def get_default_target_names(estimator, num_targets=None):
    """
    Return a vector of target names: "y" if there is only one target,
    and "y0", "y1", ... if there are multiple targets.
    """
    if num_targets is None:
        if len(estimator.coef_.shape) <= 1:
            num_targets = 1
        else:
            num_targets, _ = estimator.coef_.shape
    if num_targets == 1:
        target_names = ['y']
    else:
        target_names = ['y%d' % i for i in range(num_targets)]
    return np.array(target_names)