Guide for typing custom Fields

[martinlehoux]

Hello!

I'm struggling to make a pattern work, but I think it's a common pattern in Django. I would like help to implement this correctly, then I can add it as a FAQ item.

My question is : how can I implement the correct typing for a custom model Field in Django?

I have a simple wrapper around postgres DateRange field

class EnhancedDateRangeField(DateRangeField):
    range_type = EnhancedDateRange
    form_field = EnhancedDateRangeFormField

    def from_db_value(self, value: Range[date] | None, expression, connection) -> EnhancedDateRange | None:
        if value is None:
            return None

        return EnhancedDateRange(lower=value.lower, upper=value.upper, bounds=value.bounds)

I also have this typing override

class EnhancedDateRangeField(DateRangeField):
    _pyi_private_set_type: EnhancedDateRange
    _pyi_private_get_type: EnhancedDateRange
    _pyi_lookup_exact_type: EnhancedDateRange
    range_type: EnhancedDateRange  # type: ignore[assignment]
    def __get__(self, *args, **kwargs) -> EnhancedDateRange: ...  # type: ignore[no-untyped-def, override]

1. As you can expect, it works well until I tried to make a date_range = EnhancedDateRangeField(null=True). At this point, I get EnhancedDateRangeField is nullable but its generic get type parameter is not optional
2. I tried to replace all EnhancedDateRange in my typing override with a typevar TEnhanced = TypeVar("TEnhancedDateRange", bound=EnhancedDateRange | None), but then it requires to add type annotations to all usage of EnhancedDateRangeField
3. I also tried to remove the typing overrides, but then the field is DateRange

[Viicos]

#1900 would really help, however to be even more user friendly with subclasses, it will require some work on the mypy side. I'll see if I can make some progress here