Tutorials on writing custom Django widgets?

I'm trying to modify the Django Admin interface. I need a custom type of textbox that does some pre/post processing on the text. If I understand correctly, a custom widget is the way 开发者_如何学编程to go about this.

Any good tutorials on how to write custom widgets for Django?

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I found this: http://www.kryogenix.org/days/2008/03/28/overriding-a-single-field-in-the-django-admin-using-newforms-admin

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Found this reference for the admin site: http://www.slideshare.net/lincolnloop/customizing-the-django-admin