Lesson 11: File Handling in Python

🎯 Lesson Objective

To learn how to create, read, write, and manage files in Python using built-in file handling methods.

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🧩 1. What Is File Handling?

File handling allows Python programs to store and retrieve data from files.
Common file types: Text files (.txt), CSV files (.csv), JSON files (.json), etc.

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⚙️ 2. Opening a File

Use the open() function to open a file.

file = open("example.txt", "w")  # w → write mode

File Modes

Mode	Description
r	Read (default)
w	Write (creates new or overwrites)
a	Append
x	Create (fails if file exists)
r+	Read and write

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✏️ 3. Writing to a File

file = open("example.txt", "w")
file.write("Hello, Python File Handling!\n")
file.write("This is a second line.")
file.close()

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📖 4. Reading from a File

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

✅ Output:

Hello, Python File Handling!
This is a second line.

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🔄 5. Reading Line by Line

file = open("example.txt", "r")
for line in file:
    print(line.strip())
file.close()

Output:

Hello, Python File Handling!
This is a second line.

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➕ 6. Appending to a File

file = open("example.txt", "a")
file.write("\nAdding a new line.")
file.close()

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🛡️ 7. Using with Statement

Automatically closes the file after operations.

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

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🧮 8. Other Useful File Methods

Method	Description	Example
read(size)	Reads size characters	file.read(10)
readline()	Reads one line	file.readline()
readlines()	Reads all lines into a list	file.readlines()
tell()	Returns current cursor position	file.tell()
seek(offset)	Moves cursor to position	file.seek(0)
truncate()	Truncates file	file.truncate(10)

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🌍 9. Real-Life Example – Logging Messages

with open("log.txt", "a") as log_file:
    log_file.write("User logged in at 10:00 AM\n")

with open("log.txt", "r") as log_file:
    logs = log_file.read()
    print(logs)

Output:

User logged in at 10:00 AM