Fields on generic interfaces cannot be generic

[thearchitector]

Describe the Bug

Generics that have been decorated with @strawberry.interface are unable to define fields with generic return types. However, the same field seems to work fine if the generic is decorated as a type instead.

The following code:

from typing import Generic, TypeVar

import strawberry

T = TypeVar("T")


@strawberry.interface
class GenericInterface(Generic[T]):
    obj: strawberry.Private[T]

    @strawberry.field
    def as_list(self) -> list[T]:
        return [self.obj]


@strawberry.type
class GenericObject(GenericInterface[int]):
    pass


@strawberry.type
class Query:
    @strawberry.field
    def numbers(self) -> GenericObject:
        return GenericObject(obj=1)


res = strawberry.Schema(query=Query).execute_sync(
    """
        query {
            numbers {
                asList
            }
        }
    """
)
print(res)

produces the error:

TypeError: GenericInterface fields cannot be resolved. The type "<strawberry.type.StrawberryList object at 0x7e3c02eaece0>" is generic, but no type has been passed

If you replace @strawberry.interface with @strawberry.type, the error goes away and the query returns what I would expect:

ExecutionResult(data={'numbers': {'asList': [1]}}, errors=None, extensions={})

System Information

• Operating system: Linux
• Strawberry version (if applicable): 0.237.3

[patrick91]

Just had a quick look at this and it might not be that easy to do, at least not in a couple of hours :D but we should support this use case 😊

[thearchitector]

Just had a quick look at this and it might not be that easy to do, at least not in a couple of hours :D but we should support this use case 😊

thanks for looking at it! i agree, an interface generic seems like a really natural/perfect use case for interfaces. so much so that i figured this was a bug rather than a missing feature haha, given that it works for @types

[thearchitector]

@patrick91 wondering if this is on the roadmap?

[thearchitector]

I'm still interested in this, FWIW.

[obrazkowv]

The same problem with arguments

T = TypeVar('T')


@strawberry.type
class Base(Generic[T]):
    @strawberry.field()
    async def dummy_field(self, info: Info, value: list[T]) -> int:
        return len(value)


@strawberry.type
class Child(Base[int]):
    ...

TypeError: Child fields cannot be resolved. The type "<strawberry.types.base.StrawberryList object at 0xffffad2a6390>" is generic, but no type has been passed