Nevow: A Web Application Construction Kit

# A Web Application Construction Kit

## Summary

Nevow is a next-generation web application templating system, based on the ideas developed in the Twisted Woven package. Its main focus is on separating the HTML template from both the business logic and the display logic, while allowing the programmer to write pure Python code as much as possible. It separates your code into 'data' and 'render' functions, a simplified implementation of traditional MVC. It has various parts which can be used individually or as a whole, integrated web solution:

- XHTML templates: contain no programming logic, only nodes tagged with nevow attributes
- data/render methods: simplified MVC
- stan: An s-expression-like syntax for expressing xml in pure python
- formless: For describing the types of objects which may be passed to methods of your classes, validating and coercing string input from either web or command-line sources, and calling your methods automatically once validation passes
- formless.webform: For rendering web forms based on formless type descriptions, accepting form posts and passing them to formless validators, and rendering error forms in the event validation fails
- livepage: Cross-browser JavaScript glue for sending client side events to the server and server side events to the client after the page has loaded, without causing the entire page to refresh

## Disk based templates

Nevow includes the ability to load templates off disk. These templates may have processing directives which cause the execution of python methods at render time. The attribute technique was inspired by the attributes used by ZPT. However, no actual code may be embedded in the HTML template:

    <html xmlns:nevow="http://nevow.com/ns/nevow/0.1">
      <head>
        <title>Greetings!</title>
      </head>
      <body>
        <h1 style="font-size: large">Now I will greet you:</h1>
        <span nevow:render="greet" />
      </body>
    </html>

This template can then be loaded and rendered like so:

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    class Greeter(rend.Page):
        docFactory = loaders.xmlfile("Greeting.html")

        def render_greet(self, context, data):
            return random.choice(["Hello", "Greetings", "Hi"]), " ", data

    Greeter("My name is").renderString()

## data/render methods

To allow clean isolation between code which fetches data from a data source and code which renders the data into HTML, nevow allows you to write both 'data' methods and 'render' methods. These concepts are inspired by MVC, but simpler, since the framework can handle most of the controller aspect. An example:

    <html xmlns:nevow="http://nevow.com/ns/nevow/0.1">
      <body>
        <span nevow:data="name" nevow:render="colorful" />
        <span nevow:data="fun" nevow:render="colorful" />
      </body>
    </html>

This template can be loaded and rendered using a class such as this:

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    class Colorful(rend.Page):
        docFactory = loaders.xmlfile("Colorful.html")

        def render_colorful(self, context, data):
            color = random.choice(['red', 'green', 'blue'])
            return context.tag(style="color: %s" % color)

        def data_name(self, context, data):
            return "Your name here"

        def data_fun(self, context, data):
            return "Are we having fun yet?"

## Stan

One of the most powerful things about nevow is stan, an s-expression-like syntax for producing XML fragments in pure Python syntax. Stan is not required for using nevow, but it is both a simple and powerful way to both lay out one's XHTML templates and express one's display logic. A brief example will illustrate its utility:

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    import random
    from nevow import rend, tags

    class Greeter(rend.Page):
        def greet(self, context, data):
            return random.choice(["Hello", "Greetings", "Hi"]), " ", data

        docFactory = loaders.stan(
            tags.html[
            tags.head[ tags.title[ "Greetings!" ]],
            tags.body[
                tags.h1(style="font-size: large")[ "Now I will greet you:" ],
                greet
            ]
        ])

When the Greeter class is constructed, it is passed a Python object which will be used as that page's data:

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    Greeter("Your name here").renderString()

## Formless

Python is dynamically typed, which means it has no built-in controls for enforcing the types of objects which are passed to one's methods. This is great for programmers, but not necessarily great if you are going to be passing user-entered input to those methods. Formless is a simple way to describe the types of objects that can be passed to one's methods, as well as coerce from string input to those types. Other code can then accept user input from a command line or from a web form, validate the input against the types described using formless, and call the method once validation has passed. A simple example:

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    from zope.interface import implements
    from formless.annotate import TypedInterface, Integer, String

    class ISimpleMethod(TypedInterface):
        def simple(self,
                   name=String(description="Your name."),
                   age=Integer(description="Your age.")):
            """
            Simple

            Please enter your name and age.
            """

    class Implementation(object):
        implements(ISimpleMethod)

        def simple(self, name, age):
            print "Hello, %s, who is %s" % (name, age)

## Webform

Webform is a nevow module which will automatically render web forms and accept form posts based on types described using the classes in formless. Used in conjunction with the twisted.web HTTP server, the process is almost automatic:

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    from nevow import rend, tags
    from formless import webform

    class WebForm(rend.Page):
        document = rend.stan(
        tags.html[
        tags.body[
            h1["Here is the form:"],
            webform.renderForms('original')
        ]
    ])

    resource = WebForm(Implementation())

Exposing this resource instance to the web using twisted.web and visiting it will cause a form with two input boxes to be rendered. Posting the form will cause form validation to occur. Upon error, the user will be returned to the original page, with the form annotated with error messages. Upon success, the "simple" method of the Implementation instance will be called and passed a string and an integer.

## LivePage

LivePage was a Woven technology which allowed programmers to receive server-side notification of client-side JavaScript events, and to send JavaScript to the client in response to a server-side event. New for Nevow 0.3, LivePage has been updated to support Mozilla, Firefox, IE6 Win, and Safari. Using LivePage is very easy:

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    from nevow.liveevil import handler

    def greeter(client, nodeName):
        client.alert("Greetings. You clicked the %s node." % nodeName)

    # Any string arguments after the event handler function will be evaluated
    # as JavaScript in the context of the web browser and results passed to the
    # Python event handler
    handler = handler(greeter, 'node.name')

    class Live(rend.Page):
        docFactory = loaders.stan(
            tags.html[
            tags.body[
                ol[
                    li(onclick=handler, name="one")["One"]
                    li(onclick=handler, name="two")["Two"]
                    li(onclick=handler, name="three")["Three"]
                ]
            ]
        ])

## More Information

The Nevow website has more information. Starting with 0.3, it contains a simple WSGI implementation and can also be used to render CGIs. However, the recommended mode of operation is using the Twisted web server. Nevow is an active project, and many new bugfixes and features are committed to the Nevow SVN repository. Information about Nevow commits is available by subscribing to the Divmod commits mailing list. The Nevow SVN repository can be checked out using:

    svn co svn://divmod.org/svn/Nevow/trunk Nevow

Discussion of Nevow occurs on the twisted.web mailing list. The Nevow developers are also often available for real-time help on the #twisted.web channel on irc.freenode.net.

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