Python | 標準ライブラリ（csv + pathlib + collections + datetime）だけ を使って、CSVファイルを「日付ごとに集計」する実践例

import csv
from pathlib import Path
from collections import defaultdict
from datetime import datetime

csv_path = Path.home() / "Documents" / "sales.csv"

# 日付ごとの集計用辞書
summary = defaultdict(lambda: {"数量": 0, "金額": 0})

with csv_path.open("r", encoding="utf-8") as f:
    reader = csv.DictReader(f)
    for row in reader:
        # 日付を正規化（datetimeに変換してからdateにする）
        date = datetime.strptime(row["日付"], "%Y-%m-%d").date()
        qty = int(row["数量"])
        price = int(row["金額"])
        summary[date]["数量"] += qty
        summary[date]["金額"] += price

# 結果を日付順に表示
for date, data in sorted(summary.items()):
    print(f"{date}: 数量={data['数量']}, 金額={data['金額']}")

import csv
from pathlib import Path
from collections import defaultdict
from datetime import datetime

csv_path = Path.home() / "Documents" / "sales.csv"

# 日付ごとの集計用辞書
summary = defaultdict(lambda: {"数量": 0, "金額": 0})

with csv_path.open("r", encoding="utf-8") as f:
    reader = csv.DictReader(f)
    for row in reader:
        # 日付を正規化（datetimeに変換してからdateにする）
        date = datetime.strptime(row["日付"], "%Y-%m-%d").date()
        qty = int(row["数量"])
        price = int(row["金額"])
        summary[date]["数量"] += qty
        summary[date]["金額"] += price

# 結果を日付順に表示
for date, data in sorted(summary.items()):
    print(f"{date}: 数量={data['数量']}, 金額={data['金額']}")

weekday_summary = defaultdict(int)

for date, data in summary.items():
    weekday = date.strftime("%A")  # Monday, Tuesday...
    weekday_summary[weekday] += data["金額"]

for wd, total in weekday_summary.items():
    print(f"{wd}: 売上合計={total}")

weekday_summary = defaultdict(int)

for date, data in summary.items():
    weekday = date.strftime("%A")  # Monday, Tuesday...
    weekday_summary[weekday] += data["金額"]

for wd, total in weekday_summary.items():
    print(f"{wd}: 売上合計={total}")

output_path = csv_path.parent / "daily_summary.csv"

with output_path.open("w", newline="", encoding="utf-8") as f:
    writer = csv.writer(f)
    writer.writerow(["日付", "数量合計", "金額合計"])
    for date, data in sorted(summary.items()):
        writer.writerow([date.isoformat(), data["数量"], data["金額"]])

print("日付ごとの集計結果を保存しました:", output_path)

output_path = csv_path.parent / "daily_summary.csv"

with output_path.open("w", newline="", encoding="utf-8") as f:
    writer = csv.writer(f)
    writer.writerow(["日付", "数量合計", "金額合計"])
    for date, data in sorted(summary.items()):
        writer.writerow([date.isoformat(), data["数量"], data["金額"]])

print("日付ごとの集計結果を保存しました:", output_path)