django-stubs-ext: Export RelatedManager, ManyRelatedManager stub-only classes

[intgr]

I have made things!

As discussed in #1814 (comment), RelatedManager, ManyRelatedManager are sometimes useful for type hinting. But these classes don't exist as is at Django runtime, rather they are defined inside function bodies.

• When TYPE_CHECKING, we re-export django-stubs fake classes.

• At runtime, we define these as Protocol[_T]. This has the advantage that Python prevents them being used with isinstance(). It fails with error:

  TypeError: Instance and class checks can only be used with @runtime_checkable protocols

Usage before:

if TYPE_CHECKING:
    from django.db.models.manager import RelatedManager  # before PR #1814

def get_manager() -> "RelatedManager[MyModel]": ...

Usage after:

from django_stubs_ext.db.models.manager import RelatedManager

def get_manager() -> RelatedManager[MyModel]: ...

Related issues

• Move RelatedManager to django.db.models.fields.related_descriptors #1814

[intgr]

Bikeshedding question: Right now I've defined these as django_stubs_ext.db.models.RelatedManager.

But Django itself defines these in django.db.models.fields.related_descriptors. Do we want to mirror this structure in django-stubs-ext?

E.g. django_stubs_ext.db.models.fields.related_descriptors.RelatedManager (Seems that Django itself considers related_descriptors like an implementation detail)

Or maybe django_stubs_ext.db.models.managers.RelatedManager is more future-proof if we add more manager-related things in the future?

I actually slightly prefer the last one: django_stubs_ext.db.models.managers.RelatedManager

[flaeppe]

Nice!

I don't see it necessary to keep a corresponding import path in this case, as it's typing specific and the classes are not importable from Django anyways.

django_stubs_ext.db.models.managers.RelatedManager looks like a relevant import path to me!

[intgr]

Moved to django_stubs_ext.db.models.managers.

Also had to rename db/models.py -> db/models/__init__.py

[intgr]

Oh wait, Django itself has django.db.models.manager (singular), I'll use singular here too for consistency: django_stubs_ext.db.models.managers -> manager

[flaeppe]

Side note: adding the django-stubs-ext label could probably be automated with https://github.com/actions/labeler (one of my favourite actions)

[intgr]

adding the django-stubs-ext label could probably be automated

Cool, yes. Also mypy-plugin label would be useful (so far we've mostly used it for issues, not PRs).

[UnknownPlatypus]

Side note: adding the django-stubs-ext label could probably be automated with https://github.com/actions/labeler (one of my favourite actions)

This looks really nice 😯