Type hints are optional Python annotations for variable values, function parameters, and return values. Python ignores them during ordinary execution; tools such as Mypy use them to detect incompatible types before running the program. section 22.1.3 section 22.4
A typed Tic-Tac-Toe state can describe both fields and their defaults:
- Use NamedTuple fields such as board: List[str], player: str, quit: bool, and winner: Optional[str]. section 22.1.3
- Annotate functions with parameter types and return types, such as format_board(board: List[str]) -> str. section 22.1.6
- Use custom types such as State in function signatures so Mypy can check the values passed between functions. section 22.1.6
Runtime acceptance, static rejection
State(quit='True') still runs and stores the string because Python does not enforce the annotation. Running Mypy instead reports that quit expects bool; changing the value to True produces no issues. section 22.1.4
$ mypy typehints.py typehints.py:16: error: Argument "quit" to "State" has incompatible type "str"; expected "bool" Found 1 error in 1 file (checked 1 source file)
The diagnostic output makes the practical role of type hints visible: Mypy identifies the incompatible argument and expected type. section 22.1.4
Type hints work especially well with structured state. NamedTuple supplies named, immutable fields, while TypedDict and NamedTuple can define reusable types for later function annotations. section 22.1.1 section 22.4
