Use assignment with bound TypeVar in conditional.

bound_base_conditional.py

from typing import Type, TypeVar


class Base:
    common: str = "Data"


class ImportModel(Base):
    pass


class RunModel(Base):
    pass


ModelType = TypeVar("ModelType", bound=Base)


def process(model: Type[ModelType]) -> ModelType:
    return model()


def use_process(is_run: bool, is_imported: bool) -> str:
    if is_run and is_imported:
        raise ValueError("Ambigious")

    # This is required in this case as mypy can't infer the type of x properly
    # in this case and will infer it to RunModel unless explicitly defined
    # before.
    #
    # To do this will result in the following error
    # bound_base_conditional.py:37: error: Incompatible types in assignment (expression has type "ImportModel", variable has type "RunModel")
    x: Base
    if is_run:
        x = process(RunModel)
    elif is_imported:
        x = process(ImportModel)
    else:
        raise ValueError("Need to be either imported or run")
    return x.common


print(__name__)
print(
    use_process(
        is_run=(__name__ == "__main__"),
        is_imported=(__name__ == "bound_base_conditional"),
    )
)

I got caught with this today where the inferred type worked in some cases but not when the conditional was based on external/run-time arguments. In those cases the variable needs to be declared with the base-class type before assignment or mypy will assume it to the first assignment type.

Easy fix when one sees it but a bit tricky semantics when it suddenly stops working. I was first thinking I had done something weird with my TypeVar.