def check_for_choice_col_based_on_return_long_probs(return_long_probs,
    choice_col):
    """
    Ensure that if return_long_probs is False then choice_col is not None.
    Raise a helpful ValueError if otherwise.

    Parameters
    ----------
    return_long_probs : bool.
            Indicates whether or not the long format probabilities (a 1D numpy
            array with one element per observation per available alternative)
            should be returned.
    choice_col : str or None.
        Denotes the column in `data` which contains a one if the
        alternative pertaining to the given row was the observed outcome
        for the observation pertaining to the given row and a zero
        otherwise.

    Returns
    -------
    None.
    """
    if not return_long_probs and choice_col is None:
        msg = 'If return_long_probs == False, then choice_col cannote be None.'
        raise ValueError(msg)
    else:
        return None
