    from Common import *
    from HTTPServlet import HTTPServlet
    from WebUtils import Funcs
    from Application import EndResponse

    class HTTPContentError(Exception):
        pass

    class HTTPContent(HTTPServlet):
        """Content producing HTTP servlet.

        HTTPContent is a type of HTTPServlet that is more convenient for
        Servlets which represent content generated in response to
        GET and POST requests.  If you are generating HTML content, you
        you probably want your servlet to inherit from Page, which contains
        many HTML-related convenience methods.

        If you are generating non-HTML content, it is appropriate to inherit
        from this class directly.

        Subclasses typically override defaultAction().

        In `awake`, the page sets self attributes: `_transaction`,
        `_response` and `_request` which subclasses should use as
        appropriate.

        For the purposes of output, the `write` and `writeln`
        convenience methods are provided.

        If you plan to produce HTML content, you should start by looking
        at Page instead of this lower-level class.

        """

        ## Transactions ##

        def awake(self, transaction):
            """Let servlet awake.

            Makes instance variables from the transaction. This is
            where Page becomes unthreadsafe, as the page is tied to
            the transaction. This is also what allows us to
            implement functions like `write`, where you don't
            need to pass in the transaction or response.

            """
            HTTPServlet.awake(self, transaction)
            self._response    = transaction.response()
            self._request     = transaction.request()
            self._session     = None # don't create unless needed
            assert self._transaction is not None
            assert self._response    is not None
            assert self._request     is not None

        def respondToGet(self, transaction):
            """Respond to GET.

            Invoked in response to a GET request method. All methods
            are passed to `_respond`.
            """
            self._respond(transaction)

        def respondToPost(self, transaction):
            """Respond to POST.

            Invoked in response to a POST request method. All methods
            are passed to `_respond`.

            """
            self._respond(transaction)

        def _respond(self, transaction):
            """Respond to action.

            Handles actions if an ``_action_`` or ``_action_name`` field is
            defined, otherwise invokes `writeHTML`. This implementation makes
            sure that exactly one action per request is handled. ``_action_``
            takes precedence over ``_action_name``; and if there are multiple
            ``action_name`` fields, the precedence is given by the order of
            the names in the actions() method. If no action field matches,
            the default action is run. The value of the ``_action_`` field
            is transformed to a method name using the methodNameForAction(),
            whereas ``name`` in ``_action_name`` is left unchanged.

            Invoked by both `respondToGet` and `respondToPost`.

            """
            req = transaction.request()
            prefix = self._actionPrefix
            if prefix:
                # First check whether there is an _action_ field:
                if req.hasField(prefix):
                    action = self.methodNameForAction(req.field(prefix))
                    if action in self.actions():
                        self.handleAction(action)
                        return
                # Next, check whether there is an _acion_name field:
                for action in self.actions():
                    name = prefix + action
                    if req.hasField(name) or (req.hasField(name + '.x')
                            and req.hasField(name + '.y')):
                        self.handleAction(action)
                        return
                # If no action was found, run the default:
            self.defaultAction()

        def defaultAction(self):
            """Default action.

            The core method that gets called as a result of requests.
            Subclasses should override this.

            """
            pass

        def sleep(self, transaction):
            """Let servlet sleep again.

            We unset some variables. Very boring.

            """
            self._session = None
            self._request  = None
            self._response = None
            self._transaction = None
            HTTPServlet.sleep(self, transaction)

        ## Access ##

        def application(self):
            """The `Application` instance we're using."""
            return self._transaction.application()

        def transaction(self):
            """The `Transaction` we're currently handling."""
            return self._transaction

        def request(self):
            """The request (`HTTPRequest`) we're handling."""
            return self._request

        def response(self):
            """The response (`HTTPResponse`) we're handling."""
            return self._response

        def session(self):
            """The session object.

            This provides a state for the current user
            (associated with a browser instance, really).
            If no session exists, then a session will be created.

            """
            if not self._session:
                self._session = self._transaction.session()
            return self._session

        ## Writing ##

        def write(self, *args):
            """Write to output.

            Writes the arguments, which are turned to strings (with `str`)
            and concatenated before being written to the response.
            Unicode strings must be encoded before they can be written.

            """
            for arg in args:
                self._response.write(arg)

        def writeln(self, *args):
            """Write to output with newline.

            Writes the arguments (like `write`), adding a newline after.
            Unicode strings must be encoded before they can be written.

            """
            for arg in args:
                self._response.write(arg)
            self._response.write('\n')

        ## Threading ##

        def canBeThreaded(self):
            """Declares whether servlet can be threaded.

            Returns 0 because of the instance variables we set up in `awake`.
            """
            return 0

        ## Actions ##

        _actionPrefix = '_action_'

        def handleAction(self, action):
            """Handle action.

            Invoked by `_respond` when a legitimate action has
            been found in a form. Invokes `preAction`, the actual
            action method and `postAction`.

            Subclasses rarely override this method.

            """
            self.preAction(action)
            getattr(self, action)()
            self.postAction(action)

        def actions(self):
            """The allowed actions.

            Returns a list of method names that are allowable
            actions from HTML forms. The default implementation
            returns [].  See `_respond` for more about actions.

            """
            return []

        def preAction(self, actionName):
            """Things to do before action.

            Invoked by self prior to invoking a action method.
            The `actionName` is passed to this method,
            although it seems a generally bad idea to rely on
            this. However, it's still provided just in case you
            need that hook.

            By default this does nothing.
            """
            pass

        def postAction(self, actionName):
            """Things to do after action.

            Invoked by self after invoking a action method.
            Subclasses may override to
            customize and may or may not invoke super as they see
            fit. The `actionName` is passed to this method,
            although it seems a generally bad idea to rely on
            this. However, it's still provided just in case you
            need that hook.

            By default this does nothing.

            """
            pass

        def methodNameForAction(self, name):
            """Return method name for an action name.

            Invoked by _respond() to determine the method name for a given action
            name which has been derived as the value of an ``_action_`` field.
            Since this is usually the label of an HTML submit button in a form,
            it is often needed to transform it in order to get a valid method name
            (for instance, blanks could be replaced by underscores and the like).
            This default implementation of the name transformation is the identity,
            it simply returns the name. Subclasses should override this method
            when action names don't match their method names; they could "mangle"
            the action names or look the method names up in a dictionary.

            """
            return name

        def urlEncode(self, s):
            """Alias for `WebUtils.Funcs.urlEncode`.

            Quotes special characters using the % substitutions.

            """
            # @@: urllib.quote, or
            return Funcs.urlEncode(s)

        def urlDecode(self, s):
            """Alias for `WebUtils.Funcs.urlDecode`.

            Turns special % characters into actual characters.

            """
            return Funcs.urlDecode(s)

        def forward(self, url):
            """Forward request.

            Forwards this request to another servlet.
            See `Application.forward` for details.
            The main difference is that here you don't have
            to pass in the transaction as the first argument.

            """
            self.application().forward(self.transaction(), url)

        def includeURL(self, url):
            """Include output from other servlet.

            Includes the response of another servlet
            in the current servlet's response.
            See `Application.includeURL` for details.
            The main difference is that here you don't have
            to pass in the transaction as the first argument.

            """
            self.application().includeURL(self.transaction(), url)

        def callMethodOfServlet(self, url, method, *args, **kwargs):
            """Call a method of another servlet.

            See `Application.callMethodOfServlet` for details.
            The main difference is that here you don't have
            to pass in the transaction as the first argument.

            """
            return self.application().callMethodOfServlet(
                self.transaction(), url, method, *args, **kwargs)

        def endResponse(self):
            """End response.

            When this method is called during `awake` or
            `respond`, servlet processing will end immediately,
            and the accumulated response will be sent.

            Note that `sleep` will still be called, providing a
            chance to clean up or free any resources.

            """
            raise EndResponse

        def sendRedirectAndEnd(self, url, status=None):
            """Send redirect and end.

            Sends a redirect back to the client and ends the response.
            This is a very popular pattern.

            """
            self.response().sendRedirect(url, status)
            self.endResponse()

        def sendRedirectPermanentAndEnd(self, url):
            """Send permanent redirect and end."""
            self.response().sendRedirectPermanent(url)
            self.endResponse()

        def sendRedirectSeeOtherAndEnd(self, url):
            """Send redirect to a URL to be retrieved with GET and end.

            This is the proper method for the Post/Redirect/Get pattern.

            """
            self.response().sendRedirectSeeOther(url)
            self.endResponse()

        def sendRedirectTemporaryAndEnd(self, url):
            """Send temporary redirect and end."""
            self.response().sendRedirectTemporary(url)
            self.endResponse()

        ## Utility ##

        def sessionEncode(self, url=None):
            """Utility function to access `Session.sessionEncode`.

            Takes a url and adds the session ID as a parameter.
            This is for cases where you don't know if the client
            will accepts cookies.

            """
            if url is None:
                url = self.request().uri()
            return self.session().sessionEncode(url)

        ## Exception Reports ##

        def writeExceptionReport(self, handler):
            """Write extra information to the exception report.

            The `handler` argument is the exception handler, and
            information is written there (using `writeTitle`,
            `write`, and `writeln`).  This information is added
            to the exception report.

            See `WebKit.ExceptionHandler` for more information.

            """
            handler.writeln('''
    <p>Servlets can provide debugging information here by overriding
    <tt>writeExceptionReport()</tt>.</p><p>For example:</p>
    <pre>
        exceptionReportAttrs = 'foo bar baz'.split()
        def writeExceptionReport(self, handler):
            handler.writeTitle(self.__class__.__name__)
            handler.writeAttrs(self, self.exceptionReportAttrs)
            handler.write('any string')
    </pre>
    <p>See WebKit/ExceptionHandler.py for more information.</p>
    ''')