    import traceback

    from Common import *

    class Transaction(Object):
        """The Transaction container.

        A transaction serves as:

            * A container for all objects involved in the transaction. The
              objects include application, request, response, session and
              servlet.

            * A message dissemination point. The messages include awake(),
              respond() and sleep().

        When first created, a transaction has no session. However, it will
        create or retrieve one upon being asked for session().

        The life cycle of a transaction begins and ends with Application's
        dispatchRequest().

        """

        ## Init ##

        def __init__(self, application, request=None):
            Object.__init__(self)
            self._application = application
            self._request = request
            self._response = None
            self._session = None
            self._servlet = None
            self._error = None
            self._nested = 0

        def __repr__(self):
            names = self.__dict__.keys()
            names.sort()
            s = []
            for name in names:
                attr = getattr(self, name)
                if isinstance(attr, Object) or isinstance(attr, Exception):
                    s.append('%s=%r' % (name, attr))
            s = ' '.join(s)
            return '<%s %s>' % (self.__class__.__name__, s)

        ## Access ##

        def application(self):
            return self._application

        def request(self):
            return self._request

        def response(self):
            return self._response

        def setResponse(self, response):
            self._response = response

        def hasSession(self):
            """Return true if the transaction has a session."""
            id = self._request.sessionId()
            return id and self._application.hasSession(id)

        def session(self):
            """Return the session for the transaction.

            A new transaction is created if necessary. Therefore, this method
            never returns None. Use hasSession() if you want to find out if
            a session already exists.

            """
            if not self._session:
                self._session = self._application.createSessionForTransaction(self)
                self._session.awake(self) # give the new servlet a chance to set up
            return self._session

        def setSession(self, session):
            self._session = session

        def servlet(self):
            """Return the current servlet that is processing.

            Remember that servlets can be nested.

            """
            return self._servlet

        def setServlet(self, servlet):
            self._servlet = servlet
            if servlet and self._request:
                servlet._serverSidePath = self._request.serverSidePath()

        def duration(self):
            """Return the duration, in seconds, of the transaction.

            This is basically the response end time minus the request start time.

            """
            return self._response.endTime() - self._request.time()

        def errorOccurred(self):
            """Check whether a server error occured."""
            return isinstance(self._error, Exception)

        def error(self):
            """Return Exception instance if there was any."""
            return self._error

        def setError(self, err):
            """Set Exception instance.

            Invoked by the application if an Exception is raised to the
            application level.

            """
            self._error = err

        ## Transaction stages ##

        def awake(self):
            """Send awake() to the session (if there is one) and the servlet.

            Currently, the request and response do not partake in the
            awake()-respond()-sleep() cycle. This could definitely be added
            in the future if any use was demonstrated for it.

            """
            if not self._nested and self._session:
                self._session.awake(self)
            self._servlet.awake(self)
            self._nested += 1

        def respond(self):
            if self._session:
                self._session.respond(self)
            self._servlet.respond(self)

        def sleep(self):
            """Send sleep() to the session and the servlet.

            Note that sleep() is sent in reverse order as awake()
            (which is typical for shutdown/cleanup methods).

            """
            self._nested -= 1
            self._servlet.sleep(self)
            if not self._nested and self._session:
                self._session.sleep(self)
                self._application.sessions().storeSession(self._session)

        ## Debugging ##

        def dump(self, file=None):
            """Dump debugging info to stdout."""
            if file is None:
                file = sys.stdout
            wr = file.write
            wr('>> Transaction: %s\n' % self)
            for attr in dir(self):
                wr('%s: %s\n' % (attr, getattr(self, attr)))
            wr('\n')

        ## Die ##

        def die(self):
            """End transaction.

            This method should be invoked when the entire transaction is
            finished with. Currently, this is invoked by AppServer. This method
            removes references to the different objects in the transaction,
            breaking cyclic reference chains and allowing either older versions
            of Python to collect garbage, or newer versions to collect it faster.

            """
            for name in self.__dict__.keys():
                attr = getattr(self,  name)
                if isinstance(attr, Object) and not name.startswith('_app'):
                    attr.resetKeyBindings()
                delattr(self, name)

        ## Exception handling ##

        _exceptionReportAttrNames = \
            'application request response session servlet'.split()

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

            for name in self._exceptionReportAttrNames:
                obj = getattr(self, '_' + name, None)
                if obj:
                    try:
                        obj.writeExceptionReport(handler)
                    except Exception:
                        handler.writeln('<p>Uncaught exception while asking'
                            ' <b>%s</b> to write report:</p>\n<pre>' % name)
                        traceback.print_exc(file=handler)
                        handler.writeln('</pre>')