pytest_fixture.md

Pytest Fixture

• Fixture可以用来处理一些多个tests公用的setup
• Fixture可以定义在test_xxx.py中, 或者定义在conftest.py中, fixture会在调用它的测试函数之前被调用
• 一个测试函数能调用多个fixtures, 自定义的fixture也能调用其他的fixture

@pytest.fixture
def my_fixture():
    return some_obj

Bultin fixtures

• capsys 捕获系统的输出, stdout/stderr

def test_capsys(capsys):
    print("hello")
    out, err = capsys.readouterr()
    assert out == "hello\n"

• monkeypatch 用一个虚假的函数去代替一个真实的函数, 从而避免在test时避免一些高计算或者高延迟的操作

def test_monkeypatch(monkeypatch):
    def fake_add(a, b):
        return 42
    monkeypatch.setattr(demo, "add", fake_add)
    assert demo.add(2, 3) == 42

另一种类似的方法是利用unittest.mock.patch装饰器

from unittest.mock import patch

def mocked_current_utc_time():
    return datetime.datetime(2018, 3, 26, 12)

@patch('demo.current_utc_time', new=mocked_current_utc_time)
def test_that_mock_works():
    result = demo.current_utc_time()
    truth = datetime.datetime(2018, 3, 26, 12)
    assert result == truth

• tmpdir 创建一个临时文件夹, 语法类似os.path

def test_tmpdir(tmpdir):
    some_file = tmpdir.join("something.txt")
    some_file.write('{"hello": "world"}')

    result = demo.read_json(str(some_file))
    assert result["hello"] == "world"

• tmp_path 创建一个临时文件夹, 语法类似pathlib

Scopes

Pytest fixtures have five different scopes

• function
• class
• module
• package
• session

Function

• Function is the default scope without explicitly adding scope="function"
• The fixture will be executed per test function
• It works for fixtures that
  • used only once
  • contain a very light operation
  • you want a different value each time

import pytest
import json
from datetime import datetime

@pytest.fixture()
def only_used_once():
    with open("app.json") as f:
        config = json.load(f)
    return config

@pytest.fixture()
def light_operation():
    return "I'm a constant"

@pytest.fixture()
def need_different_value_each_time():
    return datetime.now()

Class

• The scope class runs the fixture per test class
• To use a fixture with class score, put @pytest.mark.usefixtures("fixture-name") before the test class, and the fixture will be executed before any test function.

Reference

Understand 5 Scopes of Pytest Fixtures | by Xiaoxu Gao | Better Programming