaseHandler, AbstractDigestAuthHandler):

    auth_header = 'Proxy-Authorization'
    handler_order = 490  # before Basic auth

    def http_error_407(self, req, fp, code, msg, headers):
        host = req.get_host()
        retry = self.http_error_auth_reqed('proxy-authenticate',
                                           host, req, headers)
        self.reset_retry_count()
        return retry

class AbstractHTTPHandler(BaseHandler):

    def __init__(self, debuglevel=0):
        self._debuglevel = debuglevel

    def set_http_debuglevel(self, level):
        self._debuglevel = level

    def do_request_(self, request):
        host = request.get_host()
        if not host:
            raise URLError('no host given')

        if request.has_data():  # POST
            data = request.get_data()
            if not request.has_header('Content-type'):
                request.add_unredirected_header(
                    'Content-type',
                    'application/x-www-form-urlencoded')
            if not request.has_header('Content-length'):
                request.add_unredirected_header(
                    'Content-length', '%d' % len(data))

        scheme, sel = splittype(request.get_selector())
        sel_host, sel_path = splithost(sel)
        if not request.has_header('Host'):
            request.add_unredirected_header('Host', sel_host or host)
        for name, value in self.parent.addheaders:
            name = name.capitalize()
            if not request.has_header(name):
                request.add_unredirected_header(name, value)

        return request

    def do_open(self, http_class, req):
        """Return an addinfourl object for the request, using http_class.

        http_class must implement the HTTPConnection API from httplib.
        The addinfourl return value is a file-like object.  It also
        has methods and attributes including:
            - info(): return a mimetools.Message object for the headers
            - geturl(): return the original request URL
            - code: HTTP status code
        """
        host = req.get_host()
        if not host:
            raise URLError('no host given')

        h = http_class(host) # will parse host:port
        h.set_debuglevel(self._debuglevel)

        headers = dict(req.headers)
        headers.update(req.unredirected_hdrs)
        # We want to make an HTTP/1.1 request, but the addinfourl
        # class isn't prepared to deal with a persistent connection.
        # It will try to read all remaining data from the socket,
        # which will block while the server waits for the next request.
        # So make sure the connection gets closed after the (only)
        # request.
        headers["Connection"] = "close"
        try:
            h.request(req.get_method(), req.get_selector(), req.data, headers)
            r = h.getresponse()
        except socket.error, err: # XXX what error?
            raise URLError(err)

        # Pick apart the HTTPResponse object to get the addinfourl
        # object initialized properly.

        # Wrap the HTTPResponse object in socket's file object adapter
        # for Windows.  That adapter calls recv(), so delegate recv()
        # to read().  This weird wrapping allows the returned object to
        # have readline() and readlines() methods.

        # XXX It might be better to extract the read buffering code
        # out of socket._fileobject() and into a base class.

        r.recv = r.read
        fp = socket._fileobject(r)

        resp = addinfourl(fp, r.msg, req.get_full_url())
        resp.code = r.status
        resp.msg = r.reason
        return resp


class HTTPHandler(AbstractHTTPHandler):

    def http_open(self, req):
        return self.do_open(httplib.HTTPConnection, req)

    http_request = AbstractHTTPHandler.do_request_

if hasattr(httplib, 'HTTPS'):
    class HTTPSHandler(AbstractHTTPHandler):

        def https_open(self, req):
            return self.do_open(httplib.HTTPSConnection, req)

        https_request = AbstractHTTPHandler.do_request_

class HTTPCookieProcessor(BaseHandler):
    def __init__(self, cookiejar=None):
        import cookielib
        if cookiejar is None:
            cookiejar = cookielib.CookieJar()
        self.cookiejar = cookiejar

    def http_request(self, request):
        self.cookiejar.add_cookie_header(request)
        return request

    def http_response(self, request, response):
        self.cookiejar.extract_cookies(response, request)
        return response

    https_request = http_request
    https_response = http_response

class UnknownHandler(BaseHandler):
    def unknown_open(self, req):
        type = req.get_type()
        raise URLError('unknown url type: %s' % type)

def parse_keqv_list(l):
    """Parse list of key=value strings where keys are not duplicated."""
    parsed = {}
    for elt in l:
        k, v = elt.split('=', 1)
        if v[0] == '"' and v[-1] == '"':
            v = v[1:-1]
        parsed[k] = v
    return parsed

def parse_http_list(s):
    """Parse lists as described by RFC 2068 Section 2.

    In particular, parse comma-separated lists where the elements of
    the list may include quoted-strings.  A quoted-string could
    contain a comma.  A non-quoted string could have quotes in the
    middle.  Neither commas nor quotes count if they are escaped.
    Only double-quotes count, not single-quotes.
    """
    res = []
    part = ''

    escape = quote = False
    for cur in s:
        if escape:
            part += cur
            escape = False
            continue
        if quote:
            if cur == '\\':
                escape = True
                continue
            elif cur == '"':
                quote = False
            part += cur
            continue

        if cur == ',':
            res.append(part)
            part = ''
            continue

        if cur == '"':
            quote = True

        part += cur

    # append last part
    if part:
        res.append(part)

    return [part.strip() for part in res]

class FileHandler(BaseHandler):
    # Use local file or FTP depending on form of URL
    def file_open(self, req):
        url = req.get_selector()
        if url[:2] == '//' and url[2:3] != '/':
            req.type = 'ftp'
            return self.parent.open(req)
        else:
            return self.open_local_file(req)

    # names for the localhost
    names = None
    def get_names(self):
        if FileHandler.names is None:
            try:
                FileHandler.names = (socket.gethostbyname('localhost'),
                                    socket.gethostbyname(socket.gethostname()))
            except socket.gaierror:
                FileHandler.names = (socket.gethostbyname('localhost'),)
        return FileHandler.names

    # not entirely sure what the rules are here
    def open_local_file(self, req):
        import email.Utils
        import mimetypes
        host = req.get_host()
        file = req.get_selector()
        localfile = url2pathname(file)
        stats = os.stat(localfile)
        size = stats.st_size
        modified = email.Utils.formatdate(stats.st_mtime, usegmt=True)
        mtype = mimetypes.guess_type(file)[0]
        headers = mimetools.Message(StringIO(
            'Content-type: %s\nContent-length: %d\nLast-modified: %s\n' %
            (mtype or 'text/plain', size, modified)))
        if host:
            host, port = splitport(host)
        if not host or \
           (not port and socket.gethostbyname(host) in self.get_names()):
            return addinfourl(open(localfile, 'rb'),
                              headers, 'file:'+file)
        raise URLError('file not on local host')

class FTPHandler(BaseHandler):
    def ftp_open(self, req):
        import ftplib
        import mimetypes
        host = req.get_host()
        if not host:
            raise IOError, ('ftp error', 'no host given')
        host, port = splitport(host)
        if port is None:
            port = ftplib.FTP_PORT
        else:
            port = int(port)

        # username/password handling
        user, host = splituser(host)
        if user:
            user, passwd = splitpasswd(user)
        else:
            passwd = None
        host = unquote(host)
        user = unquote(user or '')
        passwd = unquote(passwd or '')

        try:
            host = socket.gethostbyname(host)
        except socket.error, msg:
            raise URLError(msg)
        path, attrs = splitattr(req.get_selector())
        dirs = path.split('/')
        dirs = map(unquote, dirs)
        dirs, file = dirs[:-1], dirs[-1]
        if dirs and not dirs[0]:
            dirs = dirs[1:]
        try:
            fw = self.connect_ftp(user, passwd, host, port, dirs)
            type = file and 'I' or 'D'
            for attr in attrs:
                attr, value = splitvalue(attr)
                if attr.lower() == 'type' and \
                   value in ('a', 'A', 'i', 'I', 'd', 'D'):
                    type = value.upper()
            fp, retrlen = fw.retrfile(file, type)
            headers = ""
            mtype = mimetypes.guess_type(req.get_full_url())[0]
            if mtype:
                headers += "Content-type: %s\n" % mtype
            if retrlen is not None and retrlen >= 0:
                headers += "Content-length: %d\n" % retrlen
            sf = StringIO(headers)
            headers = mimetools.Message(sf)
            return addinfourl(fp, headers, req.get_full_url())
        except ftplib.all_errors, msg:
            raise IOError, ('ftp error', msg), sys.exc_info()[2]

    def connect_ftp(self, user, passwd, host, port, dirs):
        fw = ftpwrapper(user, passwd, host, port, dirs)
##        fw.ftp.set_debuglevel(1)
        return fw

class CacheFTPHandler(FTPHandler):
    # XXX would be nice to have pluggable cache strategies
    # XXX this stuff is definitely not thread safe
    def __init__(self):
        self.cache = {}
        self.timeout = {}
        self.soonest = 0
        self.delay = 60
        self.max_conns = 16

    def setTimeout(self, t):
        self.delay = t

    def setMaxConns(self, m):
        self.max_conns = m

    def connect_ftp(self, user, passwd, host, port, dirs):
        key = user, host, port, '/'.join(dirs)
        if key in self.cache:
            self.timeout[key] = time.time() + self.delay
        else:
            self.cache[key] = ftpwrapper(user, passwd, host, port, dirs)
            self.timeout[key] = time.time() + self.delay
        self.check_cache()
        return self.cache[key]

    def check_cache(self):
        # first check for old ones
        t = time.time()
        if self.soonest <= t:
            for k, v in self.timeout.items():
                if v < t:
                    self.cache[k].close()
                    del self.cache[k]
                    del self.timeout[k]
        self.soonest = min(self.timeout.values())

        # then check the size
        if len(self.cache) == self.max_conns:
            for k, v in self.timeout.items():
                if v == self.soonest:
                    del self.cache[k]
                    del self.timeout[k]
                    break
            self.soonest = min(self.timeout.values())

class GopherHandler(BaseHandler):
    def gopher_open(self, req):
        # XXX can raise socket.error
        import gopherlib  # this raises DeprecationWarning in 2.5
        host = req.get_host()
        if not host:
            raise GopherError('no host given')
        host = unquote(host)
        selector = req.get_selector()
        type, selector = splitgophertype(selector)
        selector, query = splitquery(selector)
        selector = unquote(selector)
        if query:
            query = unquote(query)
            fp = gopherlib.send_query(selector, query, host)
        else:
            fp = gopherlib.send_selector(selector, host)
        return addinfourl(fp, noheaders(), req.get_full_url())