prog13_4.py

import csv

with open("population.csv", encoding="utf-8", newline="") as file_in:
    reader_obj = csv.reader(file_in)
    next(reader_obj)  # ヘッダ行を捨てる
    ave_data = []
    for row in reader_obj:
        country = row[0]
        average = sum([float(pop) for pop in row[-3:]]) / 3
        ave_data.append([country, round(average, 1)])
with open("average.csv", "w", encoding="utf-8", newline="") as file_out:
    writer_obj = csv.writer(file_out)
    writer_obj.writerow(["国別コード", "平均人口（百万人）"])
    for data in ave_data:
        writer_obj.writerow(data)

# プログラム13-4（CSVファイルの出力 ②）