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Lesson 20: Python File Handling and Advanced I/O | Coding Class Series

Introduction

Welcome to Lesson 20!
In this lesson, we will learn file handling in Python, including reading, writing, appending, and advanced I/O operations.
File handling is essential to store and retrieve data for programs.

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1. Opening a File

Python uses the open() function to work with files.

Syntax:

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

Modes:

• "r" – Read (default)
• "w" – Write (creates new file or overwrites)
• "a" – Append (adds content at the end)
• "r+" – Read and Write

Example:

file = open("sample.txt", "w")
file.write("Hello, World!")
file.close()

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2. Reading from a File

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

Other reading methods:

• readline() – reads one line at a time
• readlines() – reads all lines into a list

file = open("sample.txt", "r")
print(file.readline())
print(file.readlines())
file.close()

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3. Writing and Appending

• Writing overwrites the file:

file = open("sample.txt", "w")
file.write("New content here.")
file.close()

• Appending adds data to the end:

file = open("sample.txt", "a")
file.write("\nThis line is appended.")
file.close()

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4. Using with Statement

The with statement automatically closes the file:

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

with open("sample.txt", "a") as file:
    file.write("\nAdding new line safely.")

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5. Advanced I/O

• Binary files – "rb" and "wb" modes for reading/writing binary files.
• File positioning – seek() and tell() methods.

with open("sample.txt", "r") as file:
    print(file.tell())  # Current position
    print(file.read(5)) # Read 5 characters
    file.seek(0)        # Move pointer to start

• Checking file existence using os module:

import os
if os.path.exists("sample.txt"):
    print("File exists")
else:
    print("File does not exist")

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6. Practice Exercises

1. Create a text file and write 5 lines of your choice.
2. Read the file and print each line one by one.
3. Append 3 more lines to the same file.
4. Use seek() and tell() to read specific parts of the file.
5. Try opening a file in binary mode and read/write some data.

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