Support disabled form fields

[Claude Paroz]

I'd like to propose adding support for disabled form fields in Django.
If it were only a matter of adding widget attributes, like it's feasible with the readonly attribute, it wouldn't need special treatment. However, the disabled attribute is particular in that user agents are not transmitting their values when POSTing form data.

A typical use case is a ModelForm where you'd like to disable some fields while still displaying all values. So currently, if you simply add disabled to the widget attributes, you'll have a problem: the form will clear the field as the value is not present in the data (and the field is not in excluded fields).

[Claude Paroz]

POC pull request: ​https://github.com/django/django/pull/4958

[Tim Graham]

I'd suggest to write to the mailing list to get some ideas for this. An implementation that reduces the amount of admin-specific code needed for ModelAdmin.readonly_fields might be interesting (#342 / [bcd9482a20]).

[Claude Paroz]

ModelAdmin.readonly_fields has chosen another route, that is displaying the value of the field instead of displaying a disabled form widget containing the value. I can write to the mailing list, but I'm not sure about what sort of ideas I would ask for.

[Tim Graham]

Do you prefer or require disabled form widgets as opposed to displaying the values? Just curious.

[Claude Paroz]

Displaying a value is triggering a whole different machinery, as the readonly_fields admin patch shows. I think what we address in this issue is a lot simpler as we simply rely on a HTML attribute. I think that displaying the disabled form widget makes sense as we are still in the context of displaying a form, but for whatever reason, some field is not editable by the current user.

[Claude Paroz <claude@…>]

In 1ef4aeab:

Fixed #25078 -- Added support for disabled form fields

Thanks Keryn Knight and Tim Graham for the reviews.