Handling Extra Request Parameters With CherryPy Filters

Table of Contents

• Handling Extra Request Parameters With CherryPy Filters
  • The Problem
  • The Solution
  • The Filter Class
  • Adding the Filter to the Controller

The Problem

You want to add the same GET/POST parameter to each controller method, but using **kwargs on every controller method is cumbersome. Imagine you want to be able to change the “skin” variable on any controller.

The Solution

Using CherryPy filters you can intercept the request before it reaches the controller method and add/remove parameters from the request object. You could then, for example, put this request into the session.

So the way to go about this is:

1. Write a CherryPy filter.

2. Append the filter to a controller.

   Important

   Note that the filter will be also active for all sub-controllers inside that controller!

The Filter Class

from cherrypy.filters.basefilter import BaseFilter
from cherrypy import request, session

class SkinFilter(BaseFilter):
    """If the request parameters contain a "skin" variable, pull it out of
    the request and put it into the session.

    """
    __detected_skin = None

    def before_request_body(self):
        try:
            if "skin" in request.params:
                skin = request.params.pop("skin")
                if skin is not None:
                    self.__detected_skin = skin
        except Exception, exc:
           log.warning("Error in %s:\n%s", self.__class__.__name__, exc)

    def before_main(self):
        if self.__detected_skin is not None:
            session['skin'] = self.__detected_skin

Adding the Filter to the Controller

Top activate the filter, you have to add it to the _cp_filters attribute (a list) of your root controller:

class Root(controllers.RootController):

    # attach cherrypy filters
    _cp_filters = [SkinFilter()]

   ...

The filter can also be attached to non-root controllers so only parts of your application are affected by the filter.