Django class based views

Django Class-BasedViews
CoMo Rich Web, October 2010

Agenda
• Python 101
• RegularViews
• GenericViews
• Class-BasedViews

# defining a function
def say_hello(subject):
print 'Hello, %s' % subject
# calling a function
say_hello('World')

# defining a function
def say_hello(subject):
print 'Hello, %s' % subject
# calling a function
say_hello('World')
# defining a class
class HelloSayer(object):
def __init__(self, name):
self.name = name
def say_hello(self):
print 'Hello, %s' % self.name
# creating an object instance
i = HelloSayer('World')
i.say_hello()

First-Class Everything
• ﬁrst-class functions
• can be stored in variables
• can be passed into other functions
• can be returned from functions
• ﬁrst-class classes
• ditto all the above

f = say_hello
f('Everybody')
cls = MyClass
j = cls('Everybody')
j.say_hello()
Text

class FakeFunction(object):
def __init__(self):
print 'Constructing...'
def __call__(self):
print 'You called?'
f = FakeFunction() # Constructing...
f() # You called?

result = x()
# is `x()` calling a function?
# creating an object?
# or calling a method on an object?

• a view is a function that takes (at minimum)
an HttpRequest and returns an
HttpResponse
• views usually live in the ﬁle views.py
• URLs are routed to views using patterns in
the ﬁle urls.py
View Basics

# __init__.py intentionally left blank
# models.py intentionally left blank
# views.py
from django.http import HttpResponse
def homepage(request):
return HttpResponse('<html><body><h1>Welcome to website</h1></body></html>')
# urls.py
from django.conf.urls.defaults import *
urlpatterns = patterns('',
(r'^$', 'myapp.views.homepage'),
)

# views.py
def user_profile(request, username):
# do useful stuff here...
return HttpResponse('''<html><body>
<h1>Profile for user %s</h1>
</body></html>''' % username)
# urls.py
urlpatterns = patterns('',
(r'^user/(w+)/profile$', 'myapp.views.user_profile'),
)

Paving the Cow Paths
• “CRUD” apps have well-known patterns
• dynamic typing allows Django to provide
common view patterns as reusable
functions

# hypothetical URL: /users?page=3
def user_list(request):
page = int(request.GET.get('page', '1')) # sloppy parsing
users = User.objects.all()
per_page = 20
num_pages = math.ceil(len(users) / per_page) # buggy math
if page <= 0 or page > num_pages:
raise Http404()
start_index = (page - 1) * per_page # buggy math
end_index = page * per_page # buggy math
users_on_page = users[start_index:end_index]
context = {
'users': users_on_page,
'page': page,
'num_pages': num_pages
# next or prev pages?
}
return render_to_response('myapp/user_list.html', context)

from django.views.generic import list_detail
# hypothetical URL: /users?page=3
return list_detail.object_list(request,
queryset=User.objects.all(),
paginate_by=20)

from django.views.generic import list_detail
# hypothetical URL: /users?staff=y
if request.GET.get('staff', 'n').lower() == 'y':
qs = Users.objects.filter(staff=True)
else:
qs = Users.objects.all()
return list_detail.object_list(request,
queryset=qs,
paginate_by=20)

Going off the Beaten Path
• at some point you’ll want to customize the
generic views even more
• wrapping only gets you so far
• what about deeper behavior changes?
• e.g,“delete” should actually ﬂag as
“inactive”

Beneﬁts
• control not just parameters but behavior
• more code reuse

# completely made-up example
from django.views.generic.list_detail import ObjectListView
class UserListView(ObjectListView):
def get_queryset(self):
if self.request.GET.get('staff', 'n').lower() == 'y':
return Users.objects.filter(staff=True)
else:
return Users.objects.all()
# meanwhile, in urls.py...
(r'^users$', 'myapp.views.UserListView')

# completely made-up example
from django.views.generic.create_update import DeleteObjectView
class DeleteUserView(DeleteObjectView):
def perform_deletion(self, user):
user.is_active = False
user.save()

Not a Silver Bullet
• still only allows customization the original
author thought of
• can introduce multithreading issues
• “ofﬁcial” Django views aren’t ﬁnished yet

Questions?
Thanks for listening!

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