Mock exception with side effect raised in class method and caught in calling method gives 'did not raise'

问题

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.

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

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

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'

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'

回答1:

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:

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

来源：https://stackoverflow.com/questions/58842817/mock-exception-with-side-effect-raised-in-class-method-and-caught-in-calling-met

标签

python-3.x

unit-testing

exception

mocking

pytest