Difference between add_form and form

Question

I'm currently reading a book on Django and reached the point where the author is creating a custom user model using the following code:

from django.contrib import admin
from django.contrib.auth.admin import UserAdmin
from .forms import CustomUserCreationForm, CustomUserChangeForm from .models import CustomUser

class CustomUserAdmin(UserAdmin): 
    add_form = CustomUserCreationForm 
    form = CustomUserChangeForm 
    model = CustomUser

admin.site.register(CustomUser, CustomUserAdmin)

could someone possibly explain to me what is the difference between add_form and form?

(if anyone could also tell me why we include both CustomUser and CustomUserAdminin admin.site.register(CustomUser, CustomUserAdmin) that would be great)

thank you very much for your help!

Answer 1

In short: the add_form is used when you construct a new CustomUser, whereas the simple form is used to change data for an existing CustomerUser object.

The UserAdmin class overrides the get_form(..) method, as we can see in the source code [GitHub]:

def get_form(self, request, obj=None, **kwargs):
    """
    Use special form during user creation
    """
    defaults = {}
    if obj is None:
        defaults['form'] = self.add_form
    defaults.update(kwargs)
    return super().get_form(request, obj, **defaults)

Now the get_form itself of the ModelAdmin class (the superclass of UserAdmin) invokes the modelform_factory(..) with a dictionary of attributes, as we can see in the source code [GitHub]:

def get_form(self, request, obj=None, change=False, **kwargs):
    # ...

    defaults = {
        'form': form,
        'fields': fields,
        'exclude': exclude,
        'formfield_callback': partial(self.formfield_for_dbfield, request=request),
        **kwargs,
    }

    if defaults['fields'] is None and not modelform_defines_fields(defaults['form']):
        defaults['fields'] = forms.ALL_FIELDS

    try:
        return modelform_factory(self.model, **defaults)
    except FieldError as e:
        raise FieldError(
            '%s. Check fields/fieldsets/exclude attributes of class %s.'
            % (e, self.__class__.__name__)
        )

That dictionary is updated with parameters that are passed as named arguments. So with form=self.add_form. This thus results in using a different form: the add_form.

This can be useful if we, for example, only want to be able to update specific fields when we update a CustomUser.

Note that only the UserAdmin has such get_form override, a ModelAdmin itself has no add_form to the best of my knowledge, and thus specifying such attribute will not make a difference.