Validate the input and output types on a function.

def FUNCTION(VAR: TYPE, VAR2: TYPE, VAR3: TYPE = DEFAULT) -> RETURN_TYPE:
    EXPRESSION

e.g.

def greeting(name: str, count: int) -> str:
    return f"Hello {name} - your count is {count}"

Default values

If you have a default value, you can set the type too:

def greet(v: int = 123):
    print(v)


greet(456)

greet("456")
# Argument 1 to "greet" has incompatible type "str"; expected "int"

Warning

If you omit the type, then Mypy will not infer it.

def greet(v=123):
    print(v)


greet(456)

greet("456")
# No error raised unfortunately.

Data structures

Python 3.9 - list, tuple, dict, etc.

def greet_all(names: list[str]) -> str:
    msg = ','.join(names)
    
    return f"Hello, {msg}"

Python 3.8 - list, Tuple, Dict, etc.

from typing import List


def greet_all(names: List[str]) -> str:
    msg = ','.join(names)
    
    return f"Hello, {msg}"

Custom type

Create a new reusable type called Vector, to make your code more expressive.

from typing import List

Vector = List[float]


def scale(scalar: float, vector: Vector) -> Vector:
    return [scalar * num for num in vector]

new_vector = scale(2.0, [1.0, -4.2, 5.4])

If you leave out the types on the function, they just won’t be checked.