Python型ヒント入門！typingモジュールで静的解析を活用してコードの品質を向上させよう

# 従来のコード
def calculate_tax(price, rate):
    return price * rate

# 型ヒント付きのコード
def calculate_tax(price: float, rate: float) -> float:
    return price * rate

# 基本的な型ヒント
name: str = "たなべ"
age: int = 25
height: float = 170.5
is_student: bool = True

# リストの型ヒント
numbers: list[int] = [1, 2, 3, 4, 5]
names: list[str] = ["たなべ", "リンド", "太郎"]

def greet(name: str) -> str:
    return f"こんにちは、{name}さん！"

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

# 戻り値がない場合はNoneを指定
def print_message(message: str) -> None:
    print(message)

from typing import Union

def process_id(user_id: Union[int, str]) -> str:
    return f"ID: {user_id}"

# Python 3.10以降では | 記法も使用可能
def process_id_new(user_id: int | str) -> str:
    return f"ID: {user_id}"

from typing import List, Dict, Tuple, Set

# リストの中身の型を指定
student_grades: List[int] = [85, 92, 78, 90]
product_names: List[str] = ["ノートPC", "マウス", "キーボード"]

# 辞書のキーと値の型を指定
user_ages: Dict[str, int] = {"たなべ": 25, "リンド": 22}
config: Dict[str, Union[str, int, bool]] = {
    "host": "localhost",
    "port": 8080,
    "debug": True
}

# タプルの各要素の型を指定
coordinate: Tuple[float, float] = (35.6762, 139.6503)
rgb_color: Tuple[int, int, int] = (255, 128, 0)

# セットの中身の型を指定
unique_numbers: Set[int] = {1, 2, 3, 4, 5}

from typing import Optional

def find_user(user_id: int) -> Optional[str]:
    users = {1: "たなべ", 2: "リンド"}
    return users.get(user_id)  # 見つからない場合はNoneを返す

# Python 3.10以降では | None 記法も可能
def find_user_new(user_id: int) -> str | None:
    users = {1: "たなべ", 2: "リンド"}
    return users.get(user_id)

from typing import Callable

def apply_operation(numbers: List[int], operation: Callable[[int], int]) -> List[int]:
    return [operation(num) for num in numbers]

# 使用例
def double(x: int) -> int:
    return x * 2

result = apply_operation([1, 2, 3, 4], double)  # [2, 4, 6, 8]

pip install mypy

# sample.py
def add_numbers(a: int, b: int) -> int:
    return a + b

# 正しい使用例
result1 = add_numbers(5, 3)

# 型が間違っている例
result2 = add_numbers("5", 3)  # mypyがエラーを検出

mypy sample.py

sample.py:7: error: Argument 1 to "add_numbers" has incompatible type "str"; expected "int"

# 最初は重要な関数から始める
def calculate_total(prices: List[float]) -> float:
    return sum(prices)

# 徐々に範囲を広げていく
class ShoppingCart:
    def __init__(self) -> None:
        self.items: List[str] = []
        self.quantities: List[int] = []
    
    def add_item(self, item: str, quantity: int) -> None:
        self.items.append(item)
        self.quantities.append(quantity)
    
    def get_total_items(self) -> int:
        return sum(self.quantities)

[mypy]
python_version = 3.14
warn_return_any = True
warn_unused_configs = True
disallow_untyped_defs = True

from typing import List, Optional
from dataclasses import dataclass

@dataclass
class Student:
    name: str
    age: int
    grades: List[int]
    email: Optional[str] = None
    
    def get_average_grade(self) -> float:
        if not self.grades:
            return 0.0
        return sum(self.grades) / len(self.grades)
    
    def add_grade(self, grade: int) -> None:
        if 0 <= grade <= 100:
            self.grades.append(grade)
        else:
            raise ValueError("成績は0-100の範囲で入力してください")

class Classroom:
    def __init__(self) -> None:
        self.students: List[Student] = []
    
    def add_student(self, student: Student) -> None:
        self.students.append(student)
    
    def get_class_average(self) -> float:
        if not self.students:
            return 0.0
        
        total_average = sum(student.get_average_grade() for student in self.students)
        return total_average / len(self.students)

from typing import Dict, List, Tuple

# 型エイリアスの定義
UserID = int
UserName = str
UserData = Dict[str, Union[str, int, bool]]
Coordinate = Tuple[float, float]

# 使用例
def create_user_profile(user_id: UserID, name: UserName, data: UserData) -> bool:
    # ユーザープロフィール作成処理
    return True

def calculate_distance(point1: Coordinate, point2: Coordinate) -> float:
    # 2点間の距離を計算
    return ((point2[0] - point1[0])**2 + (point2[1] - point1[1])**2)**0.5

# ❌ 悪い例: あまりにも厳密すぎる型指定
def process_data(data: List[Dict[str, Union[int, str, float, bool]]]) -> None:
    pass

# ✅ 良い例: 適度な抽象化
from typing import Any, Dict, List

ProcessableData = List[Dict[str, Any]]

def process_data(data: ProcessableData) -> None:
    pass

# ❌ 悪い例: 型ヒントの不整合
def get_user_age(user_id: int) -> str:  # 年齢なのにstrを返すのは不自然
    return "25"

# ✅ 良い例: 自然な型ヒント
def get_user_age(user_id: int) -> int:
    return 25

def get_user_age_display(user_id: int) -> str:  # 表示用なら文字列も自然
    age = get_user_age(user_id)
    return f"{age}歳"