FormMixin.get_form's form_class argument should be optional

[Simon Charette]

Just like we do with SingleObjectMixin.get_object(queryset=None) that defaults on get_queryset() we should make FormMixin.get_form(form_class) default on get_form_class to make overriding easier.

The provided patch introduces a backward compatibility issue for users that overrided get_form() on a ProcessFormView subclass and expected form_class to be not None. It could be addressed by removing the changes in ProcessFormView.get() and ProcessFormView.post().

[Simon Charette]

I'll move it to Accepted until I either get a consensus on the mailing list about the backward compatibility issues or I find time to avoid breaking the following case:

class MyFormMixin(FormMixin):
    def get_form(form_class):
        return form_class(...)

[Tim Graham]

Okay, marking as "patch needs improvement" in the meantime so this doesn't appear in the review queue.

[Simon Charette]

I think I came up with a decent solution.

If someone overrides get_form without defining a default value for form_class a deprecation warning is raised and the unbound method is wrapped in order to make sure form_class defaults to get_form_class() if not provided.

I updated the PR with the POC. I guess we should we also mention this change in the deprecated section of the release notes?

[Tim Graham]

Yes, and of course the deprecation timeline as well.

[Simon Charette]

Added the missing deprecation note and a mention in the deprecation timeline.

[Simon Charette <charette.s@…>]

In f2ddc439b1938acb6cae693bda9d8cf83a4583be:

Fixed #23656 -- Made FormMixin.get_form's form_class argument optional.

Thanks Tim Graham for the review.

[Tim Graham <timograham@…>]

In 491de4f0:

Refs #23656 -- Required FormMixin.get_form() form_class parameter to be optional.

Per deprecation timeline.