extra and extra_evaluated

Very often it’s useful to add some little bit of data on the side that you need later to customize something. We think it’s important to support this use case with minimal amounts of code. To do this we have extra and extra_evaluated. This is your place to put whatever you want in order to extend iommi for a general feature or just some simple one-off customization for a single view.

All Part derived classes have extra and extra_evaluated namespaces, for example: Page, Column, Table, Field, Form, and Action.

You use extra to put some data you want as-is:

form = Form.create(
    auto__model=Artist,
    fields__name__extra__sounds_cool=True,
    extra__is_cool=True,
)

Here we add sounds_cool to the name field, and the is_cool value to the entire Form. We can then access these in e.g. a template: {{ form.fields.name.extra.sounds_cool }} and {{ form.extra.is_cool }}.

extra_evaluated is useful when you want to use the iommi evaluation machinery to get some dynamic behavior:

form = Form.create(
    auto__model=Artist,
    fields__name__extra_evaluated__sounds_cool=lambda request, **_: request.user.is_staff,
    extra_evaluated__is_cool=lambda request, **_: request.user.is_staff,
)

These are accessed like this in the template: {{ form.fields.name.extra_evaluated.sounds_cool }}.