Pytest fixture for mocking file open

mock_file.py

# author: michal@mazurek-inc.co.uk

@pytest.fixture
def mock_file():
    """Mock file open."""

    @contextlib.contextmanager
    def contextmanager(file_name: str, content: bytes, obj):
        """
        Context manager for mocking file.

        :param file_name: file name to be mocked,
        :param content: content in bytes
        :param obj: object to be patched where `open` will be called from

        module_a.py

        def func():
            with open("some.txt") as some_file:
                return some_file.decode("utf8")

        test_module_a.py:
        from .fixtures import mock_file
        import module_a

        def test_func(mock_file):

            with mock_file("some.txt", b"test123", module_a) as mocked_open:
                assert mocked_open.called
                assert module_a.func() == "test123"

        """
        with mock.patch.object(obj, 'open') as patched_function:
            patched_function.return_value.__enter__.return_value = (
                io.BytesIO(content))
            yield patched_function

    return contextmanager