(This post is the English translation of the Dec. 24th edition of the Python Web Framework Advent Calendar 2010. Other posts can be found at: http://atnd.org/events/10465 though they will be in Japanese)

I usually use the kay framework for Appengine development as I am a developer for the framework, but recently I have been playing with the Tipfy framework (http://www.tipfy.org) written by Rodrigo Moraes. Like Kay, Tipfy is a framework made specifically for Google Appengine. While Kay has drawn a lot of it's functionality from Django, Tipfy attempts to be as close to the Appengine SDK's Webapp Framework.

Install

Tipfy uses the buildout framework to manage dependencies and deployment. Installation is covered in the Tipfy Guide's Installation Page.

First you need to download the Tipfy "Do-it-yourself pack".

$ wget http://www.tipfy.org/tipfy.zip
$ unzip tipfy.zip

Next, you will need to create the buildout environment. Buildout will download and install the Appengine SDK and everything you need to get started from pypi.

$ cd project
$ python2.5 bootstrap.py --distribute
$ ./bin/buildout

After that is over, you can run the development server by running the dev_appserver command found in the bin directory.

$ ./bin/dev_appserver

Directory Structure

At this point, you can explore the directory structure of the Tipfy project. You can find out more about this in the Tipfy Documentation.

app/ - GAE application
    apps/ - application modules
        hello_world - A default "Hello World" application
    lib/ - Third party libraries
    distlib/ - Libraries installed by buildout (buildout clears this directory every time buildout is run)
    locale/ - Translation files
    static/ - Static media files
    templates/ - HTML templates
    main.py - The main() entry point
    app.yaml - GAE application's app.yaml
    config.py - Application settings
    urls.py - URL routing
eggs/ - Python Eggs (libraries) required for development
etc/ - Settings files requried for development
    develop-eggs - Development libraries required by buildout
    downloads - Downloads (appengine SDK etc)
    parts - buildout development parts (required for buildout recipies)
bootstrap.py - buildout bootstrap script
buildout.cfg - buildout settings file
babel.cfg - translations settings file
gaetools.cfg - Settings for the GAE SDK recipe (e.g. default settings for the dev_server)
versions.cfg - Third party library versions (empty by default)

Settings

Settings are managed in the app/config.py file. These settings are contained in a python dictionary object called config. You can find out more in the Tipfy documentation for configuration.

Settings for third party modules are stored in a key specified by the module name. For instance the configuration for Tipfy's extension that provides support for sessions is stored in the key tipfy.ext.session.

apps_installed is a list of application modules. This is similar to Django in that each application can provide it's own handlers and URL routing rules. Tipfy also allows for applications to provide default configuration.

# Configurations for the 'tipfy' module.
config['tipfy'] = {
    # Enable debugger. It will be loaded only in development.
    'middleware': [
        'tipfy.ext.debugger.DebuggerMiddleware',
    ],
    # Enable the Hello, World! app example.
    'apps_installed': [
        'apps.hello_world',
    ],
}
config['tipfy.ext.session'] = {
    'secret_key': '<Secret Key>',
}

URL Routing

URL routing is specified in the app/urls.py module in the base of the project. The default urls.py will load all URL routing rules for installed applications. Applications can provide URL routing rules similarly to the base rules by implementing a function called make_rules() which returns a list of Rule objects. You can find out more by reading the Tipfy documentation on URL routing

This is what the default app/apps/hello_world/urls.py looks like:

from tipfy import Rule

def get_rules(app):
    """Returns a list of URL rules for the Hello, World! application.

    :param app:
        The WSGI application instance.
    :return:
        A list of class:`tipfy.Rule` instances.
    """
    rules = [
        Rule('/', endpoint='hello-world', handler='apps.hello_world.handlers.HelloWorldHandler'),
        Rule('/pretty', endpoint='hello-world-pretty', handler='apps.hello_world.handlers.PrettyHelloWorldHandler'),
    ]

    return rules

Request Handlers

Request handlers implement the application logic by processing HTTP requests. These request handlers are very similar to the SDK's webapp framework. Tipfy uses Jinja2 for templating by default.

from tipfy import RequestHandler, Response
from tipfy.ext.jinja2 import render_response

class HelloWorldHandler(RequestHandler):
    def get(self):
        """Simply returns a rendered template with an enigmatic salutation."""
        return render_response('hello_world.html', message='Hello, World!')

Using Request Handler Mixins

You can add functionality to request handlers by using Mixins. Mixins are classes that are meant to be extended along with other classes to allow mixing and matching of class features.

Usually you wouldn't add these mixins for every request handler but have a base request handler you use for your project that inherits from the Mixins you need for your project. However, you may have some cases where you would use mixins only for specific request handlers.

Here we will add support for session handling by adding the appropriate Mixins to our BaseHandler class:

from tipfy import RequestHandler, Response
from tipfy.ext.jinja2 import Jinja2Mixin
from tipfy.ext.session import SecureCookieMixin, SessionMixin

class BaseHandler(RequestHandler, Jinja2Mixin, SecureCookieMixin, SessionMixin):
    middleware = ['tipfy.ext.session.SessionMiddleware']

class HelloWorldHandler(BaseHandler):
    def get(self):
        """Simply returns a rendered template with an enigmatic salutation."""
        return self.render_response('hello_world.html',
            message='Hello, World!',
            somevalue=self.session.get('somevalue')
        )

Extension modules

The tipfy.ext modules is an extension namespace module which allows development of external third party extension modules. These modules can be added to your project as needed so that they don't end up cluttering the framework. For instance, handlers for receiving e-mail, and i18n support are contained in the tipfy.ext module. The extensions installed by default can be found in the app/distlib/tipfy/ext directory.

You can find out more by reading these pages in the Tipfy documentation:

  1. Extension pages
  2. Adding Or Removing Extensions
  3. Creating Extensions

Conclusion

The use of Mixins, standard python packaging and idioms might be a bit hard to understand for newcomers to Python (There are many who are using python for the first time on Appengine). However, given that appengine projects require all the needed python code to be contained within the project directory, the use of buildout to allow developers to distribute and add packages is one of Tipfy's strengths. Tipfy's use of Mixins also allows code to be divided and reused based on functionality, allowing developers to add only the required code and modules to their project.

I think that the Kay framework has a lot to learn from the Tipfy project and I suspect that we will be integrating many of the ideas in Tipfy in the future.