type-hints.md

Python by Example: Type Hints

Type hints annotate variables, parameters, and return values. They don't change runtime behavior—Python ignores them. But tools like mypy and IDEs use them to catch bugs and improve autocomplete. Hints are optional; add them gradually to existing code. Use list[int], dict[str, int] (Python 3.9+); for older Python, use List[int] from typing.

What you'll learn:

• Annotating parameters and return values
• Generic types: list[str], dict[str, int]
• Optional for values that can be None

def greet(name: str) -> str:
    return f"Hello, {name}!"


def add(a: int, b: int) -> int:
    return a + b


def process(items: list[str]) -> dict[str, int]:
    return {s: len(s) for s in items}


print(greet("Alice"))
print(add(2, 3))
print(process(["a", "bb", "ccc"]))


# Optional (can be None)
from typing import Optional


def find(items: list[str], target: str) -> Optional[int]:
    for i, x in enumerate(items):
        if x == target:
            return i
    return None

Optional[int] means int | None. In Python 3.10+, you can write int | None directly. Run mypy script.py to check types.

To run this program:

$ python source/type-hints.py
Hello, Alice!
5
{'a': 1, 'bb': 2, 'ccc': 3}

Tip: Start with function signatures—they give the biggest benefit. Add variable hints where they clarify tricky code.

Try it: Add type hints to a function you wrote earlier.

Source: type-hints.py

Next: Exceptions