『読める・速い・止まる』Pythonの制御構文｜if / for / whileの設計術

def discount(price, member, coupon):
    if price > 10000 and (member == "gold" or coupon == "VIP") and not (price % 2 == 1):
        return int(price * 0.9)
    else:
        return price

def discount(price, member, coupon):
    is_large = price > 10000
    is_special = member == "gold" or coupon == "VIP"
    is_even_price = price % 2 == 0
    if not (is_large and is_special and is_even_price):
        return price  # 早期returnでネストを浅く
    return int(price * 0.9)

users = [{"id": 1, "name": "A"}, {"id": 2, "name": "B"}]
for user in users:  # 変数名は意味が伝わるように
    print(user["name"])

for i, user in enumerate(users, start=1):
    print(i, user["name"])

names = ["A", "B", "C"]
ages = [23, 31, 27]
for name, age in zip(names, ages):
    print(name, age)

for key, value in mapping.items():
    ...

# 良い：式が1つで副作用なし
squares = [x * x for x in range(10) if x % 2 == 0]

def read_until_empty(lines):
    result = []
    for line in lines:  # 可能ならforでイテラブルを回す
        if not line.strip():
            break
        result.append(line)
    return result

while True:
    line = input(": ")
    if not line:
        break  # 明示的に終了
    process(line)

# 例：素数判定
def is_prime(n: int) -> bool:
    if n < 2:
        return False
    for p in range(2, int(n ** 0.5) + 1):
        if n % p == 0:
            return False  # 見つかったら即終了
    else:
        return True

from collections import Counter, defaultdict

# 1. 件数カウント

c = Counter(event\["type"] for event in events)

# 2. 合計/平均（辞書で蓄積）

sums, counts = defaultdict(int), defaultdict(int)
for e in events:
k = e\["type"]
sums\[k] += e\["value"]
counts\[k] += 1
avg = {k: sums\[k] / counts\[k] for k in sums}

# 条件を関数に名詞化
def is_target(u):
    return u["active"] and u["score"] >= 80

# 1. 最初の1件

found = None
for u in users:
if is\_target(u):
found = u
break

# 2. なければデフォルト

result = found or {"id": -1, "name": "N/A"}

# 1. シンプル → 内包表記
names = [u["name"].strip().title() for u in users if u["active"]]

# 2. 複雑 → for文に戻す
cleaned = []
for u in users:
    if not u["active"]:
        continue
    name = u["name"].strip().title()
    cleaned.append({**u, "name": name})

nums = [1,2,3,4,5,6]
# TODO: 偶数だけ2乗したリストを作る

[x*x for x in nums if x % 2 == 0]

users = [{"name":"A","score":70},{"name":"B","score":85}]
# TODO: 80点以上が見つかったらname、なければ"N/A"

for u in users:
    if u["score"] >= 80:
        res = u["name"]
        break
else:
    res = "N/A"

# TODO: 入力を受け取り、空行で終了して件数を表示

cnt = 0
while True:
    line = input(": ")
    if not line:
        break
    cnt += 1
print(cnt)

def is_valid(age, email):
    # TODO: 年齢が18以上、メールに"@"を含む

def is_valid(age, email):
    if age < 18:
        return False
    if "@" not in email:
        return False
    return True

items = ["a","b","c"]
# TODO: 1始まりで "1:a" の形で出力

for i, x in enumerate(items, start=1):
    print(f"{i}:{x}")

user = {"age":25, "role":"admin", "active":True}
is_admin = user["role"] == "admin"
is_active = user["active"]
if is_admin and is_active:
    ...

# NG
while True:
    x = get()
    if x is None:
        break
    if x == "ERR":
        handle()
        continue
    if len(x) > 100:
        process(x)

# → OK（ガード節＋関数化）
def should_process(x):
    return x and x != "ERR" and len(x) > 100

for x in iter(get, None):  # get() が None を返すまで
if not should\_process(x):
continue
process(x)

# NG（O(n^2)）
s = ""
for x in items:
    s += x

# → OK（O(n)）
s = "".join(items)

# NG
for i in range(len(seq)):
    print(seq[i])

# → OK
for x in seq:
    print(x)

# NG
for a in A:
    for b in B:
        if cond(a, b):
            do(a, b)

# → OK（早期continue / 先に絞る）
for a in A:
    if not cond_a(a):
        continue
    for b in (x for x in B if cond_b(x)):
        if cond(a, b):
            do(a, b)

import re

# パースに成功した行だけ処理

while (line := input(": ")):
if (m := re.match(r"^(\d+),(\w+)$", line)):
num, name = int(m.group(1)), m.group(2)
handle(num, name)

def pick_actives(users):
    return [u for u in users if u.get("active")]

def test\_pick\_actives():
users = \[{"active": True}, {"active": False}]
assert pick\_actives(users) == \[{"active": True}]