Pythonのリスト・辞書内包表記でコードを簡潔に！初心者向け基本から応用まで

numbers = [1, 2, 3, 4, 5]
doubled = []
for num in numbers:
    doubled.append(num * 2)
print(doubled)  # [2, 4, 6, 8, 10]

numbers = [1, 2, 3, 4, 5]
doubled = [num * 2 for num in numbers]
print(doubled)  # [2, 4, 6, 8, 10]

[式 for 変数 in イテラブル]

# 文字列のリストを大文字に変換
fruits = ['apple', 'banana', 'cherry']
upper_fruits = [fruit.upper() for fruit in fruits]
print(upper_fruits)  # ['APPLE', 'BANANA', 'CHERRY']

# 数値の平方を計算
squares = [x**2 for x in range(1, 6)]
print(squares)  # [1, 4, 9, 16, 25]

# 偶数のみを抽出
numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
even_numbers = [num for num in numbers if num % 2 == 0]
print(even_numbers)  # [2, 4, 6, 8, 10]

# 長さが5文字以上の単語のみを抽出
words = ['cat', 'elephant', 'dog', 'butterfly', 'fish']
long_words = [word for word in words if len(word) >= 5]
print(long_words)  # ['elephant', 'butterfly']

# 偶数は2倍、奇数は3倍にする
numbers = [1, 2, 3, 4, 5]
processed = [num * 2 if num % 2 == 0 else num * 3 for num in numbers]
print(processed)  # [3, 4, 9, 8, 15]

# 文字列の長さに応じて処理を変える
words = ['a', 'hello', 'hi', 'python']
result = [word.upper() if len(word) > 2 else word * 2 for word in words]
print(result)  # ['aa', 'HELLO', 'hihi', 'PYTHON']

{キー式: 値式 for 変数 in イテラブル}

# 数値とその平方の辞書を作成
squares_dict = {x: x**2 for x in range(1, 6)}
print(squares_dict)  # {1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

# 文字列のリストから文字数の辞書を作成
fruits = ['apple', 'banana', 'cherry']
length_dict = {fruit: len(fruit) for fruit in fruits}
print(length_dict)  # {'apple': 5, 'banana': 6, 'cherry': 6}

# 価格辞書を消費税込みに変換
prices = {'apple': 100, 'banana': 80, 'cherry': 150}
tax_included = {item: price * 1.1 for item, price in prices.items()}
print(tax_included)  # {'apple': 110.0, 'banana': 88.0, 'cherry': 165.0}

# 辞書のキーと値を入れ替える
original = {'a': 1, 'b': 2, 'c': 3}
swapped = {value: key for key, value in original.items()}
print(swapped)  # {1: 'a', 2: 'b', 3: 'c'}

# 偶数のキーのみを持つ辞書を作成
numbers = range(1, 11)
even_squares = {x: x**2 for x in numbers if x % 2 == 0}
print(even_squares)  # {2: 4, 4: 16, 6: 36, 8: 64, 10: 100}

# 特定の条件を満たすアイテムのみを抽出
students = {'Alice': 85, 'Bob': 92, 'Charlie': 78, 'Diana': 96}
high_scorers = {name: score for name, score in students.items() if score >= 90}
print(high_scorers)  # {'Bob': 92, 'Diana': 96}

# CSVから読み込んだデータの処理例
csv_data = [
    ['Alice', '25', '85'],
    ['Bob', '30', '92'],
    ['Charlie', '22', '78']
]

# 名前と年齢の辞書を作成
people = {row[0]: int(row[1]) for row in csv_data}
print(people)  # {'Alice': 25, 'Bob': 30, 'Charlie': 22}

# スコアが80以上の人の名前のリストを作成
high_performers = [row[0] for row in csv_data if int(row[2]) >= 80]
print(high_performers)  # ['Alice', 'Bob']

# JSON APIからのレスポンス例
api_response = [
    {'id': 1, 'name': 'Product A', 'price': 1000, 'in_stock': True},
    {'id': 2, 'name': 'Product B', 'price': 1500, 'in_stock': False},
    {'id': 3, 'name': 'Product C', 'price': 800, 'in_stock': True}
]

# 在庫ありの商品名のリストを作成
available_products = [item['name'] for item in api_response if item['in_stock']]
print(available_products)  # ['Product A', 'Product C']

# 商品IDと価格の辞書を作成
price_map = {item['id']: item['price'] for item in api_response}
print(price_map)  # {1: 1000, 2: 1500, 3: 800}

# 悪い例：複雑すぎて読みにくい
result = [func(x) for sublist in data for x in sublist if condition(x) and other_condition(x)]

# 良い例：適度に分割して可読性を保つ
filtered_data = [x for sublist in data for x in sublist if condition(x)]
result = [func(x) for x in filtered_data if other_condition(x)]

# 大きなデータセットの場合はジェネレータ式を検討
# リスト内包表記（メモリを多く使用）
large_list = [expensive_function(x) for x in huge_dataset]

# ジェネレータ式（メモリ効率が良い）
large_generator = (expensive_function(x) for x in huge_dataset)

# 内包表記に適している例
squared_evens = [x**2 for x in numbers if x % 2 == 0]

# 従来のfor文が適している例（副作用がある処理）
for item in items:
    if validate(item):
        save_to_database(item)
        send_notification(item.user)