Doing magic with python metaclasses

Python Meta classes and how Django uses them Lakshman Prasad

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

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

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

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

`__metaclass__` defines metaclases

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

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

Django Models

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

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`

Django Forms

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

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

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

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

Thank You! lakshman@uswaretech.com : Build Cutting Edge Web Apps. Small Team of expert Django Developers. Exclusively develop in Django, Python stack. @becomingGuru, @uswaretech

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Doing magic with python metaclasses

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