1. What Are CSV Files?

CSV stands for Comma-Separated Values. It is one of the most common formats for storing tabular data (data organized in rows and columns). You can open CSV files in spreadsheet programs like Excel or Google Sheets.

name,age,grade
Alice,20,A
Bob,22,B
Charlie,21,A
Diana,23,B+

Python's built-in csv module handles the tricky parts of reading and writing CSV files, like dealing with commas inside quoted values.

2. Reading CSV Files with csv.reader

The csv.reader reads each row as a list of strings:

import csv

with open("students.csv", "r") as file:
    reader = csv.reader(file)

    # Read the header row first
    header = next(reader)
    print("Columns:", header)

    # Read each data row
    for row in reader:
        print(row)

import csv

with open("students.csv", "r") as file:
    reader = csv.reader(file)
    next(reader)  # Skip header

    for row in reader:
        name = row[0]
        age = row[1]
        grade = row[2]
        print(f"{name} is {age} years old with grade {grade}")

3. Writing CSV Files with csv.writer

import csv

# Data to write
students = [
    ["Alice", 95, "A"],
    ["Bob", 87, "B+"],
    ["Charlie", 92, "A-"],
    ["Diana", 78, "C+"]
]

with open("grades.csv", "w", newline="") as file:
    writer = csv.writer(file)

    # Write the header
    writer.writerow(["Name", "Score", "Grade"])

    # Write all data rows
    writer.writerows(students)

print("CSV file created!")

4. DictReader: Access Columns by Name

DictReader reads each row as a dictionary, using the header row as keys. This makes your code much more readable:

import csv

with open("students.csv", "r") as file:
    reader = csv.DictReader(file)

    for row in reader:
        # Access values by column name!
        print(f"{row['name']} - Age: {row['age']}, Grade: {row['grade']}")

Using row['name'] is much clearer than row[0] because you do not have to remember which column number corresponds to which data.

5. DictWriter: Write with Column Names

import csv

students = [
    {"name": "Alice", "score": 95, "grade": "A"},
    {"name": "Bob", "score": 87, "grade": "B+"},
    {"name": "Charlie", "score": 92, "grade": "A-"}
]

with open("results.csv", "w", newline="") as file:
    fieldnames = ["name", "score", "grade"]
    writer = csv.DictWriter(file, fieldnames=fieldnames)

    writer.writeheader()   # Write the column names
    writer.writerows(students)  # Write all rows

print("Results saved!")

6. Real-World Example: Analyzing Data

import csv

highest_score = 0
top_student = ""

with open("grades.csv", "r") as file:
    reader = csv.DictReader(file)

    for row in reader:
        score = int(row["Score"])
        if score > highest_score:
            highest_score = score
            top_student = row["Name"]

print(f"Top student: {top_student} with {highest_score} points")