def ensure_unique_obs_ids_in_wide_data(obs_id_col, wide_data):
    """
    Ensures that there is one observation per row in wide_data. Raises a
    helpful ValueError if otherwise.

    Parameters
    ----------
    obs_id_col : str.
        Denotes the column in `wide_data` that contains the observation ID
        values for each row.
    wide_data : pandas dataframe.
        Contains one row for each observation. Should contain the specified
        `obs_id_col` column.

    Returns
    -------
    None.
    """
    if len(wide_data[obs_id_col].unique()) != wide_data.shape[0]:
        msg = 'The values in wide_data[obs_id_col] are not unique, '
        msg_2 = 'but they need to be.'
        raise ValueError(msg + msg_2)
    return None
