【保存版】Python f-string書式一覧｜よく使うパターンとNG例

name = "Alice"
age = 30

# f-stringの基本
message = f"My name is {name}, I am {age} years old."
print(message)

name = "Alice"
score = 95.5

# 従来のformat
msg1 = "Name: {}, Score: {:.1f}".format(name, score)

# f-string
msg2 = f"Name: {name}, Score: {score:.1f}"

print(msg1)
print(msg2)

x = 10
y = 20

# Python 3.8以降で使えるデバッグ用f-string
print(f"{x=}, {y=}, {x + y=}")

f"{式:フォーマット指定}"

s = "abc"

print(f"右寄せ: [{s:10}]")
print(f"左寄せ: [{s:<10}]")
print(f"中央寄せ: [{s:^10}]")
print(f"長さ制限: [{s:.2}]")
print(f"埋め文字変更: [{s:*>10}]")

n = 42
big = 1234567

print(f"10進数: {n:d}")
print(f"幅5 右寄せ: [{n:5d}]")
print(f"幅5 ゼロ埋め: [{n:05d}]")
print(f"符号付き: {n:+d}")
print(f"符号固定+ゼロ埋め: [{n:=+5d}]")
print(f"3桁区切り: {big:,}")

x = 123.456

print(f"固定小数点(デフォルト): {x:f}")
print(f"小数点以下2桁: {x:.2f}")
print(f"幅8かつ小数2桁: [{x:8.2f}]")
print(f"有効数字(デフォルト): {x:g}")
print(f"有効数字3桁: {x:.3g}")
print(f"指数表記: {x:e}")

rate = 0.1234

print(f"デフォルト%: {rate:%}")
print(f"小数2桁%: {rate:.2%}")
print(f"幅8かつ小数1桁: [{rate:8.1%}]")

n = 255

print(f"2進数: {n:b}")
print(f"8進数: {n:o}")
print(f"16進数(小文字): {n:x}")
print(f"16進数(大文字): {n:X}")
print(f"接頭辞付き16進数: {n:#x}")
print(f"接頭辞付き2進数: {n:#b}")

from datetime import datetime

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

print(f"ISO風: {now:%Y-%m-%d %H:%M:%S}")
print(f"日付のみ: {now:%Y/%m/%d}")
print(f"時刻のみ: {now:%H:%M}")
print(f"和風表記: {now:%Y年%m月%d日(%a) %H時%M分}")

x = 10
y = 20
z = x * y

# 3.8以降で使える
print(f"{x=}, {y=}, {z=}")

print(f"x={x}, y={y}, z={z}")

user = {"name": "Alice", "age": 30}
scores = [80, 90, 100]

print(f"ユーザー: {user['name']} ({user['age']}歳)")
print(f"最初のスコア: {scores[0]}")
print(f"平均スコア: {sum(scores) / len(scores):.1f}")

config = {
    "db": {
        "host": "localhost",
        "port": 5432,
    },
    "debug": True,
}

print(f"DB: {config['db']['host']}:{config['db']['port']} (debug={config['debug']})")

score = 75
status = "OK" if score >= 60 else "NG"

print(f"点数: {score} ({'合格' if score >= 60 else '不合格'})")
print(f"ステータス: {status}")

value = 12.3456
width = 8
precision = 3

# 幅と精度を変数で指定
print(f"幅={width}, 精度={precision}: [{value:{width}.{precision}f}]")

title = "レポート"
value = 123.456

report = f"""
=== {title} ===
値: {value:.2f}
2倍: {value * 2:.2f}
"""

print(report)

import textwrap

title = "レポート"
value = 123.456

report = f"""
    === {title} ===
    値: {value:.2f}
    2倍: {value * 2:.2f}
"""
print(textwrap.dedent(report))

name = "data.txt"

# NG例: \t がタブとして解釈される
path = f"C:\temp\{name}"
print(path)

name = "data.txt"

# 方法1: バックスラッシュをエスケープする
path1 = f"C:\\temp\\{name}"

# 方法2: os.path.joinを使う(推奨)
import os
path2 = os.path.join("C:", "temp", name)

# 方法3: Raw文字列とf-stringを分ける(Python 3.12以前では f-r は制約が多い)
path3 = r"C:\temp" + "\\" + name  # もしくは pathlib.Path を使う

print(path1)
print(path2)
print(path3)

x = 10

# NG: 構文エラーになる
# text = f"値は {x}, プレースホルダは {value} です"

# OK: {{ と }} で { と } を表す
text = f"値は {x}, プレースホルダは {{value}} です"
print(text)

user = {"name": "Alice", "age": 30, "tags": ["admin", "editor"]}

# NG例: 複雑すぎる
print(
    f"Name={user['name'].upper()}, "
    f"Age={user['age']}, "
    f"Role={','.join(t for t in user['tags'] if t != 'editor') or 'none'}"
)

user = {"name": "Alice", "age": 30, "tags": ["admin", "editor"]}

name_upper = user["name"].upper()
age = user["age"]
roles = ",".join(t for t in user["tags"] if t != "editor") or "none"

print(f"Name={name_upper}, Age={age}, Role={roles}")

import locale

value = 1234.56

# ロケールに依存しない: f-stringの標準フォーマット
print(f"標準: {value:,.2f}")

# ロケール依存: locale.format_string を使う例
locale.setlocale(locale.LC_ALL, "")  # 環境のロケールに依存
formatted = locale.format_string("%.2f", value, grouping=True)
print(f"ロケール依存: {formatted}")

user_input = "1; DROP TABLE users;"  # 例

# NG: SQLインジェクションの危険
query = f"SELECT * FROM users WHERE id = {user_input}"
print(query)

import sqlite3

conn = sqlite3.connect(":memory:")
cursor = conn.cursor()

user_input = "1; DROP TABLE users;"

# OK: プレースホルダとパラメータで安全に扱う
safe_query = "SELECT * FROM users WHERE id = ?"
cursor.execute(safe_query, (user_input,))

# NG: ループ内で逐次連結
result = ""
for i in range(10000):
    result += f"{i},"

# OK: リストに貯めて最後にjoin
parts = []
for i in range(10000):
    parts.append(f"{i}")  # f-string自体はOK

result = ",".join(parts)

result = ",".join(f"{i}" for i in range(10000))