Under the hood Marty Alchin http://martyalchin.com/ PyCon 2008, Chicago 

So you know Django Declarative syntax for models and forms Easy-to-use ORM Extensible template language Automatic admin interface It slices! It dices! It makes Julienne fries! 

But wait! There’s more! 

Pluggable features HTTP middleware Database connections Cache mechanisms Authentication providers Context processors Template loaders Management commands 

And coming soon to SVN near you... 

Upcoming features Easy admin customizations Flexible QuerySet subclassing File storage options No doubt more to follow 

How do they do it? 

Tools of the trade Metaclasses Signals using PyDispatcher Several utility functions A ton of other stuff beyond today’s talk 

Metaclasses 

[Metaclasses] are deeper magic than 99% of users should ever worry about. If you wonder whether you need them, you don’t (the people who actually need them know with certainty that they need them, and don’t need an explanation about why). Tim Peters, comp.lang.python, December 22, 2002 “ “ “ 

Welcome to the 1% 

Uses of metaclasses Tracking subclasses Discovering inheritance schemes Providing custom methods Inspecting class declarations Plenty more 

Uses within Django Caching a list of known models Moving Meta options into the special _meta class attribute Moving model fields out to an internal list Keeping track of model inheritance Collecting form fields into an internal list Merging fields from inherited forms 

Model Model metaclasses Model SlugField ForeignKey CharField IntegerFIeld __unicode__() get_absolute_url() Meta verbose_name verbose_name_plural ordering order_with_respect_to Model __unicode__() get_absolute_url() _meta verbose_name verbose_name_plural ordering order_with_respect_to fields SlugField ForeignKey CharField IntegerFIeld 

contribute_to_class(cls, name) 

Signals 

How signals work post_init listener listener listener listener() listener() listener() Model() 

Signals in the box pre_init post_init pre_save post_save pre_delete post_delete class_prepared post_syncdb request_started request_finished got_request_exception template_rendered 

And you can too! my_signal = object() Document its location Document when it fires Document what parameters are sent along 

Utility functions 

Functional utilities Curried functions preload arguments Wrapped functions preserve attributes Memoized functions improve performance Lazy functions evaluate when accessed Turn middleware into per-view decorators Standard decorators are fair game Don’t forget inspect 

Text utilities Capitalize the first letter of a string Split text, preserving quoted passages Truncate text, preserving words or HTML Display a list of values in natural English Convert text to its phone number equivalent Powerful template engine is available Standard string formatting still works 

Just remember... 

Django is Python 

Questions? You know you want to