Python File Handling

Learning Objectives

Until now, most of the data used by your Python programs existed only while the program was running. File handling allows your programs to store and retrieve information from files.

What Is File Handling?

File handling means using a program to create, read, modify, or manage files stored on a computer.

Files allow information to remain available even after your Python program stops running.

Example

name = "Ali"

with open("student.txt", "w") as file:
    file.write(name)

This creates or replaces student.txt and stores the student's name inside it.

Why Files Matter

Files are useful whenever information needs to survive after the program finishes.

Examples include:

• Saving customer information.
• Storing application settings.
• Saving reports.
• Keeping logs.
• Reading configuration data.
• Importing datasets.
• Exporting results.
• Storing notes and text.

Python Program
      ↓
   Write Data
      ↓
    File
      ↓
  Read Later
      ↓
Python Program

Common File Types

Python can work with many types of files.

.txt   → Text files
.csv   → Spreadsheet-style data
.json  → Structured data
.py    → Python source code
.log   → Log files
.jpg   → Image file
.png   → Image file
.pdf   → Document file

Different file formats require different techniques. In this lesson, we focus primarily on text file handling.

CSV and JSON files will be covered in more detail in later lessons.

The open() Function

Python provides the built-in open() function for working with files.

Basic syntax

open(filename, mode)

For example:

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

This opens notes.txt in read mode.

File Modes

The mode tells Python what you intend to do with the file.

"r"   Read
"w"   Write
"a"   Append
"x"   Create
"b"   Binary
"t"   Text

Read mode

"r" opens a file for reading.

Write mode

"w" opens a file for writing. If the file already exists, its previous contents are replaced.

Append mode

"a" adds new content to the end of an existing file.

Create mode

"x" creates a new file and raises an error if the file already exists.

Reading a File

The read() method reads file content.

Python is powerful.
Python is readable.
Python is versatile.

with open("notes.txt", "r") as file:
    content = file.read()

print(content)

Python is powerful.
Python is readable.
Python is versatile.

Without an argument, read() attempts to read the remaining contents of the file.

Reading a specific number of characters

with open("notes.txt", "r") as file:
    content = file.read(6)

print(content)

Python

readline()

The readline() method reads one line at a time.

with open("notes.txt", "r") as file:
    first_line = file.readline()

print(first_line)

You can call readline() repeatedly to read subsequent lines.

with open("notes.txt", "r") as file:
    line1 = file.readline()
    line2 = file.readline()

print(line1)
print(line2)

readlines()

The readlines() method reads the lines and returns them as a list.

with open("notes.txt", "r") as file:
    lines = file.readlines()

print(lines)

['Python is powerful.\n',
 'Python is readable.\n',
 'Python is versatile.']

Looping Through a File

A file object can be iterated over directly.

with open("students.txt", "r") as file:

    for line in file:
        print(line.strip())

This is a useful technique for processing files line by line without loading the entire file into memory.

Writing to a File

Use "w" mode and the write() method to write text to a file.

with open("message.txt", "w") as file:
    file.write("Hello from PyLearn!")

If message.txt does not exist, Python creates it.

If it already exists, its previous content is replaced.

Writing multiple pieces of text

with open("profile.txt", "w") as file:
    file.write("Name: Ali\n")
    file.write("Age: 25\n")
    file.write("City: Multan\n")

Appending to a File

Use "a" mode when you want to add information without deleting the existing content.

with open("activity.log", "a") as file:
    file.write("User logged in\n")

If the file already contains:

User opened the application
User viewed dashboard

After appending, it may contain:

User opened the application
User viewed dashboard
User logged in

Writing Multiple Lines

The writelines() method can write multiple strings from an iterable.

lines = [
    "Ali\n",
    "Sara\n",
    "Ahmed\n"
]

with open("students.txt", "w") as file:
    file.writelines(lines)

The with Statement

The with statement is the preferred way to handle ordinary Python file operations.

with open("notes.txt", "r") as file:
    content = file.read()

print(content)

When the with block finishes, Python handles closing the file automatically, including when an exception occurs during the block.

Manual approach

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

content = file.read()

file.close()

File Encoding

Text files store characters using an encoding. When working with text, explicitly specifying an encoding such as UTF-8 is often a good practice.

with open(
    "message.txt",
    "w",
    encoding="utf-8"
) as file:

    file.write("Hello Python! مرحبا")

You can also specify encoding when reading:

with open(
    "message.txt",
    "r",
    encoding="utf-8"
) as file:

    content = file.read()

Binary Files

Not every file contains ordinary text. Images, audio, videos, and many other formats contain binary data.

Python supports binary modes using b.

Reading binary data

with open("photo.jpg", "rb") as file:
    data = file.read()

Writing binary data

with open("copy.jpg", "wb") as file:
    file.write(data)

File Position & seek()

When you read from a file, Python maintains a current position within that file.

The seek() method moves that position.

with open("notes.txt", "r") as file:

    print(file.read(6))

    file.seek(0)

    print(file.read(6))

Python
Python

seek(0) moves the file position back to the beginning.

tell()

The tell() method returns the current file position.

with open("notes.txt", "r") as file:

    print(file.tell())

    file.read(6)

    print(file.tell())

0
6

The exact position can depend on the file and how it is opened, but the key idea is that tell() reports the current position and seek() changes it.

Common File Errors

FileNotFoundError

Reading a file that does not exist can raise FileNotFoundError.

with open("missing.txt", "r") as file:
    content = file.read()

PermissionError

Python may raise PermissionError when the operating system does not allow the requested operation.

IsADirectoryError

Attempting to open a directory as though it were a normal file can raise IsADirectoryError.

Working with Files & Folders

Python's os and pathlib modules provide tools for working with files and directories.

Using pathlib

pathlib provides an object-oriented approach to filesystem paths.

from pathlib import Path

file_path = Path("notes.txt")

print(file_path.exists())

Creating a directory

from pathlib import Path

folder = Path("reports")

folder.mkdir(exist_ok=True)

Reading and writing with pathlib

from pathlib import Path

file = Path("report.txt")

file.write_text(
    "Monthly sales report",
    encoding="utf-8"
)

content = file.read_text(
    encoding="utf-8"
)

print(content)

Real-World Examples

File handling becomes useful in many practical applications.

Example 1 — Application Log

message = "User opened the dashboard\n"

with open("app.log", "a", encoding="utf-8") as file:
    file.write(message)

Example 2 — Student Records

Ali,85
Sara,92
Ahmed,78

A Python program can read these records and process them.

Example 3 — Saving a Report

report = """Sales Report
-------------
Total Orders: 125
Revenue: 450000
Top Product: Laptop
"""

with open("sales_report.txt", "w", encoding="utf-8") as file:
    file.write(report)

Example 4 — Simple Notes Application

note = input("Enter a note: ")

with open("notes.txt", "a", encoding="utf-8") as file:
    file.write(note + "\n")

print("Note saved successfully.")

This small program demonstrates a real persistence workflow: user input → file storage → future retrieval.

Common Mistakes

Practice Exercises

Complete these exercises without copying the examples directly. Try to build each solution yourself.

Mini Challenge — Student Record Manager

Build a small command-line application that stores student records inside a text file.

Suggested file

students.txt

Each record could contain

Name: Ali
Age: 20
Marks: 85
----------------
Name: Sara
Age: 21
Marks: 92
----------------

Requirements

1. Ask the user for a student's name.
2. Ask for age.
3. Ask for marks.
4. Save the record to students.txt.
5. Use append mode so existing students are preserved.
6. Add an option to display all saved students.
7. Use the with statement for file operations.

Bonus challenge

Add a search option that asks for a student's name and displays matching records from the file.

Quick Knowledge Check

Test what you learned about Python file handling.

Lesson Summary

File handling allows Python programs to store and retrieve information outside the program's temporary memory.

You have now learned how Python programs can persist data using files. This is a major step toward building programs that work with real-world information.

In the next lesson, we will work with one of the most important data formats used by modern applications: JSON.