Django has very good documentation that describes how to write custom database fields and custom template tags and filters. I cannot find the document that describes how to write custom form fields and widgets. Does this document exist?
The way I've been able to write custom form fields and widgets is by reading the Django source code and imitating what I see there. I know that there are still things about implementing fields and widgets that I do not completely understand because I have not read any high level document that describes their interfaces.