pytest - monkeypatchの使い方

def create_file(word):
    # テスト対象のコード
    with open("sample.txt", "w") as f:
        f.write(f"{word}")

import production_code

def test_create_file(monkeypatch, tmp_path):
    # 差し替え先はtmp_path(/tmp/pytest-of-{os_username}/pytest-current)
    monkeypatch.chdir(tmp_path)

    # pytest-current配下でsample.txtが作成される
    create_file("test-word")

    # テストコード
    with open(tmp_path / "sample.txt", "r") as f:
        expected = "test-word"
        assert expected == f.read()

# テスト時に実行が難しい関数（差し替えたい関数）
def get_name():
    return "Yamada"


# テスト対象のコード
def greeting():
    name = get_name()
    return f"Hello, {name}!"

import production_code


def test_greeting(monkeypatch):

    # ダミー関数を用意
    def dummy_get_name():
        return "Dummy"

    # 関数の差し替え
    monkeypatch.setattr(production_code, "get_name", dummy_get_name)

    # テストコード
    expected = "Hello, Dummy!"
    assert expected == production_code.greeting()