Type hints basics

Learn type hints like int, str, list.

Type hints basics

A type hint marks the expected type of a parameter and return value:

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

Python does not enforce type hints at runtime; they are only for documentation and static analysis (mypy, pyright). Passing the wrong type still runs.

Type hints are very useful in large projects; they catch wrong usage early.

Common type hints

Common type hints are int, float, str, bool, list, dict. More complex types are written as list[int], dict[str, int]:

def count_words(words: list[str]) -> int:
    return len(words)

This clearly marks that the function expects a list[int] and returns int. IDEs use this info to give hints.

Exercise Write a function with a type hint

Define a function called square with parameter n: int and return type -> int. The body does return n * n. Assign square(5) to result. Then print print(result).

Show example solution

def square(n: int) -> int:
    return n * n
result = square(5)
print(result)

Test yourself

Complete the type hint

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

Show answer

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

What does a type hint do at runtime?
- Slows the code down
- Raises an error on wrong type
- Does nothing; it is for documentation
- Compiles the code
Show answer

Does nothing; it is for documentation — A type hint does not affect runtime; it is only for documentation and static analysis.
Match the type hints

int

Integer
str

String
list[int]

List of integers
-> int

Returns int

Next lesson: Summary - Function parameters

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