Python Typing Guide

A beginner-friendly guide to Python type hints and typing concepts commonly used in modern Python projects.

This document focuses on the typing features frequently used in 42 Python modules and real-world applications.

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Why Use Typing?

Type hints improve:

• Code readability
• Error detection
• IDE autocompletion
• Project maintainability
• Static analysis with mypy

Typing makes code easier to understand and safer to maintain.

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Basic Type Hints

Example

name: str = "Sara"
age: int = 20
height: float = 1.70
is_active: bool = True

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Function Type Hints

Example

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

Explanation

• a: int → parameter must be an integer
• -> int → function returns an integer

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list[str]

Used for lists containing only strings.

Example

names: list[str] = ["Sara", "John", "Alice"]

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dict[str, int]

Dictionary where: - keys are strings - values are integers

Example

student_scores: dict[str, int] = {
    "Sara": 18,
    "John": 15
}

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tuple[int, int]

Tuple with two integers.

Very common in: - game development - coordinates - maze projects - pathfinding systems

Example

position: tuple[int, int] = (5, 10)

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Optional

Optional means a value can be: - a specific type - or None

Example

from typing import Optional

nickname: Optional[str] = None

Equivalent to:

nickname: str | None = None

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Union

Union allows multiple possible types.

Example

from typing import Union

value: Union[int, str]

Equivalent to:

value: int | str

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Any

Any disables type checking for a variable.

Example

from typing import Any

data: Any

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Callable

Callable describes functions as types.

Example

from collections.abc import Callable

operation: Callable[[int, int], int]

Meaning: - accepts two integers - returns one integer

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Callable Function Example

from collections.abc import Callable

def calculate(
    a: int,
    b: int,
    operation: Callable[[int, int], int]
) -> int:

    return operation(a, b)

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Nested Typing

Typing structures can be combined.

Example

maze: list[list[int]]

A list containing lists of integers.

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Complex Example

player_positions: dict[str, tuple[int, int]]

Dictionary: - key → player name - value → (x, y) coordinates

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Common 42 Examples

Maze coordinates

position: tuple[int, int]

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BFS queue

queue: list[tuple[int, int]]

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Graph structure

graph: dict[str, list[str]]

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Type Aliases

Useful for improving readability.

Example

Position = tuple[int, int]

player_position: Position
enemy_position: Position

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Static Type Checking with mypy

mypy checks typing without running the code.

Run mypy

mypy .

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Common mypy Errors

Missing return statement

def get_value() -> int:
    pass

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Incompatible types

age: int = "hello"

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Best Practices

• Avoid unnecessary Any
• Use precise types
• Use Optional when needed
• Keep function return types explicit
• Prefer readable typing over overly complex typing

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Summary Table

Typing Feature	Purpose
list[str]	List of strings
dict[str, int]	Dictionary with string keys and integer values
tuple[int, int]	Tuple with two integers
Optional[T]	Type or None
Union[A, B]	Multiple possible types
Any	Disables type checking
Callable	Function as a type

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Final Notes

Typing is not only about satisfying mypy.

Good typing: - documents code - improves maintainability - helps teamwork - reduces bugs - makes large projects easier to manage

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