Working with CSV Files

Introduction

CSV (Comma-Separated Values) is one of the most common formats for exchanging tabular data. Python's built-in csv module handles the parsing so you do not have to split strings manually.

Reading a CSV File

Given a file employees.csv:

firstname,lastname,department,salary
Alice,Smith,Engineering,80000
Bob,Jones,Finance,65000
Charlie,Brown,HR,55000

import csv

with open("employees.csv", "r", newline="") as f:
    reader = csv.reader(f)
    for row in reader:
        print(row)

Output

['firstname', 'lastname', 'department', 'salary']
['Alice', 'Smith', 'Engineering', '80000']
['Bob', 'Jones', 'Finance', '65000']
['Charlie', 'Brown', 'HR', '55000']

Always pass newline="" when opening CSV files — this prevents csv.reader from mishandling line endings on Windows.

Skipping the Header Row

import csv

with open("employees.csv", "r", newline="") as f:
    reader = csv.reader(f)
    next(reader)   # skip the header row
    for row in reader:
        print(f"{row[0]} {row[1]} — {row[3]}")

DictReader: Reading Into Dictionaries

DictReader maps each row to a dictionary using the header row as keys. This is usually more readable than using column indexes.

import csv

with open("employees.csv", "r", newline="") as f:
    reader = csv.DictReader(f)
    for row in reader:
        print(f"{row['firstname']} works in {row['department']} earning £{row['salary']}")

Writing a CSV File

import csv

employees = [
    ["Alice", "Smith", "Engineering", 80000],
    ["Bob",   "Jones", "Finance",     65000],
]

with open("output.csv", "w", newline="") as f:
    writer = csv.writer(f)
    writer.writerow(["firstname", "lastname", "department", "salary"])  # header
    writer.writerows(employees)

DictWriter: Writing From Dictionaries

import csv

employees = [
    {"firstname": "Alice", "department": "Engineering", "salary": 80000},
    {"firstname": "Bob",   "department": "Finance",     "salary": 65000},
]

fields = ["firstname", "department", "salary"]

with open("output.csv", "w", newline="") as f:
    writer = csv.DictWriter(f, fieldnames=fields)
    writer.writeheader()
    writer.writerows(employees)

Practice Exercises

• Create a CSV file of five products with columns name, category, and price. Write it using csv.writer.
• Read the file back using DictReader and print only the products with a price above 20.
• Add a new column discounted_price (price * 0.9) and write the updated data to a new CSV file.
• Read a CSV, collect all unique categories into a set, and print them.