Nullable model fields have type error when trying to set None as value

[dbushy727]

Thank you so much for this package. It really has helped bring so much more visibility into my codebase. 🙏

One thing I'm struggling with is setting the right type when I have a nullable Model field. I find that when i look at an instance of the model, it shows me the value or none, but when trying to set None, i get a type error.

Django version: 3.2.10 Python version: 3.10.1

class MyModel(model.Model):
    # hover on variable shows DecimalField[Decimal]
    total_cost: Optional[models.DecimalField[Decimal]] = models.DecimalField(max_digits=12, decimal_places=2, null=True) 

my_model_instance = MyModel.objects.first()
# hover shows Decimal | None (which is great!)
my_model_instance.total_cost 

# hover shows error Cannot assign member "total_cost" for type "MyModel"  
# "None" is incompatible with "Decimal | Combinable"
my_model.total_cost = None 

Ive tried playing with the annotation and I can't seem to get it to work. I think this may be a bug in how the generic handles None.

class MyModel(model.Model):
    # hover on expression shows Expression of type "DecimalField[Decimal]" cannot be assigned to declared type 
    # "DecimalField[Decimal | None]"
    #  TypeVar "_DEC@DecimalField" is invariant
    #    Type "Decimal" cannot be assigned to type "Decimal | None"
    #      Type cannot be assigned to type "None"
    total_cost: models.DecimalField[Optional[Decimal]] = models.DecimalField()

When looking at the stub file for DecimalField, I noticed that i see a type error for the null field.

I think the Literal is the problem and if we use the one from typings it works for me, but im not sure if there are downstream effects.

I'll submit a PR to showcase what im talking about. Please let me know if this is something im doing wrong, or something we can address to fix. Any help would be super appreciated, thanks!

[sbdchd]

That's really weird pylance is erroring about Literal, I think we have some test cases that use that functionality

[sminnee]

Locally I've tried importing Literal from typing rather than typing_extensions (python 3.9) and that seems to improve things. I think maybe pyright isn't treating typing_extensions.Literal correctly any more?

[sminnee]

I've noted this on the pyright project here: https://github.com/microsoft/pyright/issues/3936

[sminnee]

Never mind, this was a bug in my environment.