I am working on a project (nevow + twisted) where lots of statistical computing is done using “R”, interfaced by rpy.py. It works very well. To be able to parameterize the computing, I defined an additional namespace and put the parameters with this namespace inside the nevow templates. It looks like:


<html xmlns:nevow="http://nevow.com/ns/nevow/0.1"



         xml:lang="de" lang="de">


  <meta content="text/html; charset=ISO-8859-1" http-equiv="content-type" />




    new_namespace:title="Fehlerklasse 2 (letzte 50)"


    new_namespace:fields="measurement_id gravity"



    new_namespace:sql="class = 2 ORDER BY _id DESC LIMIT 50"








The parameters are extracted like this:


    def data_Measurement (self, context, data):

        """Extracts the parameters from the attribute-list.”.


        self.html_params = {}

        self.script_params = None           

        todelete = []

        for theKey in context.tag.attributes:

            keyFound = False

            if type(theKey) is tuple:

                if len(theKey) == 2:

                    ns = theKey[0]

                    tag = theKey[1]

                    if ns == u'http:// new_namespace.de/ns/0.1' :

                       name = tag

                       value = context.tag.attributes.get(theKey)

                       self.script_params[name] = value


                       ## Append the key to the list of keys to remove later.

                       ## Unfortunately the parameters must be cleared, so that only valid attributes

                       ## according the html-element are present in the rendering step.




        ## Delete the param-keys from the attribule-list.

        ## Otherwise the attributes would be present in the rendering and this is for the birds.


        for theKey in todelete:

            del context.tag.attributes[theKey]


In the function render_measurement() the function is called with the parameters (in this case “errorplot”) and the resulting image is rendered. This also works very well. My question is: Is there a better and simpler way to do that? And if not, why do I have to remove the additional attributes, because later on nevow gets confused about this attributes and produces a nice stack-trace?


The next question is: In the render_whatever() functions an image is produced, and I want to write the image directly with perhaps request.write(). How can this be done using nevow?