File Handling in Python (Read, Write, and Append)

Introduction

In real-world programs, we often need to store data permanently.
Python provides simple ways to work with files using built-in functions.

With file handling, you can:
    Create and open files
    Read from files
    Write or append data
    Close files safely

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1. Opening a File

You can open a file using the built-in open() function.

Syntax:

file = open("filename", "mode")

Mode	Description
"r"	Read (default)
"w"	Write (overwrites existing content)
"a"	Append (adds data to the end)
"r+"	Read and Write

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2. Writing to a File

Example:

file = open("example.txt", "w")
file.write("Welcome to Python File Handling!")
file.close()

Output:
    A new file named example.txt is created with the text inside.

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3. Reading from a File

Example:

file = open("example.txt", "r")
content = file.read()
print(content)
file.close()

Output:

Welcome to Python File Handling!

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4. Appending Data to a File

Example:

file = open("example.txt", "a")
file.write("\nThis line is added later.")
file.close()

Output:
The new line will be added without removing old data.

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5. Using with Statement (Best Practice)

Using with automatically closes the file — even if an error occurs.

Example:

with open("example.txt", "r") as f:
    data = f.read()
    print(data)

Output:

Welcome to Python File Handling!
This line is added later.

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6. Reading Files Line by Line

with open("example.txt", "r") as f:
    for line in f:
        print(line.strip())

Output:

Welcome to Python File Handling!
This line is added later.

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7. Writing Lists to a File

fruits = ["Apple\n", "Banana\n", "Cherry\n"]
with open("fruits.txt", "w") as f:
    f.writelines(fruits)

Output:
File fruits.txt will contain:

Apple
Banana
Cherry

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8. Checking File Existence

You can check whether a file exists before using it.

import os

if os.path.exists("example.txt"):
    print("File exists!")
else:
    print("File not found!")

Output:

File exists!

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Key Points to Remember

 Always close the file (or use with)
"w" overwrites the file
"a" appends to the existing file
"r" reads existing content

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Challenge for You

Write a Python program that:

1. Creates a file named data.txt

2. Writes 5 lines into it

3. Reads and displays the content line by line

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Final Thoughts

File handling is essential for data storage and retrieval in applications.
You’ll use it frequently when working with databases, logs, and reports.

In the next lesson, we’ll explore Exception Handling in Python, which ensures your program runs smoothly even when errors occur.

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