Using mock_open to raise a FileNotFoundError and check if write is called

Question

I'm working on writing tests for a flask application using pytest. I'm trying to write a unit test for this code. Basically I want to mock open() so that it throws a FileNotFoundError when I call open the first time and check that write() is called once.

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({}))

This is my test:

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("{}")

This is the output from pytest

====================================================================== 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

It looks like the test is failing because a FileNotFoundError is being raised somewhere I'm not expecting it to, but since it's printing the contents of the real file, I would've assumed the mock wasn't taking effect.

What am I doing wrong?

Answer 1

Just to give an alternative without a separate class:

@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("{}")

Note that there are a few things in the original code that won't work:

• the mocking was reversed as soon as the fixture was returned - using the context manager version of patch together with yield prevents this (using mocker is another possibility)
• as you use a context manager with open, you have to add __enter__ to the mock (this is called by the context manager)
• using None as side effect won't work, as in this case write would be called on None - thus the second mock used for file