Class Based Views: Untangling the Mess

Presentation from DjangoCon Europe 2014

One of the big changes in Django 1.3 was the introduction of Class-Based Views. Class-Based Views represent one of the biggest stylistic changes in Django since Magic Removal, and there's been lots of confusion surrounding them.

This talk will be a brief tutorial, providing some background and dispelling some of the confusion around the use of Class-Based Views. It will also look at some interesting possibilities that are made possible by the use of the class-based view framework.

Transcript

1. Class Based Views:
   Untangling the Mess
   Dr Russell Keith-Magee
   DjangoCon Europe 2013
   Thursday, 23 May 13
   

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2. Thursday, 23 May 13
   

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3. Thursday, 23 May 13
   

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4. Class Based Views
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5. In the beginning...
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6. In the beginning...
   • It’s 2005
   • Django is for building websites
   • Views are for displaying content
   • There a lots of common patterns in views
   • We all like being DRY
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7. Thursday, 23 May 13
   

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8. Generic Views
   • Display a template
   • Display a single object
   • Display a list of objects
   • Display a form
   • Create, Update, Delete
   • Display a date archive
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9. Key features
   • Each view is a function
   • Configuration via arguments
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10. Example: Before
    def edit_book(request, book_id):
    try:
    book = Book.objects.get(pk=book_id)
    except Book.DoesNotExist:
    raise Http404
    if request.method == ‘POST’:
    form = BookForm(
    instance=book,
    data=request.POST)
    if form.is_valid():
    form.save()
    return HttpResponseRedirect(
    book.get_absolute_url())
    ...
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11. Example: Before
    ...
    else:
    form = BookForm(instance=obj)
    return render(request, ‘edit_book.html’, {
    ‘book’: book
    ‘form’: form
    })
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12. Example: Before
    urlpatterns = patterns('',
    url(
    r’^/book/(?P\d+)/$’,
    book_view,
    name=‘edit-book’
    )
    ...
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13. Example: After
    urlpatterns = patterns('',
    url(
    r’^/book/(?P\d+)/$’,
    generic.update_object, {‘model’: Book,},
    name=‘edit-book’
    )
    ...
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14. Example: After
    urlpatterns = patterns('',
    url(
    r’^/author/(?P\d+)/$’,
    generic.update_object, {‘model’: Author,},
    name=‘edit-author’
    )
    ...
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15. Problems
    • Configuration options limited by args
    • No control over logic flow
    • No re-use between views
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16. Thursday, 23 May 13
    

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17. Let’s go class-based!
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18. Django 1.3
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19. OMFGBBQ!!1!!
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20. What went wrong?
    • Fundamental confusion over purpose
    • Confusion over implementation choices
    • Ravioli Code
    • Bad documentation
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21. Purpose
    • Two distinct, but connected bodies of work
    • Class Based Views
    • Class Based Generic Views
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22. Class Based Views
    • A class based analog of a view function
    • Method-based HTTP-verb dispatch
    • ... and that’s it
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23. Example
    from django.views.generic import View
    class MyView(View):
    def get(self, request, *args, **kwargs):
    return render(request, ‘myview.html’ {})
    def post(self, request, *args, **kwargs):
    ...
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24. Class Based Views
    • Automatic OPTIONS request handling
    • Automatic naïve HEAD request handling
    • Automatic HTTP 405 on unknown verbs
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25. CB Generic Views
    • Uses Class Based View as a base
    • Creates analogs of the old generic views
    • Addresses limits of functional approach
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26. Implementation choices
    https://code.djangoproject.com/wiki/ClassBasedViews
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27. Obvious choice
    • A class that is instantiated as a view
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28. What do you mean?
    urlpatterns = pattern(‘’,
    # Deploy the Class?
    (‘myview’, MyView),
    # or an Instance of a class
    (‘myview’, MyView()),
    # or a classmethod?
    (‘myview’, MyView.as_view),
    # or a Factory?
    (‘myview’, MyView.as_view()),
    )
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29. Problems
    • What is being instantiated?
    • How is it being instantiated?
    • When is this class instantiated?
    • How do you pass in configuration?
    • What is the lifespan of an instance?
    • What is the contract with urls.py?
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30. Sidebar: Admin
    • Django’s admin is a class based view
    • Implemented using simple __call__()
    • Doesn’t have HTTP Verb support
    • Suffers from state problems
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31. Solution
    • Factory classmethod
    • Returns function that invokes .dispatch()
    • initkwargs passed to __init__
    • initkwargs must be pre-declared
    • view request, args and kwargs are stateful
    urlpatterns = pattern(‘’,
    (‘myview’, MyView.as_view(
    template_name=‘foo.html’)),
    )
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32. Sidebar: Other option
    • Change the urls.py contract
    • Currently: a callable
    • Change to: a callable or a class.
    • Decision: keep the urls.py contract clear
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33. Ravioli
    • Goal: Replace function-based generics with
    class-based generics
    • Show what class-based approach can do
    • End point views built out of mixins
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34. freakboy3742’s
    Ravioli Cooking 101
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35. Mixins
    edit_view:
    get object
    if POST:
    get form
    if form is valid:
    handle form
    redirect
    else:
    handle failure
    else:
    get form
    return response
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36. Mixins
    • UpdateView
    • BaseUpdateView
    • ModelFormMixin
    • FormMixin
    • SingleObjectMixin
    • ContextMixin
    • ProcessFormView
    • SingleObjectTemplate
    ResponseMixin
    • TemplateResponseMixin
    edit_view:
    get object
    if POST:
    get form
    if form is valid:
    handle form
    redirect
    else:
    handle failure
    else:
    get form
    return response
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37. Mixins
    •CreateView
    •BaseCreateView
    • ModelFormMixin
    • FormMixin
    • SingleObjectMixin
    • ContextMixin
    • ProcessFormView
    • SingleObjectTemplate
    ResponseMixin
    • TemplateResponseMixin
    create_view:
    instantiate object
    if POST:
    get form
    if form is valid:
    handle form
    redirect
    else:
    handle failure
    else:
    get form
    return response
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38. Mixins
    • CreateView
    • BaseCreateView
    • ModelFormMixin
    • FormMixin
    • SingleObjectMixin
    • ContextMixin
    • ProcessFormView
    •SingleObjectJSON
    ResponseMixin
    create_view:
    instantiate object
    if POST:
    get form
    if form is valid:
    handle form
    redirect
    else:
    handle failure
    else:
    get form
    return JSON response
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39. Ravioli tastes good!
    • Allows for maximum reuse of core logic
    • Extremely flexible for inserting new logic
    • Easy to add your own mixins
    • But: You need to grok all the pieces
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40. Documentation
    • Bad as originally released
    • Much better now
    • Still need framework decisions needed
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41. Where to from here?
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42. Have we solved the
    wrong problem?
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43. Generic Views
    • Display a template
    • Display a single object
    • Display a list of objects
    • Display a form
    • Create, Update, Delete
    • Display a date archive
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44. Modern web sites
    have different problems
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45. Modern problems
    • Multiple forms/formsets per page
    • Conditional forms
    • Continuous scrolling, not pagination
    • AJAX support (including in-place editing)
    • PJAX
    • Multiple “actions” per page
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46. Call to action
    • In discussion: Do you mean CBV or CBGV?
    • Docs can still be improved
    • #18830 - FormCollection
    • Experiment with APIs. Django’s admin is a
    useful case study
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47. Questions?
    http://djangoproject.org
    http://cecinestpasun.com
    russell@keith-magee.com
    @freakboy3742
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