Type Hints

Hints describe what types your functions expect and return. Python doesn't enforce them at runtime — tools like mypy, pyright, and your editor do.

Cheat sheet

• x: int, name: str, flag: bool
• list[int], dict[str, int], tuple[int, str] (Python 3.9+)
• Optional[X] / X | None — nullable
• Callable[[int, int], int] — function that takes two ints, returns int
• Iterable[X] — anything you can for ... in

Why bother

• Editor autocomplete catches typos and wrong-type calls
• Self-documenting — type hints often replace half your docstring
• Forces you to think about your interfaces

Try it

1. Add type hints to a function you've written.
2. What's the difference between list and list[int]?