What's the best way to write a type hint for any kind of model object?

Thanks for maintaining this repo! I have an issue that’s likely a documentation issue in that I can’t figure out how to create a type hint for a django model object that supports more than one model class.

I’m writing a type hint for a function that takes a model object of any type and performs an action on it, something like:

def transform_model(some_object: models.Model) -> models.Model:
    LOGGER.info("Transforming model with id %s", model.id)  # example of an error MyPy would raise
    return some_object

The issue I’m facing is that if I try to use models.Model as my type hint, I get errors like:

“Model” has no attribute “id”

as well as an error for any model methods I’m trying to use in the transformation function.

If I use a particular model class as the type hint it works fine, for instance some_object: SomeParticularModel. The issue is that I want to use this function for any type of model, not just SomeParticularModel.

Is there an appropriate way to be able to type hint a generic Django model?

[sobolevn]

my_model.say_nothing() must raise an error. Proof: https://mypy-play.net/?mypy=latest&python=3.10&gist=728d3a85fa2a39bcfb911174f4a12b75

Can you please attach a reproducable example where it does not?

[UnknownPlatypus]

If Django sees you’ve explicitly set Field.primary_key, it won’t add the automatic id column.

Because of that, django-stubs only declare the pk attribute on models.Model and not the id one because he might not exist. The presence of an id field will be correctly inferred if you give a more specific model.

@YPCrumble I think in your case you probably want to reference the pk because if one of your models were to set the primary_key on another field, you would have a runtime error:

TModel = TypeVar('TModel', bound=models.Model)

class SomeModel(models.Model):
    name = models.Charfield(primary_key=True)

def transform_model(some_object: TModel) -> TModel:
    LOGGER.info("Transforming model with id %s", model.id)
    return some_object

transform_model(SomeModel.objects.first()) # AttributeError: 'SomeModel' object has no attribute 'id'

If nonetheless, you know for sure that every models will have an id, I’m not sure how you can declare that. I guess the pk tip should do the trick.