25. File Handling and CSV Operations in Python

Working with Text Files

Python provides built-in functions to work with text files. You can open files in read (`r`), write (`w`), append (`a`), or read/write (`r+`) modes.

# Writing to a file
with open('example.txt', 'w') as file:
    file.write('Hello, Python!\n')
    file.write('This is a text file example.')

# Reading from a file
with open('example.txt', 'r') as file:
    content = file.read()
    print(content)

Appending Data to Files

You can add new content without overwriting existing data using the append mode.

with open('example.txt', 'a') as file:
    file.write('\nAppended line!')

Working with CSV Files

CSV (Comma-Separated Values) files store tabular data. Python's `csv` module makes reading and writing CSVs simple.

Reading CSV Files

import csv

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

Writing CSV Files

import csv

data = [
    ['Name', 'Age', 'Email'],
    ['Alice', 30, 'alice@example.com'],
    ['Bob', 25, 'bob@example.com']
]

with open('output.csv', 'w', newline='') as csvfile:
    writer = csv.writer(csvfile)
    writer.writerows(data)

Best Practices

• Always use `with open(...)` to automatically close files.
• Use `newline=''` when writing CSVs to prevent blank lines on Windows.
• Validate CSV data before processing to avoid runtime errors.
• Organize file operations into functions for reusable code.

Mini Project Step

Create a CSV file `contacts.csv` with columns: name, phone, and email. Write Python code to add new contacts, read all contacts, and update or delete a contact. Try to implement this using both `csv.reader`/`csv.writer` and Python lists for practice.

Python csv Module Documentation