The Django Web Application Framework

The Django Web Application Framework Simon Willison http://simonwillison.net/ ACCU, Python Track, 20th April 2006

This talk An overview of Django What's in a web framework, anyway? Framework comparison (and some pretty screenshots, too)

Origins

Lawrence, Kansas - 2003

Web development on Journalism deadlines

... in three days

The ideal framework... Clean URLs Loosely coupled components Designer-friendly templates As little code as possible Really fast development

We didn’t mean to build a framework...

... but nothing else really fitted the bill

Building a framework

HTTP handling

GET /community/ HTTP/1.0 Host: www.djangoproject.com HTTP/1.1 200 OK Date: Wed, 19 Apr 2006 23:53:29 GMT Server: Apache Vary: Cookie Connection: close Content-Type: text/html; charset=utf-8 <!DOCTYPE html PUBLIC \"-//W3C//DTD XHTML 1.0 Transitional//EN\" \"http://www.w3.org/TR/xhtml1/DTD/xhtml1- transitional.dtd\"> <html xmlns=\"http://www.w3.org/1999/xhtml\" xml:lang=\"en\" lang=\"en\"> <head> [...]

Parse the request; generate a response

GET variables, POST variables, cookies, uploaded files, caching, content negotiation

HttpRequest HttpResponse

def index(request): s = “Hello, World” return HttpResponse(s)

def index(request): r = HttpResponse( mimetype='text/plain' ) r.write(“Hello, World”) return r

def index(request): if request.GET: s = “Hi, %s” % \\ escape(request.GET.get( ’name’, ‘anon’) else: s = “Hello, World” return HttpResponse(s)

URL dispatching

http://www.example.com/poll/5/ What code shall we execute?

urlpatterns = patterns('', (r'^$', 'views.index'), (r'^hello/', ‘views.hello'), (r'^poll/(\\d+)/', ‘views.poll'), )

Database access

... (r'^poll/(\\d+)/', 'views.poll'), ... def poll(request, poll_id): poll = Poll.objects.get( pk=poll_id ) return HttpResponse( 'Title: ' + poll.title )

class Poll(Model): question = CharField(maxlength=200) pub_date = DateTimeField('date published') class Choice(Model): poll = ForeignKey(Poll) choice = CharField(maxlength=200) votes = IntegerField()

BEGIN; CREATE TABLE \"polls_poll\" ( \"id\" serial NOT NULL PRIMARY KEY, \"question\" varchar(200) NOT NULL, \"pub_date\" timestamp with time zone NOT NULL ); CREATE TABLE \"polls_choice\" ( \"id\" serial NOT NULL PRIMARY KEY, \"poll_id\" integer NOT NULL REFERENCES \"polls_polls\" (\"id\"), \"choice\" varchar(200) NOT NULL, \"votes\" integer NOT NULL ); COMMIT;

p = Poll( question = \"What's up?\", pub_date = datetime.now() ) p.save()

>>> p.id 1 >>> p.question \"What's up?\" >>> p.pub_date datetime(2005, 7, 15, 12, 00, 53)

Templating

Gluing strings together gets old fast

{ 'today': datetime.date.today(), 'edibles': ['pear', 'apple', 'orange'] }

<h1>Hello World!</h1> <p>Today is {{ today|date:”jS F, Y” }}</p> {% if edibles %} <ul> {% for fruit in edibles %} <li>{{ fruit }}</li> {% endfor %} </ul> {% endif %}

<h1>Hello World!</h1> <p>Today is 20th April, 2006</p> <ul> <li>pear</li> <li>apple</li> <li>orange</li> </ul>

def hello(request): t = get_template('hello.html') c = Context({ 'today': datetime.date.today(), 'edibles': ['pear', 'apple', 'orange'] }) return HttpResponse(t.render(c))

All you really need are variables, conditionals and loops

Essential ingredients HTTP handling URL dispatching Templating Documentation Database access (optional) ... no wonder there are so many frameworks!

Extras

Forms are boring

1. Display form 2. Validate submitted data 3. If errors, redisplay with: 3.1. Contextual error messages 3.2. Correct fields pre-filled 4. ... do something useful!

Model validation rules + the Manipulator API do all of this for you

django.contrib.admin does even more

class Poll(Model): question = CharField(maxlength=200) pub_date = DateTimeField('date published') class Admin: list_display = ('question', 'pub_date') class Choice(Model): poll = ForeignKey(Poll) choice = CharField(maxlength=200) votes = IntegerField() class Admin: pass

Smarter templating

Common headers and footers?

Template inheritance

home.html {% extends “base.html” %} {% block title %}Homepage{% endblock %} {% block main %} Main page contents goes here. {% endblock %}

Custom template tags

{{ page.content|textile }} {% comment_form for news.stories story.id with is_public yes photos_optional thumbs,200,400 ratings_optional scale:1-5|first_option|second_option %}

from django import template register = template.Library() def textile(value): try: import textile except ImportError: return value else: return textile.textile(value) register.filter(textile)

i18n and l10n

Bengali Japanese Czech Dutch Welsh Norwegian Danish Brazilian German Romanian Greek Russian English Slovak Spanish Slovenian French Serbian Galician Swedish Hebrew Ukrainian Icelandic Simpliﬁed Chinese Italian Traditional Chinese

Authentication and authorisation

Community

?

It’s a matter of taste

HTTP handling What happens to form variables? GET vs POST How to handle /path?foo=1&foo=2 How to send back non-standard responses Different Content-Type (and other) headers 404s, 500s Session support?

URL dispatching Object traversal or explicit configuration? Reversible URLs? Parameter extraction Trailing slashes?

Database handling To ORM or not to ORM? Pluralisation? Handling joins Lookup syntax When should you fall back on raw SQL?

Templating Plain text or XML? Markup visible on the page or hidden in HTML attributes? Logic in the template vs logic in the view/ controller Safe (and limited) or unsafe (and more powerful) Language extension mechanism?

Where’s the line? Authentication and authorisation? Automated admin / forms? i18n and l10n? JavaScript and Ajax?

Getting involved

Summer of Code 2006

www.djangoproject.com

Extra slides

Comparisons

$_GET, $_POST, HTTP $_REQUEST etc URL dispatch Filesystem Database handling mysql_* functions Templating <?php ... ?>

HTTP CherryPy Object path URL dispatch traversal Database handling SQLObject Kid (XML), Templating common templating API

HTTP ActionController? Object traversal, URL dispatch Routes Database handling ActiveRecord Templating ERb, Builder, RJS

The Django Web Application Framework

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