Add type annotations for primary key types by model [FC-0117]

[bradenmacdonald]

This PR has some experimental implementations of #436 as well as moving closer toward the consistent terms preferred in OEP-68.

Second Approach: fully-typed .id, avoid .pk

(Revised from my initial attempt)

• PublishableEntity, LearningPackage, Container, and Component all get a fully-typed .id field.
  • PublishableEntity.id is of type PublishableEntity.PK which is a sub-type of int.
  • Container.id is of type Container.PK which is a sub-type of PublishableEntity.PK.
  • Component.id is of type Component.PK which is a sub-type of PublishableEntity.PK.
  • LearningPackage.id is of type LearningPackage.PK which is a sub-type of int.
  • Thanks to django-stubs, these types generally get propagated and used correctly pretty much everywhere.
  • Example:
    
• Likewise, CatalogCourse and CourseRun get typed.
• Unfortunately, it is impossible to change the type of .pk on models that inherit PublishableEntityMixin (or any other models that use a OneToOneField as their primary_key, at least without modifying django-stubs or implementing a mypy plugin. It will always have type Any, circumventing all type definitions. However, although we cannot override its type, we can mark it as deprecated. As a result, and for consistency, accessing .pk on any of these will raise a deprecation warning in pytest/console and show a warning in your IDE:
  
• I did not (yet?) add a deprecation warning for CatalogCourse.pk and CourseRun.pk, because (a) they are typed correctly, and (b) mypy crashes when I do 😬.

Prior Approach: fully-typed .pk, avoid .id

Expand for pros/cons/details of this earlier approach

In this earlier approach:

• PublishableEntity's primary key is the .pk field, and it has type PublishableEntity.PK which is a sub-type of int.

  • Example:
  • In the approach I took, accessing some_entity.id will show a deprecation warning, telling you to use pk instead. And since this is an actual field called pk, any DB lookups need to be changed from e.g. entity__id=... to entity__pk=...
    
  • A slightly less disruptive approach would be to keep the ID field called id and make an alias called entity_pk, then use deprecation warnings to encourage the use of entity_pk over id and pk; this would be more consistent with what I did for containers and components, but it's slightly annoying to type entity_pk instead of pk everywhere.
  • Keeping the ID field called id and making pk a fully-typed alias is unfortunately not possible, as django-stubs forcibly overrides the type of pk unless you have a concrete field named pk as I did here.

• Container and Component get Container.PK and Component.PK types which are sub-types of PublishableEntity.PK

• Container and Component cannot have correctly-typed .pk accessors due to django-stubs limitations, so I created a .container_pk and .component_pk accessor for them:
  

  • Details: because Container and Component have a OneToOneField(PublishsableEntity, primary_key=True), django-stubs will always override their pk attribute to have type Any, and it's impossible to change this without modifying django-stubs or writing our own mypy plugin. But, we can use the @deprecated decorator to annotate the attribute with a deprecation warning which does show up in the IDE and in the pytest logs wherever .pk is used.
  • I think this is clear and explicit, but would something shorter be better (.pk_ ?)

---

General Notes

One downside of using deprecation warnings as I have here is that Django itself uses .pk, so in the case of Container and Component (but not PublishableEntity) where we've marked .pk as deprecated, even if our code doesn't use .pk itself, you'll see warnings like this when running pytest: django/db/models/fields/related.py:812: DeprecationWarning: Use component_pk instead. It is possible to suppress these deprecation warnings, and I have done so (see tox.ini) but it's an extra step required to use this library well and I'm not a big fan of it.

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[bradenmacdonald]

@ormsbee @kdmccormick Can I get your thoughts on this? It also has implications for OEP-68.

[ormsbee]

Ouch. Very timely to know this given the OEP in flight on identifiers, where we recommend using .pk.

[bradenmacdonald]

Yes, as you can read above in the edited description, I actually changed my approach here, as it turns out it's much easier to override the type of .id than .pk in general. So I'm wondering if we should just recommend .id / _id more.

[kdmccormick]

oof, good find. I've opened a thread here: openedx/openedx-proposals#773 (comment)

[kdmccormick]

If we standardize on .id, then I presume we can drop this deprecation warning? I think most developers naturally use .id anyway.

[bradenmacdonald]

I found that .pk was being used in quite a few places in this codebase, and the deprecation warnings made it easy for me to track them down and replace them with .id.

[bradenmacdonald]

If we want to also introduce typed keys for Unit/Section/Subsection subclasses, that's easy to do: #535 . Would welcome thoughts on this as I'm a bit on the fence.

[kdmccormick]

If we standardize on id, I think it'd make sense to call the types Container.ID, ContainerID, etc.

[bradenmacdonald]

Sure, updated to Container.ID/ContainerID etc.

[bradenmacdonald]

OK, good news! I found a nicer way to do this. It still requires a custom field type per model, but we can put the definitions entirely within the model class itself, and there is now one global id_fields module in openedx_django_lib rather than needing id_fields.py and id_fields.pyi alongside each app[let]'s models.

e.g. within the class LearningPackage(models.Model): definition:

openedx-core/src/openedx_content/applets/publishing/models/learning_package.py

Lines 32 to 38 in 959f9e6

	LearningPackageID = NewType("LearningPackageID", int)
	type ID = LearningPackageID
	class IDField(TypedBigAutoField[ID]):
	pass
	id = IDField(primary_key=True)

Note:

This could be made even more concise, and copy-pastable without changes:

ID = NewType("ID", int) 
class IDField(TypedBigAutoField[ID]): ...
id = IDField(primary_key=True) 

But: this creates ambiguous type names (what is "ID", without context?) in the mypy output, and the use of an ellipsis instead of pass implies that something is incomplete. So I'm recommending the more verbose version as our new standard boilerplate.

Edit: actually this works great, so I think I prefer the second version now. If you have several types called ID, mypy distinguished them in the output like so:

error: Argument 1 to "update_learning_package" has incompatible type
"openedx_content.applets.publishing.models.learning_package.LearningPackage.ID"; expected
"openedx_content.applets.publishing.models.publishable_entity.PublishableEntity.ID"  [arg-type]