File Handling

advanced

Part of Python in Practice

Theory

File handling is essential for storing and retrieving data. Python's built-in open() function provides a straightforward interface for working with files.

The open() function takes a filename and a mode:

file = open("filename.txt", "r")

File modes:

"r" — read (default). Opens file for reading. Error if file doesn't exist.
"w" — write. Creates a new file or overwrites existing.
"a" — append. Writes to the end of the file. Creates if not exists.
"r+" — read and write. File must exist.
"x" — exclusive creation. Fails if file exists.

Add "b" for binary mode ("rb", "wb") for non-text files like images.

Reading methods:

read() — reads the entire file as a string
readline() — reads one line at a time
readlines() — returns a list of all lines

Writing methods:

write(string) — writes a string to the file
writelines(list) — writes a list of strings (no newlines added)

The with statement (context manager) automatically closes the file when the block exits, even if an exception occurs. This is the recommended approach:

with open("file.txt", "r") as file:
    content = file.read()
# File is automatically closed here

Exception handling prevents crashes when file operations fail. Use try/except/finally:

try:
    with open("file.txt", "r") as f:
        data = f.read()
except FileNotFoundError:
    print("File not found!")
except PermissionError:
    print("Permission denied!")
finally:
    print("Operation attempted")

The os module provides file system utilities:

os.path.exists(path) — check if file/directory exists
os.path.isfile(path) — check if path is a file
os.path.join() — join path components safely
os.remove(path) — delete a file

The csv module handles comma-separated values files, providing csv.reader() and csv.writer() for structured data.

Practical Examples

Example 1: Reading and Writing Files

python

Example 2: Exception Handling and CSV

python

Exercises

Journal App — Write and Read

easy

Write a program that asks the user for their journal entry for today, then saves it to a file called 'journal.txt' with a timestamp. Then read and display the entire journal file.

Starter Code:

from datetime import datetime\n\nentry = input('Write your journal entry: ')\n# Save and display

Expected Output:

Write your journal entry: Had a great day learning Python!\nSaved to journal.txt\n--- Journal Entry ---\n2026-01-15 14:30: Had a great day learning Python!

File Word Counter

medium

Write a program that reads a text file (create one called 'sample.txt' with 4-5 lines of text) and counts: total words, total characters (excluding spaces), and the number of lines. Use exception handling if the file doesn't exist.

Starter Code:

# Create sample.txt first\nwith open('sample.txt', 'w') as f:\n    f.write('...')  # add your text\n\n# Then count words, chars, and lines

Expected Output:

Lines: 4\nWords: 28\nCharacters (no spaces): 142

CSV Grade Manager

medium

Create a CSV file called 'grades.csv' with columns: Name, Math, Science, English. Write at least 3 student records with numeric scores. Then read the CSV and calculate each student's average. Print a report sorted by average (highest first).

Starter Code:

import csv\n\n# Write the CSV and then read and calculate averages

Expected Output:

Charlie: 93.3\nAlice: 88.3\nBob: 75.0

Mini Quiz

Mini Project

Mini Project: Personal Diary Application

Build a diary application that allows users to write entries, read all entries, and search entries by keyword. Data is stored in a file.

Requirements:

Bonus Challenge

Export all entries to a CSV file with columns: Date, Entry.