Equivalence

In iommi there are two equivalence principles that are important to grasp:

• declarative/programmatic hybrid API

• double underscore as a short hand syntax for nesting dicts

The model used for these examples is Album:

class Album(Model):
    name = CharField(max_length=255, db_index=True)
    artist = ForeignKey(Artist, on_delete=CASCADE, related_name='albums')
    year = IntegerField(default=1980)
    genres = ManyToManyField(Genre, related_name='albums')
    published_date = DateField(null=True, blank=True)

Declarative/programmatic hybrid API

The programmatic API is pretty straightforward: you have a class constructor that takes some arguments. The interesting part is how we can mirror that exactly into a declarative style.

table = Table(
    model=Album,
    columns=dict(
        name=Column(),
    ),
)

This simple table can be written as a class definition:

class MyTable(Table):
    class Meta:
        model = Album

    name = Column()

There are two things to notice here:

1. Variables declared in class Meta in iommi means they get passed into the constructor. model = Album in Meta is exactly the same as Table(model=Album).

2. The name column is declared on the class itself, and the columns part of the argument (Table(columns=dict(...)) is implicit. For Page the same implicit name is called parts, and for Form it’s called fields.

Double underscore short form

In iommi you can have very deeply nested object structures, and because you want to customize something deep inside a graph it would be cumbersome to nest dicts a lot. So __ is used as a separator.

Say we have a table, where we want to turn on filtering for a column, but we want to insert a special CSS class (called special) on the label of the search field:

table = Table(
    auto__model=Model,
    # Enable filtering
    columns__name__filter__include=True,
    # Set the CSS class on the label
    columns__name__filter__field__label__attrs__class__special=True,
)

We could also write this without using __ for nesting:

table = Table(
    auto=dict(model=Model),
    columns=dict(
        name=dict(
            filter=dict(
                # Enable filtering
                include=True,
                # Set the CSS class on the label
                field=dict(
                    label=dict(
                        attrs={
                            # have to use a dict literal here,
                            # because `class` is a reserved keyword in Python
                            'class': dict(
                                special=True,
                            ),
                        },
                    ),
                ),
            ),
        ),
    ),
)

These two things have exactly the same meaning, but the __ syntax is a lot shorter and cleaner.

Further examples

We want to create a form to create an album for a specific artist. We already have the artist from the URL, so that field shouldn’t be in the form.

The following forms all accomplish this goal (you can use form.as_view() to create a view from a Form instance):

form = Form.create(
    auto__model=Album,
    auto__exclude=['artist'],
)

▼ Hide result

Toggle structure

form = Form.create(
    auto=dict(
        model=Album,
        exclude=['artist'],
    ),
)

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form = Form.create(
    auto__model=Album,
    fields__artist__include=False,
)

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class AlbumForm(Form):
    class Meta:
        auto__model = Album
        auto__exclude = ['artist']

form = AlbumForm.create()

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class AlbumForm(Form):
    class Meta:
        auto__model = Album
        auto__include = ['name', 'year']

form = AlbumForm.create()

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class AlbumForm(Form):
    class Meta:
        auto__model = Album
        fields__artist__include = False

form = AlbumForm.create()

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Without using the auto features:

class AlbumForm(Form):
    name = Field()
    year = Field.integer()

    class Meta:
        title = 'Create album'
        actions__submit__post_handler = create_album

form = AlbumForm()

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form = Form(
    fields__name=Field(),
    fields__year=Field.integer(),
    title='Create album',
    actions__submit__post_handler=create_album,
)

► Show result

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You can read more about this in the philosophy section under Declarative/programmatic hybrid API.