This document discusses how metaclasses are used in Python frameworks like Django. It explains that metaclasses allow for dynamically creating classes and enable flexibility. Frameworks like Django use metaclasses extensively, for example to create models and forms declaratively. When Django creates a model, the options are stored in an Options object and the model is registered in a registry. This allows Django to return the same registered model and add useful attributes like validation. Metaclasses enable this powerful and flexible functionality in Django.

1. Python Meta classes and how
Django uses them Lakshman Prasad

2. Meta Classes
 A Design pattern
 Dynamically create classes
 Enables Flexibility, Good API
 Used by Frameworks
 Perform “Magic”

3. Where Meta Classes?
 Frameworks, frameworks, frameworks.
 Django uses them extensively.
 Scrappy, a web scrapping framework.
 Flexible Django apps. Eg: Django-registration

5. Executes the code when encountered
Creates namespace and stores all variables in it

6. `type` is an inbuilt MetaClass
Dynamic class = Static classes

7. `__metaclass__` defines metaclases

8. Meta Class Magic
Define a class extending object
Examined type is string, Magic!
Enables Declarative Syntax

9. MetaClasses, not always
 Simple function decorators
 Unintuitive to end developers, unless
explained
 Python 2.6+ has Class Decorators
 But being powerful, where they are needed,
indespesible

10. Django Models

11. Django Models Creation
 Declarative Syntax, No `__metaclass__`
 New model created in place
 Options into `Options` object
 Register in registry called `AppCache`
 Add attributes to new model

12. Inside Django Models
 Return the same registered Model from
AppCache
 Original options and Meta options stored in
`_meta`
 Add more useful attributes/classes like
`DoesnotExist`, `module_name` and
`class_name`

13. Django Forms

14.  Rendering in html
 Validation for right data
 Displaying errors
 Customization options
 Provided model, storing to the database

15. Dynamic Django forms - 1
Forms API is flexible, but often needs Meta Classes
`base_fields` and `fields` list store all fields
Change field attributes while initialization

16. Dynamic Django Forms - 2
When high complexity, instantiate a form,
dynamically

17. Resources
Python, Django Documentation, Source.
Pro Django, by Marty Alchin
Django-registration v0.7 by James Bennett

18. Thank You!
lakshman@uswaretech.com
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Small Team of expert Django Developers.
Exclusively develop in Django, Python stack.
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