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Twisted Web in 60 seconds: dynamic URL dispatch
jcalderone

Welcome to the fourth installment of "Twisted Web in 60 seconds". In the previous installment, I showed how to statically configure Twisted Web to serve different content at different URLs. The goal of this installment is to show you how to do this dynamically instead. I suggest reading the previous installment if you haven't already in order to get an overview of how URLs are treated when using Twisted Web's resource APIs.

Site (the object which associates a listening server port with the HTTP implementation), Resource (a convenient base class to use when defining custom pages), and reactor (the object which implements the Twisted main loop) return once again:

  from twisted.web.server import Site
  from twisted.web.resource import Resource
  from twisted.internet import reactor

With that out of the way, here's the interesting part of this example. I'm going to define a resource which renders a whole-year calendar. The year it will render the calendar for will be the year in the request URL. So, for example, /2009 will render a calendar for 2009. So, first, here's a resource which renders a calendar for the year passed to its initializer:

  from calendar import calendar

  class YearPage(Resource):
      def __init__(self, year):
          Resource.__init__(self)
          self.year = year

      def render_GET(self, request):
          return "<html><body><pre>%s</pre></body></html>" % (calendar(self.year),)
Pretty simple - not much different from the first dynamic resource I demonstrated. Now here's the resource which handles URLs with a year in them by creating a suitable instance of this YearPage class:
  class Calendar(Resource):
    def getChild(self, name, request):
        return YearPage(int(name))
By implementing getChild here, I've just defined how Twisted Web should find children of Calendar instances when it's resolving an URL into a resource. This implementation defines all integers as the children of Calendar (and punts on error handling, more on that later).

All that's left is to create a Site using this resource as its root and then start the reactor:

  root = Calendar()
  factory = Site(root)
  reactor.listenTCP(8880, factory)
  reactor.run()

And that's all. Any resource-based dynamic URL handling is going to look basically like Calendar.getPage. Here's the full example code:

from twisted.web.server import Site
from twisted.web.resource import Resource
from twisted.internet import reactor

from calendar import calendar

class YearPage(Resource):
    def __init__(self, year):
        Resource.__init__(self)
        self.year = year

    def render_GET(self, request):
        return "<html><body><pre>%s</pre></body></html>" % (calendar(self.year),)

class Calendar(Resource):
  def getChild(self, name, request):
      return YearPage(int(name))

root = Calendar()
factory = Site(root)
reactor.listenTCP(8880, factory)
reactor.run()

Next time I'll talk about what to do when Firefox requests /favicon.ico from your web app and you don't have one to serve... (ie, error handling).


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