如何在Python上设置模拟异常行为

Question

我使用的是一个外部库(github3.py(，它定义了一个内部异常(github3.exceptions.UnprocessableEntity(。如何定义这个异常并不重要，所以我想创建一个副作用，并从这个异常中设置我使用的属性。

经过测试的代码不那么简单示例：

import github3
class GithubService:
def __init__(self, token: str) -> None:
self.connection = github3.login(token=token)
self.repos = self.connection.repositories()
def create_pull(self, repo_name: str) -> str:
for repo in self.repos:
if repo.full_name == repo_name:
break
try:
created_pr = repo.create_pull(
title="title",
body="body",
head="head",
base="base",
)
except github3.exceptions.UnprocessableEntity as github_exception:
extra = ""
for error in github_exception.errors:
if "message" in error:
extra += f"{error['message']} "
else:
extra += f"Invalid field {error['field']}. " # testing this case
return f"{repo_name}: {github_exception.msg}. {extra}"

我需要从异常中设置属性msg和errors。所以我在测试代码中尝试使用pytest-mock:

@pytest.fixture
def mock_github3_login(mocker: MockerFixture) -> MockerFixture:
"""Fixture for mocking github3.login."""
mock = mocker.patch("github3.login", autospec=True)
mock.return_value.repositories.return_value = [
mocker.Mock(full_name="staticdev/nope"),
mocker.Mock(full_name="staticdev/omg"),
]
return mock

def test_create_pull_invalid_field(
mocker: MockerFixture, mock_github3_login: MockerFixture,
) -> None:
exception_mock = mocker.Mock(errors=[{"field": "head"}], msg="Validation Failed")
mock_github3_login.return_value.repositories.return_value[1].create_pull.side_effect = github3.exceptions.UnprocessableEntity(mocker.Mock())
mock_github3_login.return_value.repositories.return_value[1].create_pull.return_value = exception_mock
response = GithubService("faketoken").create_pull("staticdev/omg")
assert response == "staticdev/omg: Validation Failed. Invalid field head."

此代码的问题在于，如果有side_effect和return_value，Python只会忽略return_value。

这里的问题是，我不想知道UnprocessableEntity的实现来调用它，并将正确的参数传递给它的构造函数。此外，我没有找到其他只使用side_effect的方法。我还尝试使用返回值，设置mock的类，并以这种方式使用它：

def test_create_pull_invalid_field(
mock_github3_login: MockerFixture,
) -> None:
exception_mock = Mock(__class__ = github3.exceptions.UnprocessableEntity, errors=[{"field": "head"}], msg="Validation Failed")
mock_github3_login.return_value.repositories.return_value[1].create_pull.return_value = exception_mock
response = GithubService("faketoken").create_pull("staticdev/omg")
assert response == "staticdev/omg: Validation Failed. Invalid field head."

这也不起作用，不会引发异常。所以我不知道如何克服这个问题，因为我不想看到UnprocessableEntity的实现。有什么想法吗？

Answer 1

因此，根据您的示例，您实际上不需要模拟github3.exceptions.UnprocessableEntity，而只需要模拟传入的resp参数。

因此，以下测试应该有效：

def test_create_pull_invalid_field(
mocker: MockerFixture, mock_github3_login: MockerFixture,
) -> None:
mocked_response = mocker.Mock()
mocked_response.json.return_value = {
"message": "Validation Failed", "errors": [{"field": "head"}]
}
repo = mock_github3_login.return_value.repositories.return_value[1]
repo.create_pull.side_effect = github3.exceptions.UnprocessableEntity(mocked_response)
response = GithubService("faketoken").create_pull("staticdev/omg")
assert response == "staticdev/omg: Validation Failed. Invalid field head."

编辑：

如果您希望github3.exceptions.UnprocessableEntity被完全抽象，那么就不可能模拟整个类，因为不允许捕获不从BaseException继承的类(请参阅文档(。但你可以通过嘲笑构造函数来绕过它：

def test_create_pull_invalid_field(
mocker: MockerFixture, mock_github3_login: MockerFixture,
) -> None:
def _initiate_mocked_exception(self) -> None:
self.errors = [{"field": "head"}]
self.msg = "Validation Failed"
mocker.patch.object(
github3.exceptions.UnprocessableEntity, "__init__", 
_initiate_mocked_exception
)
repo = mock_github3_login.return_value.repositories.return_value[1]
repo.create_pull.side_effect = github3.exceptions.UnprocessableEntity
response = GithubService("faketoken").create_pull("staticdev/omg")
assert response == "staticdev/omg: Validation Failed. Invalid field head."