Reading and Writing CSV Files

CSV stands for Comma-Separated Values. It stores a two-dimensional table as plain text: one record per row, one field per column, and usually a header in the first row.

Start with a small score table

This lesson uses a three-column table. Every record has the same fields, so rows map naturally to Python lists or dictionaries.

Name,Python,Math
Alice,92,85
Bob,78,90
Mia,88,95

Write with csv.writer

Do not manually join fields with commas. csv.writer follows CSV quoting rules. writerow() writes one row; writerows() writes many. Use newline="" so the csv module manages line endings.

import csv

rows = [
    ["Name", "Python", "Math"],
    ["Alice", 92, 85],
    ["Bob", 78, 90],
    ["Mia", 88, 95]
]

with open("scores.csv", "w", newline="", encoding="utf-8") as file:
    writer = csv.writer(file)
    writer.writerows(rows)

Read by position with csv.reader

csv.reader produces a list of strings for each row. The first row is the header, which can be consumed with next(reader). Later rows use positional indexes: row[0] is the name and row[1] is the Python score. CSV is text, so scores still need conversion to integers before arithmetic.

import csv

with open("scores.csv", "r", encoding="utf-8") as file:
    reader = csv.reader(file)
    header = next(reader)
    for row in reader:
        python_score = int(row[1])
        math_score = int(row[2])
        average = (python_score + math_score) / 2
        print(row[0], "%.1f" % average)

Alice 88.5
Bob 84.0
Mia 91.5

Read, calculate, then write a report

A clear CSV program has three stages: read and convert types with csv.reader, calculate new values, then write a result with a header using csv.writer.

1 · Read csv.reader

2 · Transformint + average

3 · Write csv.writer

import csv

report = [["Name", "Average"]]
with open("scores.csv", "r", encoding="utf-8") as file:
    reader = csv.reader(file)
    next(reader)
    for row in reader:
        average = (int(row[1]) + int(row[2])) / 2
        report.append([row[0], "%.1f" % average])

with open("report.csv", "w", newline="", encoding="utf-8") as file:
    writer = csv.writer(file)
    writer.writerows(report)

Name,Average
Alice,88.5
Bob,84.0
Mia,91.5