def ratio_col(df, df_cols):                  # Tested [Y]
    """
    This function computes the ratio between two columns and returns a Dataframe containing the ratio as a column.

    Args
        df (pd.DataFrame): Dataframe containing the columns to compute a ratio.
        df_cols (tuple): A tuple containing the names of columns in the Dataframe to use in computing the ratio.
                         Format is (<numerator>, <denominator>)
    Return
        (pd.Series) The inital dataframe entered with addition columns for the ratio of the two columns specified in the df_cols argument
    """

    df[df_cols[0]] = df[df_cols[0]].div(df[df_cols[1]].values, axis=0)
    return df[df_cols[0]]