使用mock_open引发FileNotFoundError并检查是否调用了write

Question

我正在使用 pytest 为 Flask 应用程序编写测试。我正在尝试为这段代码编写单元测试。基本上我想模拟

open()

，以便当我第一次调用 open 时它会抛出 FileNotFoundError 并检查

write()

是否被调用一次。

try:
    with open("config.json", "r") as file:
        config = json.loads(file.read())
        print(config)
except FileNotFoundError:
    with open("config.json", "w") as file:
        file.write(json.dumps({}))

这是我的测试：

import pytest
import mock
@pytest.fixture
def client(mocker):
    return app.test_client()
@pytest.fixture
def mock_no_config(mocker):
    m = mock.mock_open()
    m.side_effect = [FileNotFoundError, None]
    mock.patch("builtins.open", m)
    return m

def test_no_config(client, mock_database_exists, mock_no_config):
    response = client.get("/install/")
    mock_no_config().write.assert_called_once_with("{}")

这是 pytest 的输出

====================================================================== FAILURES =======================================================================
___________________________________________________________________ test_no_config ____________________________________________________________________

client = <FlaskClient <Flask 'main'>>, mock_database_exists = None
mock_no_config = <MagicMock name='open' spec='builtin_function_or_method' id='139985038428816'>

    def test_no_config(client, mock_database_exists, mock_no_config):
        response = client.get("/install/")
>       mock_no_config().write.assert_called_once_with("{}")

tests/test_web_installer.py:44:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
venv/lib/python3.10/site-packages/mock/mock.py:1100: in __call__
    return _mock_self._mock_call(*args, **kwargs)
venv/lib/python3.10/site-packages/mock/mock.py:1104: in _mock_call
    return _mock_self._execute_mock_call(*args, **kwargs)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

_mock_self = <MagicMock name='open' spec='builtin_function_or_method' id='139985038428816'>, args = (), kwargs = {}
self = <MagicMock name='open' spec='builtin_function_or_method' id='139985038428816'>, effect = <list_iterator object at 0x7f50ce47f880>
result = <class 'FileNotFoundError'>

    def _execute_mock_call(_mock_self, *args, **kwargs):
        self = _mock_self
        # separate from _increment_mock_call so that awaited functions are
        # executed separately from their call, also AsyncMock overrides this method

        effect = self.side_effect
        if effect is not None:
            if _is_exception(effect):
                raise effect
            elif not _callable(effect):
                result = next(effect)
                if _is_exception(result):
>                   raise result
E                   FileNotFoundError

venv/lib/python3.10/site-packages/mock/mock.py:1165: FileNotFoundError
---------------------------------------------------------------- Captured stdout call -----------------------------------------------------------------
<THE CONTENTS OF THE REAL config.json FILE HERE>
=============================================================== short test summary info ===============================================================
FAILED tests/test_web_installer.py::test_no_config - FileNotFoundError

看起来测试失败了，因为在我不希望的地方引发了 FileNotFoundError，但由于它打印了真实文件的内容，我会假设模拟没有生效。

我做错了什么？

Answer 1

只是为了提供一种无需单独课程的替代方案：

@pytest.fixture
def mock_no_config():
    open_mock = MagicMock()  # mock for open
    file_mock = MagicMock()  # mock for file returned from open
    open_mock.return_value.__enter__.side_effect = [
        FileNotFoundError, file_mock
    ]
    with mock.patch("builtins.open", open_mock):
        yield file_mock


def test_no_config(client, mock_database_exists, mock_no_config):
    response = client.get("/install/")
    mock_no_config.write.assert_called_once_with("{}")

请注意，原始代码中有一些内容不起作用：

• 夹具一返回，模拟就会逆转 - 使用上下文管理器版本的

  patch

  和

  yield

  可以防止这种情况（使用

  mocker

  是另一种可能性）
• 当您使用带有

  open

  的上下文管理器时，您必须将

  __enter__

  添加到模拟中（这由上下文管理器调用）
• 使用

  None

  作为副作用将不起作用，因为在这种情况下

  write

  将在

  None

  上调用 - 因此第二个模拟用于

  file

Answer 2

我最终只写了自己的课程。这对我有用：

class FakeOpen():
    def __init__(self, filename, mode):
        if filename == "config.json" and mode == "r":
            raise FileNotFoundError()
        self.filename = filename
        self.mode = mode
        self.write_calls = 0

    def read(self):
        return b"{}"

    def write(self, write_string):
        self.write_calls += 1
        assert self.write_calls <= 1
        assert write_string == "{}"

    def close(self):
        if self.mode == "w":
            assert self.write_calls == 1

    def __enter__(self):
        return self

    def __exit__(self, *args, **kwargs):
        pass
mocker.patch("builtins.open", FakeOpen)

当我需要它时，它会引发 FileNotFoundError 并检查 write 是否被正确调用。可能有更好的方法来做到这一点，但这确实有效！

Answer 3

MrBean Bremen 的才华并非不寻常。

他的解决方案可以稍微简化，具体取决于使用需求。这不使用夹具，只是在测试中执行此操作：

open_mock = mock.MagicMock()  
file_mock = mock.Mock()  
open_mock.return_value.__enter__.side_effect = [
    FileNotFoundError, file_mock
]
with mock.patch('builtins.open', open_mock):        
    doc.save_as()

我的实验似乎表明，确实，

MagicMock

需要一个

open_mock

，但对于

Mock

来说，普通的香草

file_mock

就可以了。也许您需要 MrBean Bremen 的解决方案中的两种魔法。