CSV File Handling in Python

Prerequisites: Text file handling, lists, basic file modes

1. What is CSV?

CSV stands for Comma Separated Values., A CSV file stores tabular data (rows and columns) as plain text., Each line is a row (record), and values within a row are separated by commas., CSV files can be opened in spreadsheet applications (Excel, Google Sheets) or any text editor.

Example CSV Content (students.csv)

RollNo,Name,Marks
101,Aman,85
102,Priya,92
103,Rahul,78

2. The csv Module

Python's built-in csv module provides functions to read and write CSV files.

import csv

3. Opening CSV Files

Why newline=''?

When opening CSV files for writing, always use newline="" to prevent extra blank lines on Windows.

# Writing - use newline=""
f = open("data.csv", "w", newline="")

# Reading - newline="" is optional but recommended
f = open("data.csv", "r")

Without newline="", Windows adds an extra \r\n resulting in blank rows between data rows.

4. Writing CSV Files

csv.writer, Create a Writer Object

import csv

f = open("data.csv", "w", newline="")
writer = csv.writer(f)

writerow, Write a Single Row

import csv

with open("data.csv", "w", newline="") as f:
 writer = csv.writer(f)
 writer.writerow(["RollNo", "Name", "Marks"]) # Header row
 writer.writerow([101, "Aman", 85]) # Data row
 writer.writerow([102, "Priya", 92])

print("Data written to data.csv")

# data.csv contents:
# RollNo,Name,Marks
# 101,Aman,85
# 102,Priya,92

writerows, Write Multiple Rows at Once

import csv

header = ["RollNo", "Name", "Marks"]
data = [
 [101, "Aman", 85],
 [102, "Priya", 92],
 [103, "Rahul", 78],
 [104, "Sneha", 95]
]

with open("data.csv", "w", newline="") as f:
 writer = csv.writer(f)
 writer.writerow(header) # Write header
 writer.writerows(data) # Write all data rows at once

print("Multiple rows written to data.csv")

# data.csv contents:
# RollNo,Name,Marks
# 101,Aman,85
# 102,Priya,92
# 103,Rahul,78
# 104,Sneha,95

5. Reading CSV Files

csv.reader, Create a Reader Object

import csv

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

# Output:
# ['RollNo', 'Name', 'Marks']
# ['101', 'Aman', '85']
# ['102', 'Priya', '92']
# ['103', 'Rahul', '78']
# ['104', 'Sneha', '95']

Note: Every value is read as a string. Numbers must be explicitly converted using int or float.

Skipping the Header Row

import csv

with open("data.csv", "r") as f:
 reader = csv.reader(f)
 header = next(reader) # Read and skip header
 print("Columns:", header)

 for row in reader:
 print(f"Roll: {row[0]}, Name: {row[1]}, Marks: {row[2]}")

# Output:
# Columns: ['RollNo', 'Name', 'Marks']
# Roll: 101, Name: Aman, Marks: 85
# Roll: 102, Name: Priya, Marks: 92
# Roll: 103, Name: Rahul, Marks: 78
# Roll: 104, Name: Sneha, Marks: 95

Formatted Display

import csv

with open("data.csv", "r") as f:
 reader = csv.reader(f)
 header = next(reader)
 print(f"{header[0]:<10}{header[1]:<15}{header[2]:<10}")
 print("-" * 35)
 for row in reader:
 print(f"{row[0]:<10}{row[1]:<15}{row[2]:<10}")

# Output:
# RollNo Name Marks
# -----------------------------------
# 101 Aman 85
# 102 Priya 92
# 103 Rahul 78
# 104 Sneha 95

6. DictReader and DictWriter (Brief)

csv.DictWriter, Write Using Dictionaries

import csv

fieldnames = ["RollNo", "Name", "Marks"]

with open("dict_data.csv", "w", newline="") as f:
 writer = csv.DictWriter(f, fieldnames=fieldnames)
 writer.writeheader
 writer.writerow({"RollNo": 101, "Name": "Aman", "Marks": 85})
 writer.writerow({"RollNo": 102, "Name": "Priya", "Marks": 92})

# dict_data.csv contents:
# RollNo,Name,Marks
# 101,Aman,85
# 102,Priya,92

csv.DictReader, Read as Dictionaries

import csv

with open("dict_data.csv", "r") as f:
 reader = csv.DictReader(f)
 for row in reader:
 print(f"Roll: {row['RollNo']}, Name: {row['Name']}, Marks: {row['Marks']}")

# Output:
# Roll: 101, Name: Aman, Marks: 85
# Roll: 102, Name: Priya, Marks: 92

Advantage: You access values by column name instead of index, more readable and less error-prone.

Practice Programs

Program 1: Create CSV with Headers and Data

import csv

header = ["Name", "Class", "Section", "Marks"]
students = [
 ["Aman", "XII", "A", 85],
 ["Priya", "XII", "B", 92],
 ["Rahul", "XII", "A", 78],
 ["Sneha", "XII", "C", 95],
 ["Vikram", "XII", "B", 88]
]

with open("students.csv", "w", newline="") as f:
 writer = csv.writer(f)
 writer.writerow(header)
 writer.writerows(students)

print("CSV file 'students.csv' created successfully!")
print(f"Total records written: {len(students)}")

# Output:
# CSV file 'students.csv' created successfully!
# Total records written: 5

# students.csv contents:
# Name,Class,Section,Marks
# Aman,XII,A,85
# Priya,XII,B,92
# Rahul,XII,A,78
# Sneha,XII,C,95
# Vikram,XII,B,88

Program 2: Read CSV and Display

import csv

print("Student Data:")
print("=" * 50)
print(f"{'Name':<12}{'Class':<8}{'Section':<10}{'Marks':<8}")
print("=" * 50)

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

 for row in reader:
 print(f"{row[0]:<12}{row[1]:<8}{row[2]:<10}{row[3]:<8}")

print("=" * 50)

# Output:
# Student Data:
# ==================================================
# Name Class Section Marks
# ==================================================
# Aman XII A 85
# Priya XII B 92
# Rahul XII A 78
# Sneha XII C 95
# Vikram XII B 88
# ==================================================

Program 3: Create CSV with User-ID and Password

import csv

with open("users.csv", "w", newline="") as f:
 writer = csv.writer(f)
 writer.writerow(["UserID", "Password"])

 n = int(input("How many users to register? "))
 for i in range(n):
 print(f"\nUser {i+1}:")
 uid = input(" Enter User ID: ")
 pwd = input(" Enter Password: ")
 writer.writerow([uid, pwd])

print("\nUser data saved to users.csv")

# Test Run:
# How many users to register? 3
#
# User 1:
# Enter User ID: admin
# Enter Password: admin123
#
# User 2:
# Enter User ID: student1
# Enter Password: pass456
#
# User 3:
# Enter User ID: teacher
# Enter Password: teach789
#
# User data saved to users.csv

# users.csv:
# UserID,Password
# admin,admin123
# student1,pass456
# teacher,teach789

Program 4: Search Password for Given User-ID

import csv

search_id = input("Enter User ID to search: ")
found = False

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

 for row in reader:
 if row[0] == search_id:
 print(f"Password for '{search_id}': {row[1]}")
 found = True
 break

if not found:
 print(f"User ID '{search_id}' not found.")

# Test Run 1:
# Enter User ID to search: student1
# Password for 'student1': pass456

# Test Run 2:
# Enter User ID to search: unknown
# User ID 'unknown' not found.

Program 5: Add New Records to Existing CSV

import csv

n = int(input("How many records to add? "))

with open("students.csv", "a", newline="") as f:
 writer = csv.writer(f)

 for i in range(n):
 print(f"\nStudent {i+1}:")
 name = input(" Name: ")
 cls = input(" Class: ")
 sec = input(" Section: ")
 marks = input(" Marks: ")
 writer.writerow([name, cls, sec, marks])

print(f"\n{n} record(s) added to students.csv")

# Test Run:
# How many records to add? 2
#
# Student 1:
# Name: Kavya
# Class: XII
# Section: A
# Marks: 91
#
# Student 2:
# Name: Rohan
# Class: XII
# Section: C
# Marks: 82
#
# 2 record(s) added to students.csv

Program 6: Count Records in CSV

import csv

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

 count = 0
 total_marks = 0

 for row in reader:
 count += 1
 total_marks += int(row[3])

print(f"Total records : {count}")
print(f"Total marks : {total_marks}")
if count > 0:
 print(f"Average marks : {total_marks / count:.2f}")

# Output:
# Total records : 7
# Total marks : 611
# Average marks : 87.29

Quick Reference: csv Module Functions

Function	Purpose	Example
`csv.writer(file)`	Create writer object	`w = csv.writer(f)`
`writer.writerow(list)`	Write one row	`w.writerow([1, "A", 90])`
`writer.writerows(list_of_lists)`	Write multiple rows	`w.writerows(data)`
`csv.reader(file)`	Create reader object	`r = csv.reader(f)`
`next(reader)`	Read/skip one row	`header = next(r)`
`csv.DictWriter(file, fieldnames)`	Dict-based writer	`w = csv.DictWriter(f, fields)`
`writer.writeheader`	Write header from fieldnames	`w.writeheader`
`csv.DictReader(file)`	Dict-based reader	`r = csv.DictReader(f)`

Common Mistakes

Mistake	Correct Approach
Forgetting `newline=""` when writing	Always use `open("file.csv", "w", newline="")`
Forgetting `import csv`	Must import before using `csv.reader` or `csv.writer`
Treating CSV values as numbers	`csv.reader` returns everything as strings, convert with `int` or `float`
Using `pickle.dump/load` for CSV	CSV uses `csv.writer/reader`, not pickle
Using `writerow` with a string instead of list	`writerow("Hello")` writes `H,e,l,l,o` - always pass a list
Opening with `"w"` to add records	Use `"a"` (append) mode to add to existing file

$1$2 Quick Tips

csv vs pickle: CSV stores tabular text data; pickle stores Python objects in binary format. Know the differences.
writerow vs writerows: writerow writes ONE row (a list), writerows writes MULTIPLE rows (a list of lists).
newline="" is a frequently asked MCQ, know why it is used (prevents extra blank lines).
csv.reader returns strings - remember to convert to int/float when needed.
next(reader) is the standard way to skip the header, commonly tested.
Code writing questions typically ask: create a CSV, read a CSV, or search in a CSV.
Delimiter: By default, the delimiter is a comma. You can change it:

writer = csv.writer(f, delimiter="\t") # Tab-separated
reader = csv.reader(f, delimiter="|") # Pipe-separated

Remember the correct opening patterns:

Write: open("file.csv", "w", newline="")
Read: open("file.csv", "r")
Append: open("file.csv", "a", newline="")