Allow model signal sender to be specified lazily

[Simon Charette]

With the introduction of the swappable User model ​there is no sane way for a third party application to connect signals to it.

Hence I propose to make the Signal.connect method of model signals (pre|post_init, pre|post_save, pre|post_delete, m2m_changed) accept a model name ('app_label.ModelName') as its sender argument.

This should allow third party application to connect signals to the swappable user model by referring to it using settings.AUTH_USER_MODEL and is consistent with the suggest way of referring to it as the target of related fields.

Will be attaching a POC with missing documentation.

[Simon Charette]

Heres the ​POC.

[Anssi Kääriäinen]

Seems OK to me.

[Simon Charette]

Updated patch with documentation. It should be ready for review.

[loic84]

This patch is pretty clever in its implementation and as someone who always use full application label it has bugged me a number of times that signals didn't support this notation.

Other than the minor comments I've made on the PR, I think it's RFC.

[Anssi Kääriäinen]

Looks good to me. The only question is that could we somehow detect if there typoed references waiting - maybe in model validation code. At that point models should be loaded, so unresolved references there should be errors.

This isn't critical to do. But loud failure is better than silent ignore if it is easy enough to do.

[Simon Charette]

@akaariai I thought of it when writing the initial patch and felt like this could be a feature for third party applications. (registering signals for application that might or might not be installed).

Looking at it back those applications can always look into settings.INSTALLED_APPS before attaching lazy signals while typoed reference is less of an edgecase and would be consistent with how related fields missing reference are validated.

I'll add validation.

[Simon Charette]

Added validation and removed obvious warning.

[Simon Charette <charette.s@…>]

In eb38257e5199ca06b8b32f82dd50f64fd6b0d98d:

Fixed #21391 -- Allow model signals to lazily reference their senders.