Answers and explanations · Files, CSV and errors

Module 5

The conversion rejects malformed and negative values. The original text and record position remain in the reject list. DictWriter quotes a topic containing a comma, so the reader reconstructs it correctly.

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# Reference answer. Attempt the exercise first.
import csv
from io import StringIO

def parse_minutes(raw):
    value = int(raw)
    if value < 0:
        raise ValueError("minutes must be non-negative")
    return value

def read_sessions(csv_text):
    accepted, rejected = [], []
    with StringIO(csv_text) as source:
        for line_number, row in enumerate(csv.DictReader(source), start=2):
            try:
                value = parse_minutes(row["minutes"])
            except ValueError:
                rejected.append({"line": line_number, "raw": row["minutes"]})
            else:
                accepted.append({"topic": row["topic"], "minutes": value})
    return accepted, rejected

def encode_summary(summary):
    output = StringIO()
    writer = csv.DictWriter(output, fieldnames=["topic", "minutes"])
    writer.writeheader()
    writer.writerows(summary)
    return output.getvalue()

text = "topic,minutes\npython,20\ndata,unknown\ncharts,-1\npython,0\n"
accepted, rejected = read_sessions(text)
assert len(accepted) == 2 and len(rejected) == 2
assert sum(row["minutes"] for row in accepted) == 20
print(encode_summary(accepted))
quoted = encode_summary([{"topic": "data, charts", "minutes": 10}])
assert list(csv.DictReader(StringIO(quoted)))[0]["topic"] == "data, charts"
print("CSV round-trip check passed")

Exit-check guide: A value can parse successfully and still violate the task rule. Missing minutes are unknown, not zero. A missing required header should remain a visible error. The with block closes the source on normal or exceptional exit.