def get_sampling_weights(alternatives, sampling_weights_col=None):
    """
    Returns sampling weights for the alternative set.

    Parameters:
    -----------
    alternatives: pandas.DataFrame
        Data frame of alternatives to sample from.
    sampling_weights_col: str
        Name of column containing sampling weights.

    Returns:
    --------
    pandas.Series, None if `sampling_weights_col` is None

    """
    if sampling_weights_col is None:
        return None
    return alternatives[sampling_weights_col]