Python CSV headers: reject duplicate names before row mapping

Operation contract

The fixture reads a header row first and checks that it is exactly the expected two-column set with no duplicate spelling. It then hands the remaining text to DictReader using those names. A repeated amount column is rejected before any row is accepted. The normal rows retain text values so a later step can parse amounts under a separate numeric rule.

Failure and ownership boundary

Opening a real file needs an encoding, newline policy, byte cap, row cap and field-size bound. Checking after converting a row to a dict is too late. Column names that differ only by case are distinct here; choose a canonicalization rule before changing that contract. Python CSV imports: parse quoted fields before validating rows, Python CSV ingestion: cap bytes, rows and fields before publication and Python JSON duplicate keys: reject ambiguous object fields cover adjacent formats.

Working program

import csv
import io

def receipt_rows(text):
    if type(text) is not str or len(text) > 200:
        raise ValueError("input size")
    stream = io.StringIO(text, newline="")
    header = next(csv.reader(stream), None)
    if header is None or len(header) != 2 or set(header) != {"id", "minor"}:
        raise ValueError("header contract")
    rows = list(csv.DictReader(stream, fieldnames=header))
    if len(rows) > 5 or any(None in row or any(value is None for value in row.values()) for row in rows):
        raise ValueError("row width")
    return rows

print(receipt_rows("id,minor\nR41,125\nR42,75\n"))
try:
    receipt_rows("id,minor,minor\nR41,125,0\n")
except ValueError:
    print("duplicate header rejected")

Output

[{'id': 'R41', 'minor': '125'}, {'id': 'R42', 'minor': '75'}]
duplicate header rejected

Costs and limits

Parsing is O(n) in bounded text length and materializes O(n) row text here. DictReader does not grant a streaming memory guarantee once its result is converted to a list.

Common Mistakes

• Validate header names before row mapping.
• A parsed numeric string is not yet a validated amount.

Connected lessons

Python CSV imports: parse quoted fields before validating rows, Python CSV ingestion: cap bytes, rows and fields before publication, Python JSON duplicate keys: reject ambiguous object fields.