Python関数とスコープの設計術：I/O分離×型ヒントで再利用性とテスト容易性を最大化

from typing import Iterable, Sequence

def top\_k(values: Sequence\[float], k: int, \*, descending: bool = True) -> list\[float]:
"""値の上位k件を返す（安定ソート不要想定）。"""
if k <= 0:
return \[]
sorted\_vals = sorted(values, reverse=descending)
return sorted\_vals\[:k]

from typing import Iterable, NamedTuple

class Stat(NamedTuple):
mean: float
std: float

def describe(xs: Iterable\[float]) -> Stat:
xs = list(xs)
m = sum(xs) / len(xs)
v = sum((x - m) \*\* 2 for x in xs) / len(xs)
return Stat(mean=m, std=v \*\* 0.5)

# 悪い例：I/Oと計算が混在しテスト不能

def analyze\_and\_save(path: str) -> None:
rows = open(path).read().splitlines()  # I/O
nums = \[int(r) for r in rows]
avg = sum(nums) / len(nums)            # 計算
print(avg)                             # I/O
open("out.txt", "w").write(str(avg))  # I/O

# 良い例：I/Oを端に、計算を中心に

from pathlib import Path

def read\_numbers(p: Path) -> list\[int]:  # I/O
return \[int(x) for x in p.read\_text().splitlines()]

def mean(xs: list\[int]) -> float:        # 計算（純粋）
return sum(xs) / len(xs)

def write\_text(p: Path, text: str) -> None:  # I/O
p.write\_text(text)

# “アプリ層”でつなぐだけ

nums = read\_numbers(Path("in.txt"))
write\_text(Path("out.txt"), f"{mean(nums):.2f}")

def make_counter():
    count = 0
    def inc():
        nonlocal count
        count += 1
        return count
    return inc

# NG
def append_item(x, bucket=[]):
    bucket.append(x)
    return bucket

# OK

def append\_item(x, bucket=None):
if bucket is None:
bucket = \[]
bucket.append(x)
return bucket

from collections.abc import Callable
import json

def fetch\_json(url: str, get: Callable\[\[str], str]) -> dict:
"""HTTPクライアント(get)を注入してテスト容易に"""
return json.loads(get(url))

# 本番

import urllib.request
fetch\_json("[https://ex](https://ex)", lambda u: urllib.request.urlopen(u).read().decode())

# テスト（ネットに出ない）

def fake\_get(\_):
return '{"ok"\:true}'
assert fetch\_json("x", fake\_get)\["ok"] is True

from functools import lru_cache

@lru\_cache(maxsize=1024)
def fib(n: int) -> int:
if n < 2:
return n
return fib(n-1) + fib(n-2)

from functools import partial

def discount(price: int, rate: float, \*, cap: int | None = None) -> int:
y = int(price \* (1 - rate))
return min(y, cap) if cap is not None else y

black\_friday = partial(discount, rate=0.3, cap=10000)
black\_friday(12000)  # => 10000

from dataclasses import dataclass
from datetime import date
from typing import Iterable

@dataclass(frozen=True)
class Order:
id: int
price: int
created: date

def total\_price(orders: Iterable\[Order]) -> int:
return sum(o.price for o in orders)

# tests/test_core.py
from core import mean

def test\_mean():
assert mean(\[1,2,3]) == 2
assert mean(\[10,0]) == 5

pip install pytest
pytest -q