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:

.. code-block:: python

    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:

.. code-block:: python

    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 }}`.