My talk on Django Design patterns http://in.pycon.org/funnel/2014/227-django-design-patterns

1. Django Design Patterns
Arun Ravindran

2. About Me
Arun Ravindran
Blogs and screencasts at arunrocks.com

3. Outline
●Why Design Patterns?
●Does Django follow MVC pattern?
●Model Patterns
●View Patterns
●Template Patterns
●Gyaan on Applying Patterns

4. “
A design pattern is a general reusable solution to a commonly occurring problem within a given context in software design

5. ●Pattern are often elegant and beautiful code
●Repeatable solutions
●Gives a Common Lingo
●Reduces Trial and Error
●Formalizes best practices, say -
○Don’t Repeat Yourself
○Separation of Concerns
○SOLID…
Why Design Patterns?

6. Django and Patterns
GoF Pattern
Django Component
Explanation
Observer pattern
Signals
When one object changes state, all its listeners are notified and updated automatically
Template Method
Class based generic views
Steps of an algorithm can be redefined by subclassing without changing the algorithm’s structure
Fowler Pattern
Django Component
Explanation
Active Record
Django Models
Encapsulates the database access, and adds domain logic on that data
Identity Field
Id Field
Saves a database ID field in an object to maintain identity
Template View
Django Templates
Renders into HTML by embedding markers in HTML

7. Does Django follow MVC?
●MVC is easily misunderstood
●Creators of Django prefer the term MTV
●In reality, Django is an MVC variant
●Compared to classic MVC –
●“Model” is comparable to Django’s Models,
●“View” is usually Django’s Templates
●“Controller” is the entire framework itself

8. Model Patterns
●Pattern: User Profiles
●Pattern: Service Objects
●Pattern: Custom Model Managers

9. User Profiles

10. User Profiles (contd.)
1.Define the profile model
2.Listen for User model’s post_save signal

11. User Profiles (contd.)
3.Mention your app’s AppConfig
4.Import signals in the AppConfig’s ready
5.(Optional) Inline the User model’s admin

12. User Profiles (contd.)
Usage:
UserProfile.objects.get_or_create(user=u)

13. Service Objects
●The adage “Fat Models, Thin Views” can lead to frighteningly obese models.
●Service Objects are POPOs that encapsulate a ‘service’ or interactions with a system.
●They are usually kept in a separate file named services.py
●Common use cases: Interactions with external services, Helper tasks, Long- running tasks

14. Service Objects (contd.)
1.Your model delegates to service objects
2.Services are specialized classes:

15. Custom Model Managers

16. Custom Model Managers (contd.)
1.Use custom manager for common queries
2.Add it to your model

17. View Patterns
●Pattern: Context Enhancers
●Pattern: Access Controlled Views

18. Detour: What is a Mixin?
•Mixin is a class used to add properties and methods to other classes
•Composition or Inheritance? Sort of both.
TemplateView
View
ContextMixin
Template
ResponseMixin

19. Context Enhancers
●Several views might need the same queryset or values in context
●Overriding get_context_data everywhere isn’t DRY
●Instead, use a view mixin:

20. Access Controlled Views

21. Access Controlled Views (contd.)
1.By using a decorator on a function based view or a class based view
2.Or, by overriding the dispatch method through a mixin

22. Template Patterns
●Pattern: Template Inheritance Tree
●Pattern: Active Link

23. Template Inheritance Tree

24. Template Inheritance Tree (contd.)
1.Build a base structure, with placeholders
2.Similar pages are extended from base

25. Active Link

26. Active Link (contd.)
Here is a simple template-only approach:
1.Every template has the following line
2.The snippet in _navbar.html will be:
Custom tags can also be used

27. And the list goes on...

28. Gyaan on Patterns
●How not to use patterns:
oDon’t blindly apply. Explain why.
oDon’t apply a pattern if you language supports a direct solution
oDon’t try to retro-fit everything in terms of patterns
oDon’t be afraid to create new patterns.

29. PATTERNS ARE EVERYWHERE

30. Thank you!

31. Image Credits
•http://fooyoh.com/geekapolis_gadgets_wishlist/657945
•http://www.bigalspets.ca/multi-stage- canister-filter-c-360.html
•https://wrapbootstrap.com/theme/sanoor- navbar-WB037R7G3
•Monty Python and the Holy Grail
•Warner Bros