How to make the choices argument of a TypedChoiceField in a form dynamically changeable?

Question

Django

I'm trying to make an application - the editor of the main page of the showcase site. The main page should display a maximum of 9 products. But I want the user to display a selection with position numbers no greater than the number of the next available cell. Let's say the user has already added 3 products, which means that a maximum of 4 positions should be displayed for selection, three positions for replacement and the fourth for a new product.

class ItemOnMainPageForm(forms.Form):
    item_on_main_page = forms.ModelChoiceField(
        queryset=Item.objects.all(),
        label='Выберете товар'
    )
    position = forms.TypedChoiceField(
        choices=[
            (1, 1), (2, 2), (3, 3),
            (4, 4), (5, 5), (6, 6),
            (7, 7), (8, 8), (9, 9)
        ],
        coerce=int,
        label='Позиция на главной'
    )

The current implementation allows you to display all 9 cells at once for selection. What approach can be taken to change this behavior and force the field positionto be dynamic and match the state of the system? I suspect some workaround needs to be used since the class is read once when the server starts, but which one?

Answer 1

in __init__
self.fields['position'].choices =