def cv_classes():
    """all objects to be used as a cross-validation generator"""
    all_types = ("<class 'sklearn.model_selection._split.KFold'>",
        "<class 'sklearn.model_selection._split.ShuffleSplit'>",
        "<class 'sklearn.model_selection._split.StratifiedShuffleSplit'>",
        "<class 'sklearn.model_selection._split.LeaveOneOut'>")
    return all_types
