    """Implementation of client sessions."""

    import re
    from time import time, localtime

    from MiscUtils import NoDefault
    from MiscUtils.Funcs import uniqueId

    class SessionError(Exception):
        """Client session error"""

    class Session(object):
        """Implementation of client sessions.

        All methods that deal with time stamps, such as creationTime(),
        treat time as the number of seconds since January 1, 1970.

        Session identifiers are stored in cookies. Therefore, clients
        must have cookies enabled.

        Unlike Response and Request, which have HTTP subclass versions
        (e.g., HTTPRequest and HTTPResponse respectively), Session does
        not. This is because there is nothing protocol specific in
        Session. (Is that true considering cookies? @@ 2000-04-09 ce)
        2000-04-27 ce: With regards to ids/cookies, maybe the notion
        of a session id should be part of the interface of a Request.

        Note that the session id should be a string that is valid
        as part of a filename. This is currently true, and should
        be maintained if the session id generation technique is
        modified. Session ids can be used in filenames.

        FUTURE

          * invalidate()
          * Sessions don't actually time out and invalidate themselves.
          * Should this be called 'HTTPSession'?
          * Should "numTransactions" be exposed as a method? Should it
            be common to all transaction objects that do the
            awake()-respond()-sleep() thing? And should there be an
            abstract super class to codify that?

        """

        ## Init ##

        def __init__(self, trans, identifier=None):
            self._lastAccessTime = self._creationTime = time()
            self._isExpired = self._dirty = False
            self._numTrans = 0
            self._values = {}
            app = trans.application()
            self._timeout = app.sessionTimeout(trans)
            self._prefix = app.sessionPrefix(trans)
            self._sessionName = app.sessionName(trans)

            if identifier:
                if re.search(r'[^\w\.\-]', identifier) is not None:
                    raise SessionError("Illegal characters in session identifier")
                if len(identifier) > 80:
                    raise SessionError("Session identifier too long")
                self._identifier = identifier
            else:
                attempts = 0
                while attempts < 10000:
                    self._identifier = self._prefix + ''.join(
                        map(lambda x: '%02d' % x,
                            localtime()[:6])) + '-' + uniqueId(self)
                    if not app.hasSession(self._identifier):
                        break
                    attempts += 1
                else:
                    raise SessionError(
                        "Can't create valid session id after %d attempts." % attempts)

            if app.setting('Debug')['Sessions']:
                print ('>> [session] Created session, '
                    'timeout = %s, id = %s, self = %s'
                    % (self._timeout, self._identifier, self))

        ## Access ##

        def creationTime(self):
            """Return the time when this session was created."""
            return self._creationTime

        def lastAccessTime(self):
            """Get last access time.

            Returns the last time the user accessed the session through
            interaction. This attribute is updated in awake(), which is
            invoked at the beginning of a transaction.

            """
            return self._lastAccessTime

        def identifier(self):
            """Return a string that uniquely identifies the session.

            This method will create the identifier if needed.

            """
            return self._identifier

        def isDirty(self):
            """Check whether the session is dirty (has unsaved changes)."""
            return self._dirty

        def setDirty(self, dirty=True):
            """Set the dirty status of the session."""
            self._dirty = dirty

        def isExpired(self):
            """Check whether the session has been previously expired.

            See also: expiring()

            """
            return getattr(self, '_isExpired', False) or self._timeout == 0

        def isNew(self):
            """Check whether the session is new."""
            return self._numTrans < 2

        def timeout(self):
            """Return the timeout for this session in seconds."""
            return self._timeout

        def setTimeout(self, timeout):
            """Set the timeout on this session in seconds."""
            self._timeout = timeout

        ## Invalidate ##

        def invalidate(self):
            """Invalidate the session.

            It will be discarded the next time it is accessed.

            """
            self._lastAccessTime = 0
            self._values = {}
            self._dirty = False
            self._timeout = 0

        ## Values ##

        def value(self, name, default=NoDefault):
            if default is NoDefault:
                return self._values[name]
            else:
                return self._values.get(name, default)

        def hasValue(self, name):
            return name in self._values

        def setValue(self, name, value):
            self._values[name] = value
            self._dirty = True

        def delValue(self, name):
            del self._values[name]
            self._dirty = True

        def values(self):
            return self._values

        def __getitem__(self, name):
            return self.value(name)

        def __setitem__(self, name, value):
            self.setValue(name, value)

        def __delitem__(self, name):
            self.delValue(name)

        def __contains__(self, name):
            return self.hasValue(name)

        ## Transactions ##

        def awake(self, trans):
            """Let the session awake.

            Invoked during the beginning of a transaction, giving a Session an
            opportunity to perform any required setup. The default implementation
            updates the 'lastAccessTime'.

            """
            self._lastAccessTime = time()
            self._numTrans += 1

        def respond(self, trans):
            """Let the session respond to a request.

            The default implementation does nothing, but could do something
            in the future. Subclasses should invoke super.

            """
            pass

        def sleep(self, trans):
            """Let the session sleep again.

            Invoked during the ending of a transaction, giving a Session an
            opportunity to perform any required shutdown. The default
            implementation does nothing, but could do something in the future.
            Subclasses should invoke super.

            """
            pass

        def expiring(self):
            """Let the session expire.

            Called when session is expired by the application.
            Subclasses should invoke super.
            Session store __delitem__()s should invoke if not isExpired().

            """
            self._isExpired = True

        ## Utility ##

        def sessionEncode(self, url):
            """Encode the session ID as a parameter to a url."""
            import urlparse
            url = list(urlparse.urlparse(url)) # make a list
            if url[4]:
                url[4] += '&'
            url[4] += '%s=%s' % (self._sessionName, self.identifier())
            url = urlparse.urlunparse(url)
            return url

        ## Exception reports ##

        _exceptionReportAttrNames = \
            'isDirty isExpired lastAccessTime numTrans timeout values'.split()

        def writeExceptionReport(self, handler):
            handler.writeTitle(self.__class__.__name__)
            handler.writeAttrs(self, self._exceptionReportAttrNames)