Type Hints¶

Python allows functions to include type hints that describe the expected types of parameters and return values.

The Problem¶

Consider this function:

def area(length, width):
    return length * width

What types are length and width? Are they integers, floats, or something else?

Without additional context, the reader cannot tell.

The Solution¶

Type hints make the expected types explicit.

def area(length: float, width: float) -> float:
    return length * width

A reader can immediately see the intended types.

Syntax¶

Type hints appear after parameter names and after the function arrow.

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

Here:

a: int means a should be an integer
b: int means b should be an integer
-> int means the function returns an integer

A slightly richer example:

def average(a: float, b: float) -> float:
    return (a + b) / 2

result = average(3.0, 7.0)
print(result)

Output

5.0

Returning None¶

Functions that do not return a value can be annotated with -> None.

def greet(name: str) -> None:
    print("Welcome,", name)

This tells the reader — and any analysis tool — that greet is called for its side effect (printing), not for a return value.

Type Hints Are Optional¶

Python does not enforce type hints at runtime.

This means the following will still run without error:

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

print(add("2", "3"))

Output

The function was annotated for int, but Python happily accepted two strings and concatenated them. The result "23" is valid Python but almost certainly not what the caller intended.

Static analysis tools such as mypy or pyright can examine the code without running it and warn about mismatches like this before they cause bugs.

Key Ideas¶

Type hints document the expected types of parameters (: type) and return values (-> type). Python does not enforce them at runtime, but they make code easier to read and allow static analysis tools to catch type errors early. For functions that only perform side effects and return nothing, use -> None.

Next: Function Examples.