1. Django Heresies
Simon Willison @simonw
EuroDjangoCon http://simonwillison.net/
4th May 2009

2. http://www.flickr.com/photos/ianlloyd/264753656/

3. DJANGO IS AWESOME

4. Selling bacon on the internet

5. Pulitzer prize winning journalism

6. Saving children’s lives in Kenya

7. Heresy
“An opinion at variance with the
orthodox or accepted doctrine”

8. Templates

9. {% if %} tags SUCK

10. {% if %} tags SUCK
• Every time you {% endifnotequal %},
God kicks the Django Pony

11. {% if %} tags SUCK
• Every time you {% endifnotequal %},
God kicks the Django Pony

12. http://docs.djangoproject.com/en/dev/misc/design-philosophies/
“ Don’t invent a programming language
The template system intentionally doesn’t allow the
following:
•
Assignment to variables
•
Advanced logic
The goal is not to invent a programming language.
The goal is to offer just enough programming-esque
functionality, such as branching and looping, that is
”
essential for making presentation-related decisions.

15. {% if photo.width > 390 %}
...
{% endif %}

16. http://www.djangosnippets.org/snippets/1350/
Chris Beaven (aka SmileyChris)
'''
A smarter {% if %} tag for django templates.
While retaining current Django functionality, it also
handles equality, greater than and less than operators.
Some common case examples::
{% if articles|length >= 5 %}...{% endif %}
{% if quot;ifnotequal tagquot; != quot;beautifulquot; %}...{% endif %}
'''
{% load smartif %} replaces the Django {% if %} tag

17. http://www.djangosnippets.org/snippets/1350/
Chris Beaven (aka SmileyChris)
'''
A smarter {% if %} tag for django templates.
While retaining current Django functionality, it also
handles equality, greater than and less than operators.
Some common case examples::
{% if articles|length >= 5 %}...{% endif %}
{% if quot;ifnotequal tagquot; != quot;beautifulquot; %}...{% endif %}
'''
{% load smartif %} replaces the Django {% if %} tag

18. Silencing errors

19. • 2003: “template authors shouldn’t be able to
break the site”
• 2008: “I can't think of a single time this
feature has helped me, and plenty of
examples of times that it has tripped me up.”
• Silent {{ foo.bar }} is OK, silent tags are evil
• django-developers: http://bit.ly/silentfail

20. Project layout
http://www.flickr.com/photos/macrorain/2789698166/

21. Relocatable
TEMPLATE_DIRS = (
# Don't forget to use absolute paths,
# not relative paths.
)
import os
OUR_ROOT = os.path.realpath(
os.path.dirname(__file__)
)
...
TEMPLATE_DIRS = os.path.join(OUR_ROOT, 'templates')

22. Relocatable
TEMPLATE_DIRS = (
# Don't forget to use absolute paths,
# not relative paths.
)
import os
OUR_ROOT = os.path.realpath(
os.path.dirname(__file__)
)
...
TEMPLATE_DIRS = os.path.join(OUR_ROOT, 'templates')

23. local_settings.py
• svn:ignore local_settings.py ?
• Can’t easily test your production settings
• Configuration isn’t in source control!

24. Environments
zoo/configs/common_settings.py
zoo/configs/alpha/app.wsgi
zoo/configs/alpha/manage.py
zoo/configs/alpha/settings.py
zoo/configs/testing/app.wsgi
zoo/configs/testing/manage.py
zoo/configs/testing/settings.py

25. zoo/configs/alpha/settings.py
from zoo.configs.common_settings import *
DEBUG = True
TEMPLATE_DEBUG = DEBUG
# Database settings
DATABASE_NAME = 'zoo_alpha'
DATABASE_USER = 'zoo_alpha'

26. Reusable code
http://www.flickr.com/photos/ste3ve/521083510/

28. Generic views
def object_detail(request, queryset, object_id=None,
slug=None, slug_field='slug',
template_name=None, template_name_field=None,
template_loader=loader, extra_context=None,
context_processors=None,
template_object_name='object', mimetype=None
):

29. object_detail drawbacks
• You can’t swap the ORM for something else
(without duck typing your own queryset)
• You have to use RequestContext
• You can’t modify something added to the
context; you can only specify extra_context
• That’s despite a great deal of effort going in
to making the behaviour customisable

30. newforms-admin
• De-coupled admin from the rest of Django
• A new approach to customisation
• Powerful subclassing pattern

31. Fine grained permissions
class Entry(models.Model):
title = models.CharField(max_length=255)
author = models.ForeignKey('auth.User')
class EntryAdmin(admin.ModelAdmin):
exclude = ('author',)
def queryset(self, request):
queryset = super(EntryAdmin, self).queryset(request)
return queryset.filter(author = request.user)
def save_model(self, request, obj, form, change):
obj.author = request.user
obj.save()
def has_change_permission(self, request, axj=None):
if not obj:
return True # access to change list
return obj.author == request.user
has_delete_permission = has_change_permission
admin.site.register(Entry, EntryAdmin)

32. Fine grained permissions
class Entry(models.Model):
title = models.CharField(max_length=255)
author = models.ForeignKey('auth.User')
class EntryAdmin(admin.ModelAdmin):
exclude = ('author',)
def queryset(self, request):
queryset = super(EntryAdmin, self).queryset(request)
return queryset.filter(author = request.user)
def save_model(self, request, obj, form, change):
obj.author = request.user
obj.save()
def has_change_permission(self, request, obj=None):
if not obj:
return True # access to change list
return obj.author == request.user
has_delete_permission = has_change_permission
admin.site.register(Entry, EntryAdmin)

33. Fine grained permissions
class Entry(models.Model):
title = models.CharField(max_length=255)
author = models.ForeignKey('auth.User')
class EntryAdmin(admin.ModelAdmin):
exclude = ('author',)
def queryset(self, request):
queryset = super(EntryAdmin, self).queryset(request)
return queryset.filter(author = request.user)
def save_model(self, request, obj, form, change):
obj.author = request.user
obj.save()
def has_change_permission(self, request, obj=None):
if not obj:
return True # access to change list
return obj.author == request.user
has_delete_permission = has_change_permission
admin.site.register(Entry, EntryAdmin)

34. Fine grained permissions
class Entry(models.Model):
title = models.CharField(max_length=255)
author = models.ForeignKey('auth.User')
class EntryAdmin(admin.ModelAdmin):
exclude = ('author',)
def queryset(self, request):
queryset = super(EntryAdmin, self).queryset(request)
return queryset.filter(author = request.user)
def save_model(self, request, obj, form, change):
obj.author = request.user
obj.save()
def has_change_permission(self, request, obj=None):
if not obj:
return True # access to change list
return obj.author == request.user
has_delete_permission = has_change_permission
admin.site.register(Entry, EntryAdmin)

35. Fine grained permissions
class Entry(models.Model):
title = models.CharField(max_length=255)
author = models.ForeignKey('auth.User')
class EntryAdmin(admin.ModelAdmin):
exclude = ('author',)
def queryset(self, request):
queryset = super(EntryAdmin, self).queryset(request)
return queryset.filter(author = request.user)
def save_model(self, request, obj, form, change):
obj.author = request.user
obj.save()
def has_change_permission(self, request, obj=None):
if not obj:
return True # access to change list
return obj.author == request.user
has_delete_permission = has_change_permission
admin.site.register(Entry, EntryAdmin)

36. Fine grained permissions
class Entry(models.Model):
title = models.CharField(max_length=255)
author = models.ForeignKey('auth.User')
class EntryAdmin(admin.ModelAdmin):
exclude = ('author',)
def queryset(self, request):
queryset = super(EntryAdmin, self).queryset(request)
return queryset.filter(author = request.user)
def save_model(self, request, obj, form, change):
obj.author = request.user
obj.save()
def has_change_permission(self, request, obj=None):
if not obj:
return True # access to change list
return obj.author == request.user
has_delete_permission = has_change_permission
admin.site.register(Entry, EntryAdmin)

37. Objects can be views
• A Django view is a function that takes a
request object and returns a response
object
A Django view is a callable that takes a
request object and returns a response
object

38. Objects can be views
• A Django view is a function that takes a
request object and returns a response
object
• A Django view is a callable that takes a
request object and returns a response
object
• Just define __call__() on the class

39. Example: restview.py
Django Snippets: http://bit.ly/restview

40. class ArticleView(RestView):
def GET(request, article_id):
return render(quot;article.htmlquot;, {
'article': get_object_or_404(
Article, pk = article_id),
})
def POST(request, article_id):
form = ...
return HttpResponseRedirect(request.path)

41. from django.http import HttpResponse
class RestView(object):
def __call__(self, request, *args, **kwargs):
if not hasattr(self, method):
return self.method_not_allowed(method)
return getattr(self, method)(request, *args, **kwargs)
def method_not_allowed(self, method):
response = HttpResponse('Not allowed: %s' % method)
response.status_code = 405
return response

42. django_openid
• Next generation of my django-openid project
• Taken a lot longer than I expected
• Extensive use of class-based customisation
GitHub: http://github.com/simonw/django-openid

43. consumer.py Consumer
LoginConsumer
CookieConsumer SessionConsumer
auth.py
AuthConsumer
registration.py
RegistrationConsumer

44. Suggestions from
django_openid
• Every decision should use a method
• Every form should come from a method
• Every model interaction should live in a method
• Everything should go through a render() method

45. render()
class Consumer(object):
...
base_template = 'django_openid/base.html'
...
def render(self, request, template, context=None):
context = context or {}
context['base_template'] = self.base_template
return TemplateResponse(
request, template, context
)
...

46. render()
class Consumer(object):
...
base_template = 'django_openid/base.html'
...
def render(self, request, template, context=None):
context = context or {}
context['base_template'] = self.base_template
return TemplateResponse(
request, template, context
)
...

47. render()
class Consumer(object):
...
base_template = 'django_openid/base.html'
...
def render(self, request, template, context=None):
context = context or {}
context['base_template'] = self.base_template
return TemplateResponse(
request, template, context
)
...

48. TemplateResponse
class MyCustom(BaseView):
def index(self):
response = super(MyCustom, self).index()
# response is a TemplateResponse
response.context['counter'] += 1
response.template = 'some/other/template.html'
return response
# Two classes
SimpleTemplateResponse(template, context)
TemplateResponse(request, template, context)

49. TemplateResponse
• Subclasses can re-use your logic and
extend or modify your context
• So can middleware and unit tests
• GZip Middleware writes to
response.content, needs work arounds
• Should HttpResponse be immutable?

50. Ticket #6735, scheduled for Django 1.2

51. Storing state on self in
a class-based generic
view is not thread safe

52. Storing state on self in
a class-based generic
view is not thread safe

53. Testing

54. Django Core
• Excellent testing culture
• Dubious “find... | grep... | xargs wc -l”:
• 74k lines of code
• 45k lines of tests
• “No new code without tests”
• Coverage = 54.4%, increasing over time

55. Django community?
• ... not so good
• even though django.test.client is great
• Many reusable apps lack tests
• need more psychology!

56. nose is more fun
• nosetests --with-coverage
• (coming to a SoC project near you)
• nosetests --pdb
• nosetests --pdb-failures

57. Test views directly
• Hooking up views to a URLconf just so
you can test them is fiddly
• ... and sucks for reusable apps
• A view function takes a request and
returns a response

58. RequestFactory
rf = RequestFactory()
get_request = rf.get('/hello/')
post_request = rf.post('/submit/', {
'foo': 'bar'
})
delete_request = rf.delete('/item/1/')
http://bit.ly/requestfactory

59. The HttpRequest constructor
isn’t doing anything useful at
the moment...

60. A web-based interface?
• Testing would be more fun with pretty graphs
• ... and animated progress meters
• ... and a “test now” button
• ... maybe the Django pony could smile at
you when your tests pass
• Cheap continuous integration: run tests every
time a file changes on disk?

61. settings.py is the root of all evil

62. Why did PHP magic_quotes suck?
• They made it impossible to write reusable code
• What if your code expects them to be on, but a
library expects them to be off?
• Check get_magic_quotes_gpc() and
unescape... but what if some other library has
done that first?

63. settings.py problems
• Middleware applies globally, even to
those applications that don’t want it
• Anything fixed in settings.py I inevitably
want to dynamically alter at runtime
• TEMPLATE_DIRS for mobile sites
• DB connections
• How about per-application settings?

64. Grr
>>> from django import db
Traceback (most recent call last):
File quot;<stdin>quot;, line 1, in <module>
...
ImportError: Settings cannot be imported,
because environment variable
DJANGO_SETTINGS_MODULE is undefined.

65. http://www.flickr.com/photos/raceytay/2977241805/
Turtles all the way down

66. “an infinite regression belief
about cosmology and the
nature of the universe”

67. The Django Contract
• A view is a callable that takes a request object and
returns a response object

68. The Django Contract
• A view is a callable that takes a request object and
returns a response object
• Primary URLconf: selects a view based on regular
expressions

69. The Django Contract
• A view is a callable that takes a request object and
returns a response object
• Primary URLconf: selects a view based on regular
expressions
• Application: sometimes has its own URLconf include()d
in to the primary

70. The Django Contract
• A view is a callable that takes a request object and
returns a response object
• Primary URLconf: selects a view based on regular
expressions
• Application: sometimes has its own URLconf include()d
in to the primary
• Middleware: a sequence of globally applied classes
process_request/process_response/process_exception

71. The Django Contract
• A view is a callable that takes a request object and
returns a response object
• Primary URLconf: selects a view based on regular
expressions
• Application: sometimes has its own URLconf include()d
in to the primary
• Middleware: a sequence of globally applied classes
process_request/process_response/process_exception
• Site: a collection of applications + settings.py + urls.py

72. Extended Contract
• A view is a callable that
takes a request and returns a response
• URLconf: a callable that
takes a request and returns a response
• Application: a callable that
takes a request and returns a response
• Middleware: a callable that
takes a request and returns a response
• Site: a callable that takes a request and returns a response

73. http://www.flickr.com/photos/enil/2440955556/
Let’s call them “turtles”

74. Three species of turtle
• Django request / response
• WSGI
• HTTP
• What if they were interchangeable?

75. Three species of turtle
• Django request / response
• WSGI
• Christopher Cahoon, SoC
• django_view_from_wsgi_app() http://bit.ly/djwsgi
• django_view_dec_from_wsgi_middleware()
• HTTP
• paste.proxy

76. http://www.flickr.com/photos/remiprev/336851630/
Micro frameworks

77. http://github.com/
juno 782 lines
breily/juno
http://github.com/
newf 144 lines
JaredKuolt/newf
http://github.com/
mnml 205 lines
bradleywright/mnml
http://github.com/
itty 406 lines
toastdriven/itty

78. djng
http://github.com/simonw/djng
(First commit at 5.47am this morning)

79. A micro framework
that depends on a
macro framework

80. class Router(object):
quot;quot;quot;
Convenient wrapper around Django's urlresolvers, allowing them to be used
from normal application code.
from django.conf.urls.defaults import url
router = Router(
url('^foo/$', lambda r: HttpResponse('foo'), name='foo'),
url('^bar/$', lambda r: HttpResponse('bar'), name='bar')
)
request = RequestFactory().get('/bar/')
print router(request)
quot;quot;quot;
def __init__(self, *urlpairs):
self.urlpatterns = patterns('', *urlpairs)
self.resolver = urlresolvers.RegexURLResolver(r'^/', self)
def handle(self, request):
path = request.path_info
callback, callback_args, callback_kwargs = self.resolver.resolve(path)
return callback(request, *callback_args, **callback_kwargs)
def __call__(self, request):
return self.handle(request)

81. Re-imagining the core
Django APIs, minus
urls.py and settings.py

82. http://github.com/simonw/djangopeople.net

83. http://www.flickr.com/photos/morgantj/2639793944/
Thank you