使用Pytest模拟Bing API方法响应失败问题排查

问题背景 #

测试geocode_postcode函数时，为避免调用真实Bing API，使用unittest.mock.patch和自定义StubResponse类模拟响应，但测试返回的是MagicMock对象而非预期的(0,0)，导致断言失败。

待测试函数 #

def geocode_postcode(postcode: str) -> tuple:
    """
    Turn the postcode into a longitude and latitude point

    Inputs
        postocde: postocde for the UK

    Outputs
        point: longitude and latitude as a tuple (longitude, latitude) else None
    """
    try:

        if not postcode or not isinstance(postcode, str):
            raise ValueError("Invalid postcode format")
        
        with Bing(api_key=CONFIG.BING_API_KEY) as geolocator:
            point_location = geolocator.geocode(
                query={"postalCode":postcode,
                        "countryRegion":"UK"},
                exactly_one=True
            )

        if point_location:
            point = (point_location.longitude, point_location.latitude)
        else:
            point = None

        return point
    
    except (GeocoderTimedOut, GeocoderServiceError) as e:
        print(f"Error geocoding postcode {postcode}: {e}")
        return None
    except ValueError as e:
        print(e)
        return None

测试代码 #

from petrol_locations.helpers import geocode_postcode
from unittest.mock import MagicMock, patch
import pytest

class StubResponse():
    def __init__(self, longitude, latitude):
        self.longitude = longitude
        self.latitude = latitude

@patch("petrol_locations.helpers.Bing")
def test_geocode_postcode_success(mock_geocode):
    mock_response = StubResponse(0,0)
    mock_geocode.return_value.geocode.return_value = mock_response
    response = geocode_postcode("SW1A 2AA")
    print(response)
    assert response == (0, 0)

错误信息 #

========================================================================================================= FAILURES ==========================================================================================================
_______________________________________________________________________________________________ test_geocode_postcode_success _______________________________________________________________________________________________

mock_geocode = <MagicMock name='Bing' id='139699539770480'>

    @patch("petrol_locations.helpers.Bing")
    def test_geocode_postcode_success(mock_geocode):
        mock_response = StubResponse(0,0)
        mock_geocode.return_value.geocode.return_value = mock_response
        response = geocode_postcode("SW1A 2AA")
        print(response)
>       assert response == (0, 0)
E       AssertionError: assert (<MagicMock n...99539985984'>) == (0, 0)
E         At index 0 diff: <MagicMock name='Bing().__enter__().geocode().longitude' id='139699539973984'> != 0
E         Use -v to get more diff

tests/test_helpers.py:34: AssertionError

返回结果 #

(<MagicMock name='Bing().__enter__().geocode().longitude' id='139699539973984'>, <MagicMock name='Bing().__enter__().geocode().latitude' id='139699539985984'>)

解决方案 #

问题核心是Bing类作为上下文管理器使用，你只mock了Bing.return_value.geocode，但忽略了上下文管理器的__enter__方法。with Bing(...) as geolocator中，geolocator是Bing().__enter__()返回的对象，而非直接的Bing()实例，所以需要给__enter__的返回值设置mock。

修改后的测试代码：

@patch("petrol_locations.helpers.Bing")
def test_geocode_postcode_success(mock_geocode):
    mock_response = StubResponse(0,0)
    # 创建上下文管理器返回的mock对象
    mock_context = MagicMock()
    mock_context.geocode.return_value = mock_response
    # 让Bing实例的__enter__方法返回这个mock对象
    mock_geocode.return_value.__enter__.return_value = mock_context
    
    response = geocode_postcode("SW1A 2AA")
    assert response == (0, 0)

说明 #

• mock_geocode.return_value对应Bing()实例
• __enter__.return_value指定了with语句中geolocator变量的实际指向
• 给这个上下文对象的geocode方法设置返回值为StubResponse实例，这样point_location就是自定义的响应对象，其longitude和latitude就是预期的0和0，而非MagicMock对象。

内容的提问来源于stack exchange，提问作者Philip09