Django Design Patterns

Arun Ravindran

About Me

Arun Ravindran Blogs and screencasts at arunrocks.com

Outline

● Why Design Patterns?

● Does Django follow MVC pattern?

● Model Patterns

● View Patterns

● Template Patterns

● Gyaan on Applying Patterns

“ A design pattern is a general reusable

solution to a commonly occurring problem

within a given context in software design

● 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?

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

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

Model Patterns

● Pattern: User Profiles

● Pattern: Service Objects ● Pattern: Custom Model Managers

User Profiles

User Profiles (contd.)

1. Define the profile model

2. Listen for User model’s post_save signal

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

User Profiles (contd.)

Usage: UserProfile.objects.get_or_create(user=u)

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

Service Objects (contd.)

1. Your model delegates to service objects

2. Services are specialized classes:

Custom Model Managers

Custom Model Managers (contd.)

1. Use custom manager for common queries

2. Add it to your model

View Patterns

● Pattern: Context Enhancers ● Pattern: Access Controlled Views

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

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:

Access Controlled Views

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

Template Patterns

● Pattern: Template Inheritance Tree ● Pattern: Active Link

Template Inheritance Tree

Template Inheritance Tree (contd.)

1. Build a base structure, with placeholders

2. Similar pages are extended from base

Active Link

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

And the list goes on...

Gyaan on Patterns

● How not to use patterns:

o Don’t blindly apply. Explain why.

o Don’t apply a pattern if you language supports a

direct solution

o Don’t try to retro-fit everything in terms of patterns

o Don’t be afraid to create new patterns.

PATTERNS ARE EVERYWHERE

Thank you!

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