In admin, inlines should be allowed to be properties

[mariarchi]

Currently, if admin class defines an attribute 'inlines' which is not an instance of tuple or a list, django will raise an error.
I believe that this is wrong and contradicts the duck typing philosophy - we shouldn't care that the attribute 'inlines' is an instance of list/tuple, but rather we should care that it is iterable.

The reason why I believe that behavior is wrong is the following:
one might want to define 'inlines' as a property. This might happen when admin class A inherits from admin class B, and class B adds some inlines (that can be useful when developing a pluggable application). If class A has some inlines of its own, it can be useful to declare inlines as a property and then call 'super' to retrieve the inlines of class B.

[mariarchi]

Anyway, that behavior is quite inconsistent with the fact that for most attributes in model/admin/url definition django does not check the instance of said attribute

[Julien Phalip]

Currently a lot of ModelAdmin's attributes (list_display, readonly_fields, list_filter, etc.) are expected to be lists or tuples. ModelAdmin.inlines shouldn't be treated as a special case. If we allow it to be merely an iterable, then the same should be done for all those other attributes. (Hint: check out django.contrib.admin.validation.check_isseq())

See also #16089 which is looking at adding flexibility with inlines registration.

[Aymeric Augustin]

Change UI/UX from NULL to False.

[Baptiste Mispelon]

Turns out this got fixed in 1d8eb0cae57731b481a88dca272b2cb0d645bd8e.

I wrote a ​pull request to add a test for the specific usecase of the original ticket but I'm not 100% convinced it's necessary.
To me, the test basically checks that python properties work which seems silly. Plus if we're testing ModelAdmin.inlines then why not check all other potential ModelAdmin options?

[Mariusz Felisiak]

Thanks for checking this. I don't think that we need more tests.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 4161e35:

Refs #12679 -- Added test for using property as ModelAdmin.inlines.

Fixed in 1d8eb0cae57731b481a88dca272b2cb0d645bd8e.