code_under_test.py

    def create_or_update(self, item_id, data):
        if not self.validate(data):
            raise ValidationError('Validation error!')

        file_path = self._filepath(item_id)

        existing_data = {}
        try:
            with open(file_path, 'r') as f:
                content = f.read()
                existing_data = json.loads(content)
        except IOError:
            pass

        with open(file_path, 'w') as f:
            existing_data.update(data)
            f.write(json.dumps(existing_data, indent=4))

        return existing_data

test_storage.py

import mock
from lib.backends import FilestoreHandler


def test_filestore_create_or_update__update():
    data_dir = 'tests/instance-data/'
    fsdatastore = FilestoreHandler(data_dir)

    start_data = {
        'uuid': 'abc123',
    }
    start_datastr = json.dumps(start_data, indent=4)

    add_data = {
        'terminate_user': 'user@company.com',
    }

    final_data = dict(start_data.items() + add_data.items())

    final_datastr = json.dumps(final_data, indent=4)

    data_file_arg = os.path.join(data_dir, "abc123.json")

    with mock.patch(open_name, create=True) as mock_open:
        mock_open.return_value = mock.MagicMock(spec=file)
        file_handle = mock_open.return_value.__enter__.return_value
        file_handle.read = mock.Mock()

        file_handle.read.return_value = start_datastr

        updated_data = fsdatastore.update('abc123', add_data)

        mock_open.assert_any_call(data_file_arg, 'r')
        mock_open.assert_any_call(data_file_arg, 'w')

        file_handle.read.assert_called_once()

        file_handle.write.assert_called_with(
            final_datastr)

        assert updated_data
        assert updated_data == final_data