Other than validation, I can imagine several good reasons why one might want to wrap a primitive inside a class. For example, you may have a function: gro

Other than validation, I can imagine several good reasons why one might want to wrap a primitive inside a class. For example, you may have a function: group_by_age() -> Dict[int, List[str]] which might be perfectly good for your use case, but I can see why one might instead prefer: group_by_age() -> Dict[Age, List[CustomerId]] for self-documentation and expressiveness. Your test assertions may also become easier to read: assert group_by_age() == { Age(23): [ CustomerId("0471"), CustomerId("3390"), ], Age(42): [ CustomerId("2334"), ], }