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Twisted Web in 60 seconds: static URL dispatch
jcalderone

Welcome to the third installment of "Twisted Web in 60 seconds". The goal of this installment is to show you how to serve different content at different URLs using APIs from Twisted Web (the first and second installments covered ways in which you might want to generate this content).

Key to understanding how different URLs are handled with the resource APIs in Twisted Web is understanding that any URL can be used to address a node in a tree. Resources in Twisted Web exist in such a tree, and a request for a URL will be responded to by the resource which that URL addresses. The addressing scheme considers only the path segments of the URL. Starting with the root resource (the one used to construct the Site) and the first path segment, a child resource is looked up. As long as there are more path segments, this process is repeated using the result of the previous lookup and the next path segment. For example, to handle a request for /foo/bar, first the root's "foo" child is retrieved, then that resource's "bar" child is retrieved, then that resource is used to create the response.

With that out of the way, let's consider an example that can serve a few different resources at a few different URLs.

First things first: we need to import Site, the factory for HTTP servers, Resource, a convenient base class for custom pages, and reactor, the object which implements the Twisted main loop. We'll also import File to use as the resource at one of the example URLs.

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

Now we create a resource which will correspond to the root of the URL hierarchy: all URLs are children of this resource.

  root = Resource()

Here comes the interesting part of this example. I'm now going to create three more resources and attach them to the three URLs /foo, /bar, and /baz:

  root.putChild("foo", File("/tmp"))
  root.putChild("bar", File("/lost+found"))
  root.putChild("baz", File("/opt"))

Last, all that's required is to create a Site with the root resource, associate it with a listening server port, and start the reactor:

  factory = Site(root)
  reactor.listenTCP(8880, factory)
  reactor.run()
With this server running, http://localhost:8880/foo will serve a listing of files from /tmp, http://localhost:8880/bar will serve a listing of files from /lost+found, and http://localhost:8880/baz will serve a listing of files from /opt.

Here's the whole example uninterrupted:

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

root = Resource()
root.putChild("foo", File("/tmp"))
root.putChild("bar", File("/lost+found"))
root.putChild("baz", File("/opt"))

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

Next time I'll show you how to handle URLs dynamically. Also, hey! I want your feedback. Do you find these posts useful? Am I presenting the information clearly? Tell me about it.


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