【Python】f-stringの使い方｜便利テク・注意点・実例集

name = "Alice"
age = 20

# 変数をそのまま埋め込む
message = f"名前は{name}、年齢は{age}歳です。"
print(message)

price = 1200
tax_rate = 0.1

# f-stringの中で式をそのまま計算できる
message = f"税込価格は{price * (1 + tax_rate)}円です。"
print(message)

first_name = "Taro"
last_name = "Yamada"
age = 30

profile = f"{last_name} {first_name} さんは {age} 歳です。"
print(profile)

text = "hello"
numbers = [1, 2, 3]

# upper()などのメソッドもそのまま書ける
message = f"{text.upper()}! 要素数は {len(numbers)} 個です。"
print(message)

value = 1234.567
ratio = 0.256
count = 42

print(f"小数点以下2桁: {value:.2f}")
print(f"3桁区切り: {value:,}")
print(f"3桁区切り＋2桁: {value:,.2f}")
print(f"ゼロ埋め6桁: {count:06}")
print(f"パーセント表示: {ratio:.1%}")

from datetime import datetime

now = datetime(2024, 5, 1, 14, 30, 45)

print(f"ISO形式: {now:%Y-%m-%dT%H:%M:%S}")
print(f"日付だけ: {now:%Y/%m/%d}")
print(f"時刻だけ: {now:%H:%M}")
print(f"和風表記: {now:%Y年%m月%d日(%a) %H時%M分}")

name = "Alice"
score_math = 80
score_english = 92

report = (
    f"受験者: {name}\n"
    f"\t数学: {score_math}\n"
    f"\t英語: {score_english}"
)

print(report)

price = 1200
tax_rate = 0.1
discount_rate = 0.2

message = (
    f"元の価格: {price}円\n"
    f"税込価格: {price * (1 + tax_rate):.0f}円\n"
    f"割引後税込: {price * (1 - discount_rate) * (1 + tax_rate):.0f}円"
)

print(message)

def tax_included(price, tax_rate=0.1):
    return int(price * (1 + tax_rate))

price = 1200

message = (
    f"元の価格: {price}円\n"
    f"税込価格: {tax_included(price)}円"
)

print(message)

x = 10
y = 3

# 3.8以降で使える構文
print(f"{x=}")
print(f"{y=}")
print(f"{x + y=}")

name = "Alice"

# 外側をダブルクォート、内側をシングルクォートにする例
msg1 = f'"{name}" さんがログインしました。'
print(msg1)

# 外側をシングルクォート、内側をダブルクォートにする例
msg2 = f'"{name}" has logged in.'
print(msg2)

name = "Alice"
msg = f"\"{name}\" さんがログインしました。"
print(msg)

# 中かっこをそのまま表示したい
text = f"この辞書は {{'key': 'value'}} のような形です。"
print(text)

user = {"name": "Alice", "age": 20}

# これはSyntaxErrorになる例
# msg = f"ユーザー名: {user["name"]}、年齢: {user["age"]}"

user = {"name": "Alice", "age": 20}

# 外側をダブルクォート、内側をシングルクォートにする
msg = f"ユーザー名: {user['name']}、年齢: {user['age']}"
print(msg)

user = {"name": "Alice", "age": 20}
name = user["name"]
age = user["age"]

msg = f"ユーザー名: {name}、年齢: {age}"
print(msg)

items = [
    ("Apple", 120),
    ("バナナ", 80),
    ("みかん", 100),
]

for name, price in items:
    # 10文字幅で左寄せ、5文字幅で右寄せ
    print(f"{name:10} : {price:5}円")

# 危険な例(あくまでイメージ。実際に真似しないこと)
user_expr = input("式を入力してください: ")

# f-stringを使ってeval的なことをしてしまう
# result = eval(f"{user_expr}")

# コメント: これはあえて読みにくい例です
user = {"name": "Alice", "age": 20, "score": 92}
print(f"ユーザー名: {user['name']}、年齢: {user['age']}、評価: {'合格' if user['score'] >= 80 else '不合格'}、スコア(偏差値換算): {(user['score'] - 50) / 10 * 15:.1f}")

user = {"name": "Alice", "age": 20, "score": 92}

name = user["name"]
age = user["age"]
score = user["score"]
status = "合格" if score >= 80 else "不合格"
deviation = (score - 50) / 10 * 15

message = (
    f"ユーザー名: {name}、"
    f"年齢: {age}、"
    f"評価: {status}、"
    f"スコア(偏差値換算): {deviation:.1f}"
)

print(message)

a = 10
b = 3

result = (
    f"{a} + {b} = {a + b}\n"
    f"{a} - {b} = {a - b}\n"
    f"{a} * {b} = {a * b}\n"
    f"{a} / {b} = {a / b:.2f}"
)

print(result)

price = 1234567
tax_rate = 0.1
discount_rate = 0.15

tax_amount = int(price * tax_rate)
discount_amount = int(price * discount_rate)
final_price = price + tax_amount - discount_amount

receipt = (
    f"小計: {price:>12,} 円\n"
    f"消費税({tax_rate:.0%}): {tax_amount:>8,} 円\n"
    f"値引き({discount_rate:.0%}): -{discount_amount:>7,} 円\n"
    f"{'-' * 30}\n"
    f"合計: {final_price:>12,} 円"
)

print(receipt)

from datetime import datetime

def log_login(user_id: int, success: bool) -> None:
    now = datetime.now()
    level = "INFO" if success else "WARN"
    status = "success" if success else "failed"
    print(f"{now:%Y-%m-%d %H:%M:%S} [{level}] user_id={user_id} status={status}")

log_login(123, True)
log_login(456, False)

def divide(a: float, b: float) -> float:
    if b == 0:
        # f-stringで詳細なエラーメッセージを作る
        raise ValueError(f"0では割り算できません: a={a}, b={b}")
    return a / b

try:
    divide(10, 0)
except ValueError as e:
    print(f"エラーが発生しました: {e}")

items = [
    ("Apple", 3, 120),
    ("Banana", 5, 80),
    ("Orange", 2, 150),
]

# ヘッダ行
print(f"{'商品名':<10} {'数量':>4} {'単価':>6} {'小計':>8}")

# 区切り線
print("-" * 32)

# 各行
for name, qty, unit_price in items:
    subtotal = qty * unit_price
    print(
        f"{name:<10} "      # 左寄せ、幅10
        f"{qty:>4} "        # 右寄せ、幅4
        f"{unit_price:>6} " # 右寄せ、幅6
        f"{subtotal:>8}"    # 右寄せ、幅8
    )

import json

user = {
    "id": 123,
    "name": "Alice",
    "active": True,
    "roles": ["admin", "editor"],
}

# JSON風に整形して出力
json_text = json.dumps(user, ensure_ascii=False, indent=2)
print(f"ユーザーデータ:\n{json_text}")

def log_with_line(message: str, line_no: int) -> None:
    print(f"[line {line_no:04}] {message}")

log_with_line("処理を開始します", 1)
log_with_line("データを読み込みました", 15)

project = "report"
date = "2024-05-01"
version = 2

filename = f"{project}_{date}_v{version:02}.csv"
print(filename)

total = 100
for i in range(0, total + 1, 20):
    percent = i / total
    bar = "#" * (i // 10)
    print(f"{i:3}/{total} ({percent:>5.1%}) [{bar:<10}]")

table = "users"
user_id = 123
status = "active"

sql = (
    f"SELECT *\n"
    f"FROM {table}\n"
    f"WHERE id = {user_id} AND status = '{status}';"
)

print(sql)