Python | 標準ライブラリ（csv + pathlib + collections + datetime）だけ を使って、CSVを「商品ごと × 日付ごと」にクロス集計する実践例

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

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

# 集計用: {商品: {月: {"数量":合計, "金額":合計}}}
summary = defaultdict(lambda: 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datetimeに変換して「YYYY-MM」にまとめる
        date = datetime.strptime(row["日付"], "%Y-%m-%d")
        month = date.strftime("%Y-%m")  # 例: "2025-11"
        product = row["商品"]
        qty = int(row["数量"])
        price = int(row["金額"])
        summary[product][month]["数量"] += qty
        summary[product][month]["金額"] += price

# 表示（商品ごとに月順で出力）
for product, months in summary.items():
    print(f"=== {product} ===")
    for month, data in sorted(months.items()):
        print(f"{month}: 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"

# 集計用: {商品: {月: {"数量":合計, "金額":合計}}}
summary = defaultdict(lambda: 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datetimeに変換して「YYYY-MM」にまとめる
        date = datetime.strptime(row["日付"], "%Y-%m-%d")
        month = date.strftime("%Y-%m")  # 例: "2025-11"
        product = row["商品"]
        qty = int(row["数量"])
        price = int(row["金額"])
        summary[product][month]["数量"] += qty
        summary[product][month]["金額"] += price

# 表示（商品ごとに月順で出力）
for product, months in summary.items():
    print(f"=== {product} ===")
    for month, data in sorted(months.items()):
        print(f"{month}: 数量={data['数量']}, 金額={data['金額']}")

output_path = csv_path.parent / "monthly_cross_summary.csv"

# 全ての月を抽出してソート
all_months = sorted({m for months in summary.values() for m in months})

with output_path.open("w", newline="", encoding="utf-8") as f:
    writer = csv.writer(f)
    # ヘッダー行: 商品, 各月
    writer.writerow(["商品"] + all_months)
    for product, months in summary.items():
        row = [product]
        for m in all_months:
            row.append(months[m]["数量"] if m in months else 0)
        writer.writerow(row)

print("月ごと×商品ごとのクロス集計を保存しました:", output_path)

output_path = csv_path.parent / "monthly_cross_summary.csv"

# 全ての月を抽出してソート
all_months = sorted({m for months in summary.values() for m in months})

with output_path.open("w", newline="", encoding="utf-8") as f:
    writer = csv.writer(f)
    # ヘッダー行: 商品, 各月
    writer.writerow(["商品"] + all_months)
    for product, months in summary.items():
        row = [product]
        for m in all_months:
            row.append(months[m]["数量"] if m in months else 0)
        writer.writerow(row)

print("月ごと×商品ごとのクロス集計を保存しました:", output_path)