def METHOD_NAME(self):
    """
    Check that output has been cleaned out of all cells.
    """
    for file in _get_files():
        with self.subTest(file):
            with open(file) as f:
                json_data = json.load(f)
                for i in json_data['cells']:
                    if 'execution_count' in i and i['execution_count'] is not None:
                        msg = "Clear output with 'reset_notebook path_to_notebook.ipynb'"
                        self.fail(f"Output found in {file}.\n{msg}")