Build a CSV Expense Summarizer in Python (No Pandas Required)

Rich Polo — Fri, 23 Oct 2026 16:48:09 GMT

Spreadsheets are great until you want the same summary every month. Let's write a small, dependency-free Python CLI that reads a CSV of expenses and prints totals per category and per month.

Input format

expenses.csv:

date,category,amount
2026-09-02,food,12.50
2026-09-03,transport,3.20
2026-09-15,food,20.00
2026-10-01,rent,450.00
2026-10-04,food,8.75

The script

summarize.py:

import argparse
import csv
from collections import defaultdict
from datetime import datetime
from decimal import Decimal, InvalidOperation


def load_rows(path):
    with open(path, newline="", encoding="utf-8") as f:
        for lineno, row in enumerate(csv.DictReader(f), start=2):
            try:
                date = datetime.strptime(row["date"], "%Y-%m-%d").date()
                amount = Decimal(row["amount"])
            except (ValueError, InvalidOperation, KeyError) as e:
                print(f"skipping line {lineno}: {e}")
                continue
            yield date, row["category"].strip().lower(), amount


def summarize(rows):
    by_cat = defaultdict(Decimal)
    by_month = defaultdict(Decimal)
    for date, cat, amount in rows:
        by_cat[cat] += amount
        by_month[date.strftime("%Y-%m")] += amount
    return by_cat, by_month


def print_table(title, data):
    print(f"\n{title}")
    width = max((len(k) for k in data), default=0)
    for key, value in sorted(data.items(), key=lambda kv: -kv[1]):
        print(f"  {key:<{width}}  {value:>10.2f}")
    print(f"  {'TOTAL':<{width}}  {sum(data.values()):>10.2f}")


def main():
    parser = argparse.ArgumentParser(description="Summarize an expense CSV")
    parser.add_argument("csv_path")
    args = parser.parse_args()
    by_cat, by_month = summarize(load_rows(args.csv_path))
    print_table("By category", by_cat)
    print_table("By month", by_month)


if __name__ == "__main__":
    main()

Run it

python3 summarize.py expenses.csv

Output:

By category
  rent           450.00
  food            41.25
  transport        3.20
  TOTAL          494.45

By month
  2026-10      458.75
  2026-09       35.70
  TOTAL        494.45

Why these choices

Decimal instead of float: 0.1 + 0.2 is not 0.3 in floats. For money, use Decimal.
Generator (yield): rows stream one by one, so a huge file doesn't fill memory.
Skip bad lines, don't crash: real exports always have a broken row somewhere.

Next steps