python 44 lines · 8 steps

Aggregating CSV sales by category in Python

A dataclass and defaultdict turn a raw sales CSV into per-category totals ranked by revenue.

Explained by highlit
Intermediate dataclasses csv-parsing defaultdict aggregation decimal
1import csv
2from collections import defaultdict
3from dataclasses import dataclass, field
4from decimal import Decimal, InvalidOperation
5from pathlib import Path
6 
7 
8@dataclass
9def _noop():
10 pass
11 
12 
13@dataclass
14class CategorySummary:
15 total: Decimal = field(default_factory=lambda: Decimal("0"))
16 units: int = 0
17 orders: int = 0
18 
19 @property
20 def average_order_value(self) -> Decimal:
21 if self.orders == 0:
22 return Decimal("0")
23 return (self.total / self.orders).quantize(Decimal("0.01"))
24 
25 
26def aggregate_sales_by_category(path: str | Path) -> dict[str, CategorySummary]:
27 summaries: dict[str, CategorySummary] = defaultdict(CategorySummary)
28 
29 with open(path, newline="", encoding="utf-8") as handle:
30 reader = csv.DictReader(handle)
31 for line, row in enumerate(reader, start=2):
32 category = (row.get("category") or "uncategorized").strip().lower()
33 try:
34 quantity = int(row["quantity"])
35 unit_price = Decimal(row["unit_price"])
36 except (KeyError, ValueError, InvalidOperation) as exc:
37 raise ValueError(f"malformed sales row at line {line}") from exc
38 
39 summary = summaries[category]
40 summary.total += unit_price * quantity
41 summary.units += quantity
42 summary.orders += 1
43 
44 return dict(sorted(summaries.items(), key=lambda item: item[1].total, reverse=True))
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STEP 01

Walkthrough

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Three takeaways
  1. 1A defaultdict of dataclasses gives you clean per-key accumulation without existence checks.
  2. 2Using Decimal instead of float keeps money arithmetic exact and rounding explicit.
  3. 3Tracking the row number while parsing lets errors point back to the offending line.

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