def add_cols_df(df, cols):
    """Add columns to a data frame.
    
    Args:
        df (:obj:`pd.DataFrame`): Data frame.
        cols (Dict[str, Any]): Dictionary of ``{column: default_value}``
            to add to ``df``.

    Returns:
        :obj:`pd.DataFrame`: Data frame with columns added.

    """
    for key, val in cols.items():
        df[key] = val
    return df