Step 2: Clean the data

Write a clean function that turns raw rows into trustworthy ones: strip spaces, convert the numbers, compute each sale's revenue (units * price), and set aside rows that cannot be used, telling the user why. Never silently drop data: report what was rejected.

Examplepython

import csv
import io

RAW = """date,region,product,units,price
2026-01-05,North,Notebook,10,4.50
2026-01-09,South,Pen,50,1.20
2026-01-12,North,Backpack,3,39.90
2026-01-15, East ,Notebook,8,4.50
2026-02-01,South,Headphones,2,59.00
2026-02-03,West,Pen,n/a,1.20
2026-02-10,North,Pen,40,1.20
2026-02-14,East,Backpack,5,39.90
2026-02-20,,Mouse,4,19.50
2026-03-02,South,Notebook,12,4.50
2026-03-09,West,Headphones,3,59.00
2026-03-15,North,Mouse,6,19.50
2026-03-21,East,Pen,30,1.20"""

def clean(raw_rows):
    good, bad = [], []
    for r in raw_rows:
        region = r["region"].strip()
        try:
            units = int(r["units"])
            price = float(r["price"])
        except ValueError:
            bad.append((r["date"], "bad number"))
            continue
        if not region:
            bad.append((r["date"], "missing region"))
            continue
        good.append({"date": r["date"], "region": region, "product": r["product"].strip(),
                     "units": units, "price": price, "revenue": round(units * price, 2)})
    return good, bad

sales, rejected = clean(csv.DictReader(io.StringIO(RAW)))
print("kept:", len(sales), "rejected:", len(rejected))
for item in rejected:
    print("  rejected", item)
print(sales[3])

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Two rows were rejected (one bad number, one missing region) and eleven were kept. The try/except ValueError catches int('n/a') instead of crashing, and round(..., 2) keeps money to two decimals.

Your turn: write the code

Write to_int(text) that returns the integer in text, or None when it is not a whole number. Print to_int("12"), to_int("n/a") and to_int(" 7 ").

Quick check

What is better than silently dropping a bad row?

Counting it and saying why it was rejectedReplacing it with zeroIgnoring the error message

Finish the quick check (and any code exercise) to complete this lesson, or mark it done yourself.

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