如何使用 pytest-mock 模拟 python 类依赖项？

Question

我正在努力理解如何从 Python 和 pytest 中的类依赖项中对类进行存根/模拟所有方法。下面的清单显示了我正在测试的类。它有两个内部依赖：OWMProxy 和 PyOwmDeserializer。

被测类

class OWM:
    def __init__(self, api_key: str, units: WeatherUnits) -> None:
        self._api_key = api_key
        self._units = units    
        self._pyorm = OWMProxy.from_api_key(api_key)
        self._deserializer = PyOwmDeserializer()

    def at(self, city: str, iso_datetime: str) -> ForecastModel:
        weather = self._pyorm.for_time(city, iso_datetime)

        return self._deserializer.deserialize(weather, self._units)

    def day(self, city: str, day: str) -> ForecastModel:
        weather = self._pyorm.for_day(city, day)

        return self._deserializer.deserialize(weather, self._units)

    def now(self, city: str) -> ForecastModel:
        weather = self._pyorm.now(city) 

        return self._deserializer.deserialize(weather, self._units)

我的问题是，是否可以在使用 PyTest 进行单元测试时模拟整个类依赖项？

目前，我的单元测试使用 mocker 来模拟每个类方法，包括 init 方法。

我可以使用依赖注入方法，即为内部反序列化器和代理接口创建一个接口，并将这些接口添加到被测类的构造函数中。

或者，我可以按照here 的建议使用unittest.mock 模块进行测试。 pytest-mock中是否有等效功能？？？

到目前为止的单元测试...

@pytest.mark.skip(reason="not implemented")
  def test_owm_initialises_deserializer(
      default_weather_units: WeatherUnits, mocker: MockFixture
  ) -> None:
      api_key = "test_api_key"
  
      proxy = OWMProxy(py_OWM(api_key))
  
      patch_proxy = mocker.patch(
          "wayhome_weather_api.openweathermap.client.OWMProxy.from_api_key",
          return_value=proxy,
      )   
  
      patch_val = mocker.patch(
          "wayhome_weather_api.openweathermap.deserializers.PyOwmDeserializer",
          "__init__",
          return_value=None,
      )   
  
      owm = OWM(api_key, default_weather_units)
  
      assert owm is not None

Answer 1

您可以模拟整个类并控制其方法的返回值和/或副作用，就像它在 docs 中所做的那样。

>>> def some_function():
...     instance = module.Foo()
...     return instance.method()
...
>>> with patch('module.Foo') as mock:
...     instance = mock.return_value
...     instance.method.return_value = 'the result'
...     result = some_function()
...     assert result == 'the result'

假设被测类位于src.py。

test_owm.py

import pytest
from pytest_mock.plugin import MockerFixture

from src import OWM, WeatherUnits


@pytest.fixture
def default_weather_units():
    return 40


def test_owm_mock(
      default_weather_units: WeatherUnits, mocker: MockerFixture
) -> None:
    api_key = "test_api_key"

    #  Note that you have to mock the version of the class that is defined/imported in the target source code to run. So here, if the OWM class is located in src.py, then mock its definition/import of src.OWMProxy and src.PyOwmDeserializer
    patch_proxy = mocker.patch("src.OWMProxy.from_api_key")
    patch_val = mocker.patch("src.PyOwmDeserializer")

    owm = OWM(api_key, default_weather_units)

    assert owm is not None

    # Default patch
    print("Default patch:", owm.day("Manila", "Today"))

    # Customizing the return value
    patch_proxy.return_value.for_day.return_value = "Sunny"
    patch_val.return_value.deserialize.return_value = "Really Sunny"
    print("Custom return value:", owm.day("Manila", "Today"))
    patch_proxy.return_value.for_day.assert_called_with("Manila", "Today")
    patch_val.return_value.deserialize.assert_called_with("Sunny", default_weather_units)

    # Customizing the side effect
    patch_proxy.return_value.for_day.side_effect = lambda city, day: f"{day} in hot {city}"
    patch_val.return_value.deserialize.side_effect = lambda weather, units: f"{weather} is {units} deg celsius"
    print("Custom side effect:", owm.day("Manila", "Today"))
    patch_proxy.return_value.for_day.assert_called_with("Manila", "Today")
    patch_val.return_value.deserialize.assert_called_with("Today in hot Manila", default_weather_units)


def test_owm_stub(
      default_weather_units: WeatherUnits, mocker: MockerFixture
) -> None:
    api_key = "test_api_key"

    class OWMProxyStub:
        @staticmethod
        def from_api_key(api_key):
            return OWMProxyStub()

        def for_day(self, city, day):
            return f"{day} in hot {city}"

    class PyOwmDeserializerStub:
        def deserialize(self, weather, units):
            return f"{weather} is {units} deg celsius"


    patch_proxy = mocker.patch("src.OWMProxy", OWMProxyStub)
    patch_val = mocker.patch("src.PyOwmDeserializer", PyOwmDeserializerStub)

    owm = OWM(api_key, default_weather_units)

    assert owm is not None

    # Default patch
    print("Default patch:", owm.day("Manila", "Today"))
    # If you want to assert the calls made as did in the first test above, you can use the mocker.spy() functionality

输出

$ pytest -q -rP
================================================================================================= PASSES ==================================================================================================
______________________________________________________________________________________________ test_owm_mock ______________________________________________________________________________________________
------------------------------------------------------------------------------------------ Captured stdout call -------------------------------------------------------------------------------------------
Default patch: <MagicMock name='PyOwmDeserializer().deserialize()' id='139838844832256'>
Custom return value: Really Sunny
Custom side effect: Today in hot Manila is 40 deg celsius
______________________________________________________________________________________________ test_owm_stub ______________________________________________________________________________________________
------------------------------------------------------------------------------------------ Captured stdout call -------------------------------------------------------------------------------------------
Default patch: Today in hot Manila is 40 deg celsius
2 passed in 0.06s

如您所见，我们能够控制模拟依赖项的方法的返回值。