06 - File Handling & Exception Handling in Python

• python
• by Sn0wAlice

Introduction

File handling allows reading and writing data to files, while exception handling ensures programs handle errors gracefully. This step will cover how to work with files and manage exceptions effectively in Python.

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1. File Handling in Python

Python provides built-in functions for reading, writing, and managing files using the open() function.

Opening and Closing Files

file = open("example.txt", "r")  # Open file in read mode
print(file.read())  # Read contents
file.close()  # Close file

Using the with Statement (Automatic Closure)

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

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2. Reading Files

Reading Entire File

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

Reading Line by Line

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

Reading Specific Number of Characters

with open("example.txt", "r") as file:
    print(file.read(10))  # Reads first 10 characters

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3. Writing to Files

Writing to a New File (Overwrites Existing Content)

with open("example.txt", "w") as file:
    file.write("Hello, Python!\nThis is a new file.")

Appending Data to an Existing File

with open("example.txt", "a") as file:
    file.write("\nAppending new content.")

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4. Exception Handling in Python

Errors can crash programs if not handled properly. Python uses try-except blocks to catch exceptions.

Basic Exception Handling

try:
    x = 10 / 0
except ZeroDivisionError:
    print("Error: Division by zero is not allowed.")

Handling Multiple Exceptions

try:
    num = int(input("Enter a number: "))
    result = 10 / num
except ValueError:
    print("Error: Invalid input, enter a number.")
except ZeroDivisionError:
    print("Error: Cannot divide by zero.")

Using finally Block (Always Executes)

try:
    file = open("example.txt", "r")
    print(file.read())
finally:
    file.close()
    print("File closed.")

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5. Raising Custom Exceptions

Use raise to define custom exceptions.

def check_age(age):
    if age < 18:
        raise ValueError("Age must be 18 or above.")
    return "Access granted."

try:
    print(check_age(15))
except ValueError as e:
    print("Error:", e)

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Exercises

• Write a program that reads a file and prints its contents.
• Create a script that writes user input to a text file.
• Modify a file by appending new data instead of overwriting it.
• Write a program that handles division by zero using try-except.
• Implement a function that raises a custom exception when an invalid condition is met.

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Conclusion

You have now learned file handling (reading, writing, appending files) and exception handling (try-except, raising custom exceptions) in Python. The next step is to explore working with libraries and APIs.

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