generic type inference when combining containers with different element types

[Agostin12]

I'm trying to combine two Cache instances with different element types and preserve both types in the result.

class Cache[T]:
    def __init__(self, items: list[T]) -> None:
        self.items = items

def combine[T, U](
    first: Cache[T],
    second: Cache[U],
) -> Cache[T | U]:
    return Cache(first.items + second.items)

Is Cache[T | U] the correct return type here, or is there a better way to type this?

What I'd like to avoid is losing type information or having to use Any or cast.

[cleonaia]

You can use separate type parameters for the inputs and return their union:

def combine[T, U](
    first: Cache[T],
    second: Cache[U],
) -> Cache[T | U]:
    return Cache(first.items + second.items)

For Cache[int] and Cache[str], the result is Cache[int | str]. This keeps both element types without requiring them to be identical.

Since list is invariant, the result won't automatically be assignable to Cache[object], but Cache[T | U] is the appropriate type if the combined cache can contain values of either type. Hope that helps!!