def METHOD_NAME(self, monkeypatch):
    # Test with non-existent file
    with pytest.raises(DatasetLoadError):
        Data._load_dataset_from_string("non_existent_file.txt")
    # Test with invalid extension
    with tempfile.NamedTemporaryFile(suffix=".invalid") as temp_file:
        with pytest.raises(DatasetLoadError):
            Data._load_dataset_from_string(temp_file.name)
    # Test with invalid external dataset identifier
    with patch("datasets.load_dataset") as mock_load_dataset:
        mock_load_dataset.side_effect = ValueError("Invalid external dataset identifier")
        with pytest.raises(DatasetLoadError) as excinfo:
            Data._load_dataset_from_string("invalid_external_dataset_name")
        expected_error_substring = "Failed to load dataset from <class 'str'>.\n"
        assert expected_error_substring in str(excinfo.value)
    # Test with valid .txt, .csv, and .json files
    test_data = [
        (".txt", "sample text", "from_text"),
        (".csv", "column1,column2\nvalue1,value2", "from_csv"),
        (".json", '{"key": "value"}', "from_json"),
    ]
    mock_dataset = Dataset.from_dict({"y": [1, 2, 3]})
    for ext, content, loader_func in test_data:
        with tempfile.NamedTemporaryFile(suffix=ext, mode="w+t") as temp_file:
            temp_file.write(content)
            temp_file.flush()
            # Make sure the correct loader function is called
            def fake_loader(file_name):
                assert file_name == temp_file.name
                return mock_dataset
            with monkeypatch.context() as mp:
                mp.setattr(Dataset, loader_func, fake_loader)
                loaded_dataset = Data._load_dataset_from_string(temp_file.name)
                assert isinstance(loaded_dataset, Dataset)
                assert loaded_dataset == mock_dataset
    # Test with an external dataset
    def fake_load_dataset(data_string):
        if data_string == "external_dataset":
            return mock_dataset
        raise Exception("Not the expected dataset string")
    with monkeypatch.context() as mp:
        mp.setattr("datasets.load_dataset", fake_load_dataset)
        loaded_dataset = Data._load_dataset_from_string("external_dataset")
        assert isinstance(loaded_dataset, Dataset)
        assert loaded_dataset == mock_dataset
        with pytest.raises(DatasetLoadError) as excinfo:
            Data._load_dataset_from_string("non_external_dataset")
        expected_error_substring = "Failed to load dataset from <class 'str'>.\n"