Type hints: annotating types

Type hints (type annotations) are optional declarations that say "this variable/parameter has type X". Python does NOT verify them at runtime: they are for you, your editor (autocomplete!) and tools like mypy or pyright that perform static type-checking.

Annotating variables

nome: str = "Ada"
anni: int = 36
attivo: bool = True
prezzi: list[float] = [1.2, 3.4]

The syntax is name: type = value. Without a value it is a pure declaration:

risultato: int  # nessuna assegnazione, solo "annuncio del tipo"

Annotating functions

def saluta(nome: str, urlante: bool = False) -> str:
    s = f"Ciao {nome}"
    return s.upper() if urlante else s

def somma(numeri: list[int]) -> int:
    return sum(numeri)

Syntax: param: type for each parameter, -> type for the return. A function that returns nothing uses -> None.

Built-in generic types (Python 3.9+)

nomi: list[str] = ["a", "b"]
mappa: dict[str, int] = {"x": 1}
coppia: tuple[int, str] = (1, "ok")
unici: set[int] = {1, 2, 3}

Optional and Union

A value that may be None:

def trova_utente(id: int) -> str | None:
    if id == 1:
        return "Ada"
    return None

X | None is the modern syntax (Python 3.10+). Equivalent: Optional[X] from typing. For unions of multiple types: int | str | float.

typing: useful tools

from typing import Any, Callable

def applica(f: Callable[[int], int], x: int) -> int:
    return f(x)

valore: Any = "qualunque cosa"  # disabilita il check

• Any = "don't check" (use sparingly).
• Callable[[arg1, arg2], return] = a function that takes those parameters and returns that type.

mypy: the type-checker

You install mypy, run mypy file.py, and it tells you whether the types line up:

$ mypy script.py
script.py:5: error: Argument 1 to "somma" has incompatible type "str"; expected "int"

Checking types with mypy

Python's type hints are not enforced at runtime by the interpreter; they are ignored during execution. To validate type correctness, you must run an external static analysis tool like mypy by executing the command mypy script.py in your terminal.

Try it yourself

Review exercise

Additional challenge