Type Hints

Once a function works, you can annotate it with type hints — they describe what type each parameter and the return value should be.

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

• a: int and b: int — each parameter's expected type
• -> int — the return type

This should look very familiar coming from TypeScript:

function add(a: number, b: number): number {
  return a + b;
}

Important difference from TypeScript

TypeScript's types are checked at compile time — the compiler will refuse to build code that violates them. Python's type hints are not enforced at all by the language itself. They're purely documentation for humans and tools:

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

add("2", "3")   # runs fine and returns "23" — Python doesn't check the hints

Nothing crashes here — Python just runs the code as written. The hints only matter to tools that choose to read them.

Why they matter for FastAPI

This is the big one: FastAPI reads your type hints at runtime and uses them to validate incoming requests, convert data automatically, and generate interactive API docs — without you writing any extra validation code. So while plain Python ignores type hints, the frameworks you're heading toward absolutely do not.

Common types you'll use

def example(
    name: str,
    age: int,
    price: float,
    is_active: bool,
    tags: list,
    metadata: dict,
) -> None:
    pass

Optional values

If a value might be None, mark it explicitly:

def greet(name: str | None = None):
    if name is None:
        print("Hello stranger")
    else:
        print(f"Hello {name}")

str | None reads as "a string, or None" — the same idea as TypeScript's string | null.