模拟函数以引发异常以测试 except 块 [英] Mocking a function to raise an Exception to test an except block

问题描述

我有一个函数 (foo)，它调用另一个函数 (bar).如果调用bar()引发了HttpError，如果状态码是404我想特别处理，否则重新引发.

I have a function (foo) which calls another function (bar). If invoking bar() raises an HttpError, I want to handle it specially if the status code is 404, otherwise re-raise.

我正在尝试围绕这个 foo 函数编写一些单元测试，模拟对 bar() 的调用.不幸的是，我无法获得对 bar() 的模拟调用来引发我的 except 块捕获的异常.

I am trying to write some unit tests around this foo function, mocking out the call to bar(). Unfortunately, I am unable to get the mocked call to bar() to raise an Exception which is caught by my except block.

这是说明我的问题的代码:

Here is my code which illustrates my problem:

import unittest
import mock
from apiclient.errors import HttpError


class FooTests(unittest.TestCase):
    @mock.patch('my_tests.bar')
    def test_foo_shouldReturnResultOfBar_whenBarSucceeds(self, barMock):
        barMock.return_value = True
        result = foo()
        self.assertTrue(result)  # passes

    @mock.patch('my_tests.bar')
    def test_foo_shouldReturnNone_whenBarRaiseHttpError404(self, barMock):
        barMock.side_effect = HttpError(mock.Mock(return_value={'status': 404}), 'not found')
        result = foo()
        self.assertIsNone(result)  # fails, test raises HttpError

    @mock.patch('my_tests.bar')
    def test_foo_shouldRaiseHttpError_whenBarRaiseHttpErrorNot404(self, barMock):
        barMock.side_effect = HttpError(mock.Mock(return_value={'status': 500}), 'error')
        with self.assertRaises(HttpError):  # passes
            foo()

def foo():
    try:
        result = bar()
        return result
    except HttpError as error:
        if error.resp.status == 404:
            print '404 - %s' % error.message
            return None
        raise

def bar():
    raise NotImplementedError()

我按照 Mock docs 说您应该将 Mock 实例的 side_effect 设置为 Exception 类，以使模拟函数引发错误.

I followed the Mock docs which say that you should set the side_effect of a Mock instance to an Exception class to have the mocked function raise the error.

我还查看了一些其他相关的 StackOverflow Q&As，看起来我正在做与他们正在做的事情相同的事情，以导致他们的模拟引发异常.

I also looked at some other related StackOverflow Q&As, and it looks like I am doing the same thing they are doing to cause and Exception to be raised by their mock.

• https://stackoverflow.com/a/10310532/346561
• 如何使用 Python Mock 引发异常 - 但将 Errno 设置为给定值

为什么设置 barMock 的 side_effect 不会引发预期的 Exception?如果我在做一些奇怪的事情，我应该如何在我的 except 块中测试逻辑?

Why is setting the side_effect of barMock not causing the expected Exception to be raised? If I am doing something weird, how should I go about testing logic in my except block?

推荐答案

你的 mock 可以很好地引发异常，但是缺少 error.resp.status 值.与其使用 return_value，不如告诉 Mock status 是一个属性:

Your mock is raising the exception just fine, but the error.resp.status value is missing. Rather than use return_value, just tell Mock that status is an attribute:

barMock.side_effect = HttpError(mock.Mock(status=404), 'not found')

Mock() 的附加关键字参数被设置为结果对象的属性.

Additional keyword arguments to Mock() are set as attributes on the resulting object.

我将您的 foo 和 bar 定义放在 my_tests 模块中，并添加到 HttpError class 所以我可以使用它也一样，然后你的测试就可以成功了:

I put your foo and bar definitions in a my_tests module, added in the HttpError class so I could use it too, and your test then can be ran to success:

>>> from my_tests import foo, HttpError
>>> import mock
>>> with mock.patch('my_tests.bar') as barMock:
...     barMock.side_effect = HttpError(mock.Mock(status=404), 'not found')
...     result = my_test.foo()
... 
404 - 
>>> result is None
True

您甚至可以看到 print '404 - %s' % error.message 行运行，但我认为您想在那里使用 error.content ；无论如何，这就是 HttpError() 从第二个参数设置的属性.

You can even see the print '404 - %s' % error.message line run, but I think you wanted to use error.content there instead; that's the attribute HttpError() sets from the second argument, at any rate.

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