Django class-based views: survival guide for novices

Django class-based viewsSurvival guide for novices

v2

Django class-based views - Survival guide for novices DjangoCon 2014 #2/55

A try to help you understand

WHAT class-based views are and HOW they work

so that you may solve your problems.


Target of this talk

Django novices who completed and understood the Django tutorial.

Knowledge of the basic Python OOP syntax and concepts is useful

(classes, inheritance, method overriding, function arguments processing).

About you


Start from the basics

What are Django class-based views?



(Django) Views based on (Python) classes.



(Django) Views based on (Python) classes.

(Django) Stuff with some (Python) magic.


Views

What are views?


Django is a processor of HTTP requests.

DjangoHTTP request HTTP response

Views


A view is the part of Django that processes a specific request

(HTTP method on URL).

Views

(GET) URL1

Django

view1

view2(POST) URL2

RESPONSE1

RESPONSE2


Views

HTTP REQUEST view HTTP RESPONSE

HTTP REQUEST enhanced view HTTP RESPONSE

A view can be monolithic.

This makes hard to replace or enhance part of it.


Views

A system can be splitted in several components.

This makes easier to change part of it.

HTTP REQUEST view:step1 HTTP RESPONSEview:step2 view:step3

HTTP REQUEST view:step1 HTTP RESPONSEview:step2 view:step3


Object-oriented paradigm

Object-oriented paradigm

A way to build componentized systems that may be easily changed.


Class-based views

from django.views.generic.list import ListView

from articles.models import Article

from django.conf.urls import url

class ArticleListView(ListView):

model = Article

urlpatterns = [

url(r'ârticles/$', ArticleListView.as_view()),

]


Class-based views

This routes HTTP requests on the 'articles/' URL to the ArticleListView view.





model = Article

urlpatterns = [


]


Class-based views

This defines the view as a copy of ListView working on the Article

model.





model = Article

urlpatterns = [


]


Class-based views

● Processes incoming HTTP GET requests

● Loads all Article objects

● Renders a template called article_list.html and the list of articles is

in the object_list variable


model = Article


Class-based views

What happens behind the scenes?


Advertising

Use the source, Luke!

https://github.com/django/djangohttps://github.com/django/django


Warning

Source is a moving target

https://github.com/django/django/tree/1.5.7


A tour of a CBV

GETrequest

as_view()

url(r'ârticles/$', ArticleListView.as_view())


GETrequest

as_view()

url(r'ârticles/$', ArticleListView.as_view())

@classonlymethod

def as_view(cls, **initkwargs):

[...]

def view(request, *args, **kwargs):

[...]

self.request = request

self.args = args

self.kwargs = kwargs

return self.dispatch(request, *args, **kwargs)

[...]

return view

django/views/generic/base.py#L46

A tour of a CBV


GETrequest

dispatch()

as_view()


def dispatch(self, request, *args, **kwargs):

if request.method.lower() in self.http_method_names:

handler = getattr(self,

request.method.lower(),

self.http_method_not_allowed)

else:

handler = self.http_method_not_allowed

return handler(request, *args, **kwargs)


A tour of a CBV


GETrequest

dispatch()

as_view()







else:




A tour of a CBV

'GET' --> getattr(self, 'get', [...])'POST' --> getattr(self, 'post', [...])'PUT' --> getattr(self, 'put', [...])[...]

request.method.lower()


dispatch()

GETrequest

get()

as_view()

return self.get(request, *args, **kwargs)

A tour of a CBV


A tour of a CBV

dispatch()

GETrequest

get()

as_view()


def get(self, request, *args, **kwargs):

self.object_list = self.get_queryset()

allow_empty = self.get_allow_empty()

if not allow_empty:

if (self.get_paginate_by(self.object_list) is not None

and hasattr(self.object_list, 'exists')):

is_empty = not self.object_list.exists()

else:

is_empty = len(self.object_list) == 0

if is_empty:

raise Http404 [...]

context = self.get_context_data(object_list=self.object_list)

return self.render_to_response(context)

django/views/generic/list.py#L123class ArticleListView(ListView):


A tour of a CBV

dispatch()

GETrequest

get()

as_view()




allow_empty = self.get_allow_empty()

if not allow_empty:

if (self.get_paginate_by(self.object_list) is not None

and hasattr(self.object_list, 'exists')):

is_empty = not self.object_list.exists()

else:

is_empty = len(self.object_list) == 0

if is_empty:

raise Http404 [...]





django/views/generic/list.py#L123



dispatch()

GETrequest

get()

as_view()

get_queryset()


A tour of a CBV

def get_queryset(self):

if self.queryset is not None:

queryset = self.queryset

if hasattr(queryset, '_clone'):

queryset = queryset._clone()

elif self.model is not None:

queryset = self.model._default_manager.all()

else:

raise ImproperlyConfigured [...]

return queryset



dispatch()

GETrequest

get()

as_view()

get_queryset()


A tour of a CBV


if self.queryset is not None:

queryset = self.queryset

if hasattr(queryset, '_clone'):

queryset = queryset._clone()

elif self.model is not None:


else:

raise ImproperlyConfigured [...]

return queryset



class ArticleListView(ListView): model = Article


Override methods

How do you customize CBVs behaviour?


GETrequest

as_view()

Override methods


model = Article


Override methods


model = Article


queryset = super(ArticleListView, self).get_queryset()

return queryset.filter([...])

dispatch()

GETrequest

get()

as_view()

get_queryset()

def get_queryset(self): [...]

views/generic/list.py#L22


Arguments in class-based views


@classonlymethod

def as_view(cls, **initkwargs):

[...]

def view(request, *args, **kwargs):

[...]

self.request = request

self.args = args

self.kwargs = kwargs


[...]

return view

View parameters and request are stored in the view object.

Arguments in CBVs



model = Article


queryset = super(ArticleListView, self).get_queryset()

return queryset.filter(year=kwargs['year'])

https://docs.djangoproject.com/en/1.5/topics/http/urls/

url(r'ârticles/(?P<year>\d{4})/$', ArticleListView.as_view()),

Arguments in CBVs


CRUD operations and forms


CRUD operations

CRUD through HTTP verbs

Read: GET

Create: POST

Update: PUT

Delete: DELETE


GETrequest

dispatch()

as_view()







else:




'GET' --> getattr(self, 'get', [...])'POST' --> getattr(self, 'post', [...])'PUT' --> getattr(self, 'put', [...])[...]

request.method.lower()

CRUD operations


POSTrequest

as_view()

class NoteAdd(CreateView):

model = StickyNote

CRUD operations


dispatch()

POSTrequest

post()

as_view()

return self.post(request, *args, **kwargs)

class ProcessFormView(View):

def post(self, request, *args, **kwargs):

form_class = self.get_form_class()

form = self.get_form(form_class)

if form.is_valid():

return self.form_valid(form)

else:

return self.form_invalid(form)

CRUD operations

django/views/generic/edit.py#L157


dispatch()

POSTrequest

post()

as_view()

return self.post(request, *args, **kwargs)

class ProcessFormView(View):


form_class = self.get_form_class()

form = self.get_form(form_class)

if form.is_valid():

return self.form_valid(form)

else:

return self.form_invalid(form)

CRUD operations


form_class = self.get_form_class()form = self.get_form(form_class)

return self.form_valid(form)return self.form_invalid(form)


class FormMixin(ContextMixin):

def get_form_class(self):

return self.form_class

def get_form(self, form_class):

return form_class(**self.get_form_kwargs())

def get_form_kwargs(self):

kwargs = {'initial': self.get_initial()}

if self.request.method in ('POST', 'PUT'):

kwargs.update({'data': self.request.POST,'files': self.request.FILES,})

return kwargs

def form_valid(self, form):

return HttpResponseRedirect(self.get_success_url())

def form_invalid(self, form):

return self.render_to_response(self.get_context_data(form=form))

CRUD operations



CRUD operations

What about forms?


CRUD operations

browser serverGET request

The user browses a web page (GET)


CRUD operations


browser serverHTTP response

The server answers the GET request with a page

containing a form


CRUD operations



browser serverPOST request

The user fills the form and submits it (POST)


CRUD operations



browser serverPOST request


The server processes POST data and return a

response


Double interaction, the view is called twice

CRUD operations


from django.shortcuts import render

from django.http import HttpResponseRedirect

def contact(request):

if request.method == 'POST': # If the form has been submitted...

form = ContactForm(request.POST) # A form bound to the POST data

if form.is_valid(): # All validation rules pass

# Process the data in form.cleaned_data

# ...

return HttpResponseRedirect('/thanks/') # Redirect after POST

else:

form = ContactForm() # An unbound form

return render(request, 'contact.html', {'form': form,})

CRUD operations


GETrequest

dispatch()

as_view()







else:





CRUD operations


Managing HTTP methods

class RedirectView(View):


[...]

return http.HttpResponseRedirect(url)

def head(self, request, *args, **kwargs):




def options(self, request, *args, **kwargs):


def delete(self, request, *args, **kwargs):


def put(self, request, *args, **kwargs):




Django CBVs class hierarchy

CBVs browser

Resources

http://ccbv.co.uk/


Django CBVs class hierarchy

Django Vanilla Views

Resources

http://django-vanilla-views.org/


Where this talk comes from

Digging Up Django Class-based Views

http://lgiordani.github.io

Resources


Some links to better understand Python OOP

Google: “Some links to better understand Python OOP”

Resources

http://redd.it/226ahl


About me

Leonardo Giordanihttp://lgiordani.github.io

https://twitter.com/tw_lgiordani

https://github.com/lgiordani

https://plus.google.com/u/LeonardoGiordani

Feel free to contact me

Questions, suggestions, corrections are always warmly welcome

Technology

Django class-based views: survival guide for novices