File Handling

Learn to read, write, and manage files to save data permanently.

Part of the free Python course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

What You'll Learn in This Lesson

1. Why Learn File Handling?

When your program ends, all variables are lost. Files let you save data permanently so it survives after your program closes.

What Can You Do with Files?

• Save user data — Store settings, preferences, scores
• Read data — Load configuration, import information
• Create logs — Track events and errors
• Process data — Work with CSV, JSON, or text files

2. Opening Files with open()

Use the open() function to work with files. You specify the filename and a mode that tells Python what you want to do.

Mode

Name

What It Does

"r"

Read

Open for reading (file must exist)

"w"

Write

Create new or overwrite existing

"a"

Append

Add to end (keeps existing content)

"x"

Create

Create new (error if file exists)

3. The with Statement (Best Practice)

Always use the with statement when working with files. It automatically closes the file when you are done, even if an error occurs.

4. Reading Files

Method

What It Returns

Best For

.read()

Entire file as one string

Small files

.readline()

One line at a time

Processing line by line

.readlines()

List of all lines

When you need a list

for line in file:

Each line (loop)

Large files (memory efficient)

5. Writing to Files

Use mode "w" to write to a file. This creates a new file or overwrites an existing one.

6. Appending to Files

Use mode "a" to add content to the end of a file without deleting existing content.

Existing Content

Use Case

Deleted (overwritten)

Creating new files, saving fresh data

Kept (added to)

Logs, history, collecting data

7. Checking if a File Exists

Before reading a file, check if it exists to avoid errors:

8. Handling File Errors

Use try-except to handle errors gracefully when working with files:

Error

Cause

FileNotFoundError

File does not exist (reading mode)

PermissionError

No permission to access file

IsADirectoryError

Tried to open a folder as a file

9. Working with CSV Files

CSV (Comma-Separated Values) is a common format for spreadsheet data. Python has a built-in csv module to work with it.

10. Working with JSON Files

JSON (JavaScript Object Notation) is used for configuration files and APIs. Python can easily convert between JSON and dictionaries.

Function

json.dumps(obj)

Convert Python dict → JSON string

json.loads(str)

Convert JSON string → Python dict

json.dump(obj, file)

Write dict to JSON file

json.load(file)

Read JSON file to dict

11. Common Mistakes to Avoid

Mistake

Problem

Solution

Forgetting to close file

Data may not save, file locked

Use with statement

Using "w" instead of "a"

Existing data deleted

Double-check your mode

Forgetting \n

All text on one line

Add \n after each line

Reading non-existent file

Check with os.path.exists()

Wrong encoding

Strange characters appear

Add encoding="utf-8"

12. Practical Examples

Example 1: Simple Note Saver

Example 2: Word Counter

Example 3: Simple Log System

Summary: Quick Reference

Operation

Code

Read entire file

with open("f.txt", "r") as f: text = f.read()

Read lines

Write (overwrite)

with open("f.txt", "w") as f: f.write("text")

with open("f.txt", "a") as f: f.write("more")

Check exists

os.path.exists("f.txt")

Read CSV

csv.reader(file)

Read JSON

Write JSON

json.dump(data, file)

Lesson 9 complete — your programs can now save and load data!

Reading, writing, appending, CSV, JSON — you know how to work with files safely using the with statement. Your programs can now remember things between runs.

🚀 Up next: Exception Handling — learn to catch errors gracefully so your programs never crash unexpectedly.

Practice quiz

Continue this course

• Previous: Dictionaries
• Next: Exception Handling