def sub_col(df, df_cols):                    # Tested [Y]
    """
    This function takes the difference between two columns.

    Args
        df (pd.DataFrame):
        df_cols (tuple): A tuple containing the names of columns in the dataframe to subtract from each other.
                         Format is (<subtract_from>, <being_subtracted>)
    Return
        (pd.DataFrame) A Dataframe with one column which is the difference between the two columns passed.
    """

    df = df[[df_cols[1], df_cols[0]]]
    df = df.diff(axis=1)
    df = df.drop(df_cols[1], 1)
    return df