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# Web Application Development

## Code layout

The development of a Twisted Web application should be orthogonal to its deployment. This means is that if you are developing a web application, it should be a resource with children, and internal links. Some of the children might use Nevow , some might be resources manually using `.write` , and so on. Regardless, the code should be in a Python module, or package, outside the web tree.

You will probably want to test your application as you develop it. There are many ways to test, including dropping an `.rpy` which looks like:

    from mypackage import toplevel
    resource = toplevel.Resource(file="foo/bar", color="blue")

into a directory, and then running:

    % twistd web --path=/directory

You can also write a Python script like:

    #!/usr/bin/env python

    from twisted.web import server
    from twisted.internet import reactor
    from mypackage import toplevel

    reactor.listenTCP(8080,
        server.Site(toplevel.Resource(file="foo/bar", color="blue")))
    reactor.run()

## Web application deployment

Which one of these development strategies you use is not terribly important, since (and this is the important part) deployment is orthogonal . Later, when you want users to actually use your code, you should worry about what to do – or rather, don’t. Users may have widely different needs. Some may want to run your code in a different process, so they’ll use distributed web (twisted.web.distrib ). Some may be using the `twisted-web` Debian package, and will drop in:

    % cat > /etc/local.d/99addmypackage.py
    from mypackage import toplevel
    default.putChild("mypackage", toplevel.Resource(file="foo/bar", color="blue"))
    ^D

If you want to be friendly to your users, you can supply many examples in your package, like the above `.rpy` and the Debian-package drop-in. But the ultimate friendliness is to write a useful resource which does not have deployment assumptions built in.

## Understanding resource scripts (`.rpy` files)

Twisted Web is not PHP – it has better tools for organizing code Python modules and packages, so use them. In PHP, the only tool for organizing code is a web page, which leads to silly things like PHP pages full of functions that other pages import, and so on. If you were to write your code this way with Twisted Web, you would do web development using many `.rpy` files, all importing some Python module. This is a bad idea – it mashes deployment with development, and makes sure your users will be tied to the file-system.

We have `.rpy` s because they are useful and necessary. But using them incorrectly leads to horribly unmaintainable applications. The best way to ensure you are using them correctly is to not use them at all, until you are on your final deployment stages. You should then find your `.rpy` files will be less than 10 lines, because you will not have more than 10 lines to write.