pytestでテスト効率化: Pythonの書き方パターン10選と失敗回避

# 仮想環境の作成と有効化 (Windows)
python -m venv .venv
.venv\Scripts\activate

# 仮想環境の作成と有効化 (macOS/Linux)
python3 -m venv .venv
source .venv/bin/activate

# pytestのインストール
pip install -U pytest

# バージョン確認
pytest --version

# tests/test_math.py
# 最小のテスト例。assertだけでOKです。

def add(a: int, b: int) -> int:
    """2数の和を返す単純な関数"""
    return a + b

def test_add_returns_sum():
    # 期待値を明確に書くと読みやすいです
    assert add(2, 3) == 5

$ pytest -q
.                                                                                  [100%]
1 passed in 0.03s

your_project/
  src/
    calc.py
  tests/
    test_calc.py

# src/calc.py
def mul(a: int, b: int) -> int:
    return a * b

# tests/test_calc.py
from src.calc import mul

def test_mul_basic():
    assert mul(2, 4) == 8

# tests/test_strings.py
def shout(s: str) -> str:
    return s.upper() + "!"

def test_shout_uppercases_and_exclaims():
    assert shout("hello") == "HELLO!"

# tests/test_asserts.py
def split_csv(line: str) -> list[str]:
    return [p.strip() for p in line.split(",")]

def test_equal_and_membership_and_length():
    parts = split_csv("a, b, c")
    # 等価
    assert parts == ["a", "b", "c"]
    # 包含
    assert "b" in parts
    # 長さ
    assert len(parts) == 3

# tests/test_raises.py
import pytest

def divide(a: int, b: int) -> float:
    if b == 0:
        raise ValueError("b must not be zero")
    return a / b

def test_divide_by_zero_raises_valueerror():
    with pytest.raises(ValueError) as exc:
        # 例外を起こす最小限の呼び出しだけを書く
        divide(1, 0)
    assert "must not be zero" in str(exc.value)

# tests/test_compose.py
def full_name(user: dict) -> str:
    return f"{user['last']} {user['first']}"

def test_full_name_with_minimum_data():
    # 最小限の辞書だけを定義
    user = {"first": "Taro", "last": "Yamada"}
    assert full_name(user) == "Yamada Taro"

# src/normalize.py
def strip_and_lower(s: str) -> str:
    return s.strip().lower()

def normalize_email(email: str) -> str:
    # 小さい関数に委譲することで個別にテスト可能
    return strip_and_lower(email)

# tests/test_normalize.py
from src.normalize import strip_and_lower, normalize_email

def test_strip_and_lower():
    assert strip_and_lower("  Alice@Example.Com  ") == "alice@example.com"

def test_normalize_email():
    assert normalize_email("  Bob@Example.COM ") == "bob@example.com"

pytest -k "normalize and not strip" -q

pytest -vv -s

# tests/test_markers.py
import sys
import pytest

@pytest.mark.skipif(sys.platform.startswith("win"), reason="Windowsでは未対応の仕様")
def test_posix_only():
    assert "/" in "/tmp/path"

@pytest.mark.xfail(reason="外部APIの仕様変更対応中")
def test_known_failure():
    assert 1 / 0 == 0  # ここはまだ失敗でOKとする

# tests/helpers.py
def make_user(first: str = "Taro", last: str = "Yamada") -> dict:
    """テスト用の最小ユーザーデータを生成"""
    return {"first": first, "last": last}

# tests/test_user.py
from tests.helpers import make_user

def full_name(user: dict) -> str:
    return f"{user['last']} {user['first']}"

def test_full_name_with_helper():
    user = make_user()
    assert full_name(user) == "Yamada Taro"

# ディレクトリ単位
pytest tests/ -q

# ファイル単位
pytest tests/test_normalize.py -q

# テスト関数(ノードID)をピンポイント指定
pytest tests/test_normalize.py::test_strip_and_lower -q

$ pytest -q
collected 0 items

# tests/test_float.py
import math

def area(r: float) -> float:
    return math.pi * r * r

def test_area_close():
    assert area(1.0) == 3.141592653589793  # たまたま通るが危険

def test_area_with_tolerance():
    # 許容誤差をもたせて比較する
    assert math.isclose(area(1.0), 3.1415926535, rel_tol=1e-9, abs_tol=0.0)

import pytest

def boom():
    raise RuntimeError("boom")

def test_boom():
    with pytest.raises(RuntimeError):
        boom()  # これだけを書く

def test_debug_print():
    print("debug here")
    assert True

# tests/test_path.py
from pathlib import Path

def test_read_fixture_text():
    # テストファイルと同じ場所のfixture.txtを読む
    here = Path(__file__).parent
    text = (here / "fixture.txt").read_text(encoding="utf-8")
    assert "hello" in text

# シンプルな一例(後でフィクスチャでも改善可)
from uuid import uuid4
from pathlib import Path

def test_write_temp_file(tmp_path):
    # pytestの標準フィクスチャtmp_pathで安全に作成
    p = tmp_path / f"{uuid4().hex}.txt"
    p.write_text("ok", encoding="utf-8")
    assert p.read_text(encoding="utf-8") == "ok"

# tests/test_random_time.py
import random
from datetime import datetime

def roll():
    random.seed(1234)  # 固定化
    return random.randint(1, 6)

def format_date(dt: datetime) -> str:
    return dt.strftime("%Y-%m-%d")

def test_roll_deterministic():
    assert roll() == 4  # seed固定で再現性を持たせる

def test_format_date_fixed():
    dt = datetime(2020, 1, 2)
    assert format_date(dt) == "2020-01-02"

python -m venv .venv
source .venv/bin/activate  # Windowsは .venv\Scripts\activate
pip install -U pytest

# 名前にslowを含むテストだけ、または除外
pytest -k slow -q
pytest -k "not slow" -q