Read and write CSV

CSV stores tabular rows as text. Use Python’s csv module instead of joining fields with commas yourself: the module handles quoted fields that contain commas.

For file practice, download the Python practice files v1.0.0 bundle. The examples write their own files, so the samples stay unchanged. Work on a copy if you edit a sample.

import csv
from pathlib import Path

path = Path("my_sessions.csv")
with path.open("w", newline="", encoding="utf-8") as file:
    writer = csv.writer(file)
    writer.writerow(["subject", "minutes"])
    writer.writerow(["Python, basics", 25])

with path.open("r", newline="", encoding="utf-8") as file:
    rows = list(csv.reader(file))
print(rows)

[['subject', 'minutes'], ['Python, basics', '25']]

CSV fields are text when read

csv.reader returns strings. Convert a numeric field with int() before arithmetic. Opening in write mode replaces an existing file, so work in a practice folder.

import csv
from io import StringIO

data = StringIO("subject,minutes\nPython,25\nSQL,40\n")
reader = csv.reader(data)
next(reader)  # Skip the header.
total = sum(int(row[1]) for row in reader)
print(total)

65

Why this works

A CSV row is a sequence of fields. csv.writer adds quoting when a field contains a comma, so "Python, basics" remains one field when csv.reader loads it. The reader returns text values, including "25"; convert numeric fields before calculations.

Practice

Add a second data row to my_sessions.csv. Read the file and calculate the total minutes from its data rows, skipping the header as in the worked extension. Convert each minutes field to int. Then read the sample sessions.csv from the practice bundle the same way; its minutes add up to 75.

Try another case

After csv.reader loads the data row, what type is its minutes field and what should you do before adding it?