在类方法中引发副作用并在调用方法中捕获的模拟异常给出“未引发" [英] Mock exception with side effect raised in class method and caught in calling method gives 'did not raise'

问题描述

使用 side_effect ，我是尝试在调用模拟程序时引发异常，但出现一个我不理解的DID NOT RAISE EXCEPTION错误.

Using side_effect, I am trying to raise an exception when a mock is called but I get a DID NOT RAISE EXCEPTION error that I do not understand.

主要基于此答案，我创建了一个简单的示例，其中有一个Query类和一个类方法make_request_and_get_response会引发多个异常.这些异常是在main.py中的get_response_from_external_api方法中处理的.

Based largely on this answer, I have created a simple example where there is a Query class with a class method make_request_and_get_response which can raise several exceptions. These exceptions are being handled within the get_response_from_external_api method in main.py.

query.py

from urllib.request import urlopen
import contextlib
import urllib

class Query:
    def __init__(self, a, b):
        self.a = a
        self.b = b

        self.query = self.make_query()


    def make_query(self):
        # create query request using self.a and self.b
        return query


    def make_request_and_get_response(self):  # <--- the 'dangerous' method that can raise exceptions
        with contextlib.closing(urlopen(self.query)) as response:
            return response.read().decode('utf-8')

main.py

from foo.query import *

def get_response_from_external_api(query):
    try:
        response = query.make_request_and_get_response()
    except urllib.error.URLError as e:
        print('Got a URLError: ', e)
    except urllib.error.HTTPError as e:
        print('Got a HTTPError: ', e)
    # {{various other exceptions}}
    except Exception:
        print('Got a generic Exception!')
        # handle this exception


if __name__ == "__main__":    
    query = Query('input A', 'input B')
    result = get_response_from_external_api(query)
    return result

使用pytest，我正在尝试模拟该危险"方法(make_request_and_get_response)，并带有针对特定异常的副作用.然后，我继续创建一个模拟的Query对象，该对象将在调用make_request_and_get_response时使用，并期望最后一次调用会给出"URLError"异常.

Using pytest, I am trying to mock that 'dangerous' method (make_request_and_get_response) with a side effect for a specific exception. Then, I proceed with creating a mocked Query object to use when calling the make_request_and_get_response with the expectation that this last call give a 'URLError' exception.

test_main.py

test_main.py

import pytest
from unittest.mock import patch
from foo.query import Query
from foo.main import get_response_from_external_api


class TestExternalApiCall:
    @patch('foo.query.Query')
    def test_url_error(self, mockedQuery):
        with patch('foo.query.Query.make_request_and_get_response', side_effect=Exception('URLError')):
            with pytest.raises(Exception) as excinfo:
                q= mockedQuery()
                foo.main.get_response_from_external_api(q)
            assert excinfo.value = 'URLError'
            # assert excinfo.value.message == 'URLError' # this gives object has no attribute 'message'

上面的测试给出以下错误:

The test above gives the following error:

>       foo.main.get_response_from_external_api(q)
E       Failed: DID NOT RAISE <class 'Exception'> id='72517784'>") == 'URLError'

推荐答案

Pytest无法检测到异常，因为在get_response_from_external_api中您正在捕获所有异常.根据您的要求，您可以选择以下选项:

Pytest cannot detect an exception because in get_response_from_external_api you are catching all of them. Depending on your requirements, you have these options:

1. 不要在get_response_from_external_api中捕获异常.
2. 捕获异常，执行所需的任何操作，然后重新引发它们.
3. 使用pytest.raises异常"rel =" nofollow noreferrer> capsys 固定装置以捕获正在打印的内容并在该输出上进行断言.

1. Don't catch the exceptions in get_response_from_external_api.
2. Catch the exceptions, do whatever you want and then re-raise them.
3. Instead of detecting an exception with pytest.raises, use capsys fixture to capture what is being printed and make assertions on that output.

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