Allow None return form model Field.formfield()

[adamchainz]

I have made things!

None is allowed, for example, from AutoFieldMixin: https://github.com/django/django/blob/cdbd31960e0cf41063b3efac97292ee0ccc262bb/django/db/models/fields/__init__.py#L2828-L2829

Nones mean “ignore this field” when constructing a model form: https://github.com/django/django/blob/cdbd31960e0cf41063b3efac97292ee0ccc262bb/django/forms/models.py#L236-L248

I opened Django ticket #35748 to document this possibility.

Related issues

n/a

[sobolevn]

Thanks! TIL

[asottile-sentry]

hmmm so this is triggering a bunch of false positives after upgrading django-stubs

it seems to me that django.db.models.fields.Field.formfield always returns a Field so this annotation is wrong (?) and subclasses should override it to be | None if applicable (?) (even though it would violate substitutability -- but such is django)

[adamchainz]

Did you read the associated ticket and docs change? Any Field subclass may return None from formfield() to remove it from forms.

In Django itself, AutoFieldMixin and OneToOneField use this feature. I have used this feature at least once in my MySQL package: https://github.com/adamchainz/django-mysql/blob/1ba0471c06bf7e3b47a1ee2c544fc3738020611e/src/django_mysql/models/fields/dynamic.py#L295-L299

Adding the None option the base class fixes substitutability, no?

[asottile-sentry]

yes but fields.Field does not return None, and my subclass of fields.Field inherits the same implementation which does not return None -- and now the type checker is mad about that

[asottile-sentry]

for example this: https://github.com/adamchainz/django-jsonfield/blob/939c5a7b8c5414d2b63aa141899c9059b8ce79ec/jsonfield/tests/test_fields.py#L55-L57

[adamchainz]

Sure.. I'm happy adding assert formfield is not None to such tests. That's just type-hinted life, in my experience.