Yan Zhou | Oct2025-1 | Tools | Sprint 5 | Types and Typechecking - #8

Core theme: Catch bugs before running code — using types, mypy, classes, and enums.

✅ Self-Checklist
Added type annotations to functions and caught bugs with mypy.
Built a Person class with init, methods, and datetime.date for age.
Used @DataClass(frozen=True) to auto-generate constructors, equality, and print.
Used List[Person] generics so mypy knows what's inside a list.
Refactored preferred_operating_system from str to List[str], guided by mypy errors.
Used Enum to replace unsafe strings like "ubuntu" with OperatingSystem.UBUNTU.
Validated user input early, output errors to stderr, and exit with non-zero code.
Explored inheritance where a subclass overrides largest() with a faster implementation.

🔄 Changelist
mypy + annotations for bug detection without running code, class and @DataClass for grouping related data and methods, List[str] and List[Person] for typed containers, Enum for a fixed set of valid values, and inheritance with super().init() for reusing and extending a class.