Type derivation for a field of the dataclass containing a factory function

[Renes Noah (noahrenes)]

Hello. Is there a way to make Pyright see "the meaning in this context" of ValidatorRetType?

ValidatorRetType = TypeVar("ValidatorRetType")

@dataclass
class Parameter:
    name: str
    validate: Callable[[str, str], ValidatorRetType]
    value: ValidatorRetType = field(init=False)

(aside from using typing.Any, that is)

If possible, I would like to get a solution that works with Python 3.9.

[UnboundVariable]

According to the Python typing spec, a type variable is valid only when used in the context of a "def", "class" or type alias declaration. It must be associated with one of those scopes and has no meaning if used otherwise. That's why you're seeing the error here.

I'm not sure what your intent is here. Do you mean for the Parameter class to be generic and parameterized by the type variable?

@dataclass
class Parameter(Generic[ValidatorRetType]):
    name: str
    validate: Callable[[str, str], ValidatorRetType]
    value: ValidatorRetType = field(init=False)

[Eric Traut (erictraut)]

Yes. If you're interested in more details about the scoping rules for type variables, here's the relevant section of the typing spec.