CSV Files: Structured Data in Plain Text

The idea

A CSV file is a text file where each line is a row and each value is separated by a comma.

That's it. No special format, no proprietary software. Just commas and newlines.

name,score,active
Raven,85,True
Wolf,74,False
Ghost,91,True

Open it in Excel — you get a spreadsheet. Open it in Python — you get data you can work with.

CSV is the most common format for structured data at beginner level. If you'll ever deal with real-world data, you'll deal with CSV.

The file for this lesson

Download soldiers.csv and place it in the same folder as your script before running the examples.

Reading a CSV file — with csv.reader

Python has a built-in csv module. Import it, open the file, wrap it in csv.reader:

import csv

with open("soldiers.csv", "r", encoding="utf-8") as f:
    reader = csv.reader(f)
    for row in reader:
        print(row)

Output:

['name', 'score', 'active']
['Raven', '85', 'True']
['Wolf', '74', 'False']
['Ghost', '91', 'True']

Each row is a list. Every value is a string — including numbers. Convert when needed.

Skipping the header row

import csv

with open("soldiers.csv", "r", encoding="utf-8") as f:
    reader = csv.reader(f)
    next(reader)                # skip header
    for row in reader:
        name, score, active = row
        print(f"{name}: {int(score)}")

Output:

Raven: 85
Wolf: 74
Ghost: 91

Reading into a dict — csv.DictReader

DictReader uses the header row as keys — each row becomes a dict:

import csv

with open("soldiers.csv", "r", encoding="utf-8") as f:
    reader = csv.DictReader(f)
    for row in reader:
        print(row["name"], row["score"])

Output:

Raven 85
Wolf 74
Ghost 91

Cleaner than unpacking by position. Use this when your CSV has headers.

Writing a CSV file — csv.writer

import csv

soldiers = [
    ["name", "score", "active"],
    ["Raven", 85, True],
    ["Wolf",  74, False],
    ["Ghost", 91, True]
]

with open("output.csv", "w", encoding="utf-8", newline="") as f:
    writer = csv.writer(f)
    writer.writerows(soldiers)

Pass newline="" when writing CSV on Windows — it prevents double newlines.

writerows() writes a list of rows. writerow() writes one row at a time.

Writing from a list of dicts — csv.DictWriter

import csv

soldiers = [
    {"name": "Raven", "score": 85, "active": True},
    {"name": "Wolf",  "score": 74, "active": False},
    {"name": "Ghost", "score": 91, "active": True}
]

with open("output.csv", "w", encoding="utf-8", newline="") as f:
    writer = csv.DictWriter(f, fieldnames=["name", "score", "active"])
    writer.writeheader()
    writer.writerows(soldiers)

The header is written automatically. Each dict becomes a row.

Heads up!

• All values read from CSV are strings — convert with int(), float(), or bool() as needed
• Always pass newline="" when writing CSV files
• Use DictReader and DictWriter when your file has headers — cleaner and safer
• The csv module is built-in — no installation needed

What you should understand now

• CSV is plain text — commas separate values, newlines separate rows
• csv.reader reads rows as lists
• csv.DictReader reads rows as dicts using the header
• csv.writer writes lists as rows
• csv.DictWriter writes dicts as rows
• All values from CSV are strings — convert when needed